diff --git a/clang-tools-extra/clangd/Diagnostics.cpp b/clang-tools-extra/clangd/Diagnostics.cpp
index d5eca083eb65120..552dd36b6900bfa 100644
--- a/clang-tools-extra/clangd/Diagnostics.cpp
+++ b/clang-tools-extra/clangd/Diagnostics.cpp
@@ -579,7 +579,17 @@ std::vector<Diag> StoreDiags::take(const clang::tidy::ClangTidyContext *Tidy) {
   for (auto &Diag : Output) {
     if (const char *ClangDiag = getDiagnosticCode(Diag.ID)) {
       // Warnings controlled by -Wfoo are better recognized by that name.
-      StringRef Warning = DiagnosticIDs::getWarningOptionForDiag(Diag.ID);
+      const StringRef Warning = [&] {
+        if (OrigSrcMgr) {
+          return OrigSrcMgr->getDiagnostics()
+              .getDiagnosticIDs()
+              ->getWarningOptionForDiag(Diag.ID);
+        }
+        if (!DiagnosticIDs::IsCustomDiag(Diag.ID))
+          return DiagnosticIDs{}.getWarningOptionForDiag(Diag.ID);
+        return StringRef{};
+      }();
+
       if (!Warning.empty()) {
         Diag.Name = ("-W" + Warning).str();
       } else {
@@ -896,20 +906,23 @@ void StoreDiags::flushLastDiag() {
   Output.push_back(std::move(*LastDiag));
 }
 
-bool isBuiltinDiagnosticSuppressed(unsigned ID,
-                                   const llvm::StringSet<> &Suppress,
-                                   const LangOptions &LangOpts) {
+bool isDiagnosticSuppressed(const clang::Diagnostic &Diag,
+                            const llvm::StringSet<> &Suppress,
+                            const LangOptions &LangOpts) {
   // Don't complain about header-only stuff in mainfiles if it's a header.
   // FIXME: would be cleaner to suppress in clang, once we decide whether the
   //        behavior should be to silently-ignore or respect the pragma.
-  if (ID == diag::pp_pragma_sysheader_in_main_file && LangOpts.IsHeaderFile)
+  if (Diag.getID() == diag::pp_pragma_sysheader_in_main_file &&
+      LangOpts.IsHeaderFile)
     return true;
 
-  if (const char *CodePtr = getDiagnosticCode(ID)) {
+  if (const char *CodePtr = getDiagnosticCode(Diag.getID())) {
     if (Suppress.contains(normalizeSuppressedCode(CodePtr)))
       return true;
   }
-  StringRef Warning = DiagnosticIDs::getWarningOptionForDiag(ID);
+  StringRef Warning =
+      Diag.getDiags()->getDiagnosticIDs()->getWarningOptionForDiag(
+          Diag.getID());
   if (!Warning.empty() && Suppress.contains(Warning))
     return true;
   return false;
diff --git a/clang-tools-extra/clangd/Diagnostics.h b/clang-tools-extra/clangd/Diagnostics.h
index d4c0478c63a5cf1..c45d8dc3aa6ce6d 100644
--- a/clang-tools-extra/clangd/Diagnostics.h
+++ b/clang-tools-extra/clangd/Diagnostics.h
@@ -181,11 +181,11 @@ class StoreDiags : public DiagnosticConsumer {
 };
 
 /// Determine whether a (non-clang-tidy) diagnostic is suppressed by config.
-bool isBuiltinDiagnosticSuppressed(unsigned ID,
-                                   const llvm::StringSet<> &Suppressed,
-                                   const LangOptions &);
+bool isDiagnosticSuppressed(const clang::Diagnostic &Diag,
+                            const llvm::StringSet<> &Suppressed,
+                            const LangOptions &);
 /// Take a user-specified diagnostic code, and convert it to a normalized form
-/// stored in the config and consumed by isBuiltinDiagnosticsSuppressed.
+/// stored in the config and consumed by isDiagnosticsSuppressed.
 ///
 /// (This strips err_ and -W prefix so we can match with or without them.)
 llvm::StringRef normalizeSuppressedCode(llvm::StringRef);
diff --git a/clang-tools-extra/clangd/ParsedAST.cpp b/clang-tools-extra/clangd/ParsedAST.cpp
index a8b6cc8dd0a46f2..4491be9aa0362b2 100644
--- a/clang-tools-extra/clangd/ParsedAST.cpp
+++ b/clang-tools-extra/clangd/ParsedAST.cpp
@@ -340,7 +340,7 @@ void applyWarningOptions(llvm::ArrayRef<std::string> ExtraArgs,
       if (Enable) {
         if (Diags.getDiagnosticLevel(ID, SourceLocation()) <
             DiagnosticsEngine::Warning) {
-          auto Group = DiagnosticIDs::getGroupForDiag(ID);
+          auto Group = Diags.getDiagnosticIDs()->getGroupForDiag(ID);
           if (!Group || !EnabledGroups(*Group))
             continue;
           Diags.setSeverity(ID, diag::Severity::Warning, SourceLocation());
@@ -583,8 +583,8 @@ ParsedAST::build(llvm::StringRef Filename, const ParseInputs &Inputs,
     ASTDiags.setLevelAdjuster([&](DiagnosticsEngine::Level DiagLevel,
                                   const clang::Diagnostic &Info) {
       if (Cfg.Diagnostics.SuppressAll ||
-          isBuiltinDiagnosticSuppressed(Info.getID(), Cfg.Diagnostics.Suppress,
-                                        Clang->getLangOpts()))
+          isDiagnosticSuppressed(Info, Cfg.Diagnostics.Suppress,
+                                 Clang->getLangOpts()))
         return DiagnosticsEngine::Ignored;
 
       auto It = OverriddenSeverity.find(Info.getID());
diff --git a/clang-tools-extra/clangd/Preamble.cpp b/clang-tools-extra/clangd/Preamble.cpp
index dd13b1a9e5613d2..84e8fec342829c8 100644
--- a/clang-tools-extra/clangd/Preamble.cpp
+++ b/clang-tools-extra/clangd/Preamble.cpp
@@ -621,8 +621,8 @@ buildPreamble(PathRef FileName, CompilerInvocation CI,
   PreambleDiagnostics.setLevelAdjuster([&](DiagnosticsEngine::Level DiagLevel,
                                            const clang::Diagnostic &Info) {
     if (Cfg.Diagnostics.SuppressAll ||
-        isBuiltinDiagnosticSuppressed(Info.getID(), Cfg.Diagnostics.Suppress,
-                                      CI.getLangOpts()))
+        isDiagnosticSuppressed(Info, Cfg.Diagnostics.Suppress,
+                               CI.getLangOpts()))
       return DiagnosticsEngine::Ignored;
     switch (Info.getID()) {
     case diag::warn_no_newline_eof:
diff --git a/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp b/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp
index 4ecfdf0184ab40d..021d731f8f17680 100644
--- a/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp
@@ -298,20 +298,41 @@ TEST_F(ConfigCompileTests, DiagnosticSuppression) {
                                    "unreachable-code", "unused-variable",
                                    "typecheck_bool_condition",
                                    "unexpected_friend", "warn_alloca"));
-  EXPECT_TRUE(isBuiltinDiagnosticSuppressed(
-      diag::warn_unreachable, Conf.Diagnostics.Suppress, LangOptions()));
+  clang::DiagnosticsEngine DiagEngine(new DiagnosticIDs, nullptr,
+                                      new clang::IgnoringDiagConsumer);
+
+  using Diag = clang::Diagnostic;
+  {
+    auto D = DiagEngine.Report(diag::warn_unreachable);
+    EXPECT_TRUE(isDiagnosticSuppressed(
+        Diag{&DiagEngine}, Conf.Diagnostics.Suppress, LangOptions()));
+  }
   // Subcategory not respected/suppressed.
-  EXPECT_FALSE(isBuiltinDiagnosticSuppressed(
-      diag::warn_unreachable_break, Conf.Diagnostics.Suppress, LangOptions()));
-  EXPECT_TRUE(isBuiltinDiagnosticSuppressed(
-      diag::warn_unused_variable, Conf.Diagnostics.Suppress, LangOptions()));
-  EXPECT_TRUE(isBuiltinDiagnosticSuppressed(diag::err_typecheck_bool_condition,
-                                            Conf.Diagnostics.Suppress,
-                                            LangOptions()));
-  EXPECT_TRUE(isBuiltinDiagnosticSuppressed(
-      diag::err_unexpected_friend, Conf.Diagnostics.Suppress, LangOptions()));
-  EXPECT_TRUE(isBuiltinDiagnosticSuppressed(
-      diag::warn_alloca, Conf.Diagnostics.Suppress, LangOptions()));
+  {
+    auto D = DiagEngine.Report(diag::warn_unreachable_break);
+    EXPECT_FALSE(isDiagnosticSuppressed(
+        Diag{&DiagEngine}, Conf.Diagnostics.Suppress, LangOptions()));
+  }
+  {
+    auto D = DiagEngine.Report(diag::warn_unused_variable);
+    EXPECT_TRUE(isDiagnosticSuppressed(
+        Diag{&DiagEngine}, Conf.Diagnostics.Suppress, LangOptions()));
+  }
+  {
+    auto D = DiagEngine.Report(diag::err_typecheck_bool_condition);
+    EXPECT_TRUE(isDiagnosticSuppressed(
+        Diag{&DiagEngine}, Conf.Diagnostics.Suppress, LangOptions()));
+  }
+  {
+    auto D = DiagEngine.Report(diag::err_unexpected_friend);
+    EXPECT_TRUE(isDiagnosticSuppressed(
+        Diag{&DiagEngine}, Conf.Diagnostics.Suppress, LangOptions()));
+  }
+  {
+    auto D = DiagEngine.Report(diag::warn_alloca);
+    EXPECT_TRUE(isDiagnosticSuppressed(
+        Diag{&DiagEngine}, Conf.Diagnostics.Suppress, LangOptions()));
+  }
 
   Frag.Diagnostics.Suppress.emplace_back("*");
   EXPECT_TRUE(compileAndApply());
diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index 9a7b163b2c6da83..70fad60d4edbb56 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -3358,18 +3358,16 @@ def DiagnoseIf : InheritableAttr {
   let Spellings = [GNU<"diagnose_if">];
   let Subjects = SubjectList<[Function, ObjCMethod, ObjCProperty]>;
   let Args = [ExprArgument<"Cond">, StringArgument<"Message">,
-              EnumArgument<"DiagnosticType", "DiagnosticType",
+              EnumArgument<"DefaultSeverity",
+                           "DefaultSeverity",
                            /*is_string=*/true,
-                           ["error", "warning"],
-                           ["DT_Error", "DT_Warning"]>,
+                           ["error",    "warning"],
+                           ["DS_error", "DS_warning"]>,
+              StringArgument<"WarningGroup", /*optional*/ 1>,
               BoolArgument<"ArgDependent", 0, /*fake*/ 1>,
               DeclArgument<Named, "Parent", 0, /*fake*/ 1>];
   let InheritEvenIfAlreadyPresent = 1;
   let LateParsed = LateAttrParseStandard;
-  let AdditionalMembers = [{
-    bool isError() const { return diagnosticType == DT_Error; }
-    bool isWarning() const { return diagnosticType == DT_Warning; }
-  }];
   let TemplateDependent = 1;
   let Documentation = [DiagnoseIfDocs];
 }
diff --git a/clang/include/clang/Basic/Diagnostic.h b/clang/include/clang/Basic/Diagnostic.h
index 0c7836c2ea569cc..54b69e98540239a 100644
--- a/clang/include/clang/Basic/Diagnostic.h
+++ b/clang/include/clang/Basic/Diagnostic.h
@@ -336,10 +336,12 @@ class DiagnosticsEngine : public RefCountedBase<DiagnosticsEngine> {
     // Map extensions to warnings or errors?
     diag::Severity ExtBehavior = diag::Severity::Ignored;
 
-    DiagState()
+    DiagnosticIDs &DiagIDs;
+
+    DiagState(DiagnosticIDs &DiagIDs)
         : IgnoreAllWarnings(false), EnableAllWarnings(false),
           WarningsAsErrors(false), ErrorsAsFatal(false),
-          SuppressSystemWarnings(false) {}
+          SuppressSystemWarnings(false), DiagIDs(DiagIDs) {}
 
     using iterator = llvm::DenseMap<unsigned, DiagnosticMapping>::iterator;
     using const_iterator =
@@ -870,6 +872,8 @@ class DiagnosticsEngine : public RefCountedBase<DiagnosticsEngine> {
   /// \param FormatString A fixed diagnostic format string that will be hashed
   /// and mapped to a unique DiagID.
   template <unsigned N>
+  // TODO: Deprecate this once all uses are removed from LLVM
+  // [[deprecated("Use a CustomDiagDesc instead of a Level")]]
   unsigned getCustomDiagID(Level L, const char (&FormatString)[N]) {
     return Diags->getCustomDiagID((DiagnosticIDs::Level)L,
                                   StringRef(FormatString, N - 1));
diff --git a/clang/include/clang/Basic/DiagnosticCategories.h b/clang/include/clang/Basic/DiagnosticCategories.h
index 14be326f7515f8f..839f8dee3ca89fb 100644
--- a/clang/include/clang/Basic/DiagnosticCategories.h
+++ b/clang/include/clang/Basic/DiagnosticCategories.h
@@ -21,11 +21,12 @@ namespace clang {
     };
 
     enum class Group {
-#define DIAG_ENTRY(GroupName, FlagNameOffset, Members, SubGroups, Docs)        \
-  GroupName,
+#define DIAG_ENTRY(GroupName, FlagNameOffset, Members, SubGroups, Docs)    \
+      GroupName,
 #include "clang/Basic/DiagnosticGroups.inc"
 #undef CATEGORY
 #undef DIAG_ENTRY
+      NUM_GROUPS
     };
   }  // end namespace diag
 }  // end namespace clang
diff --git a/clang/include/clang/Basic/DiagnosticIDs.h b/clang/include/clang/Basic/DiagnosticIDs.h
index 8b976bdac6dc513..2402996ece5c94f 100644
--- a/clang/include/clang/Basic/DiagnosticIDs.h
+++ b/clang/include/clang/Basic/DiagnosticIDs.h
@@ -14,6 +14,7 @@
 #ifndef LLVM_CLANG_BASIC_DIAGNOSTICIDS_H
 #define LLVM_CLANG_BASIC_DIAGNOSTICIDS_H
 
+#include "clang/Basic/DiagnosticCategories.h"
 #include "clang/Basic/LLVM.h"
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
 #include "llvm/ADT/StringRef.h"
@@ -82,7 +83,7 @@ namespace clang {
     /// to either Ignore (nothing), Remark (emit a remark), Warning
     /// (emit a warning) or Error (emit as an error).  It allows clients to
     /// map ERRORs to Error or Fatal (stop emitting diagnostics after this one).
-    enum class Severity {
+    enum class Severity : uint8_t {
       // NOTE: 0 means "uncomputed".
       Ignored = 1, ///< Do not present this diagnostic, ignore it.
       Remark = 2,  ///< Present this diagnostic as a remark.
@@ -179,13 +180,96 @@ class DiagnosticMapping {
 class DiagnosticIDs : public RefCountedBase<DiagnosticIDs> {
 public:
   /// The level of the diagnostic, after it has been through mapping.
-  enum Level {
-    Ignored, Note, Remark, Warning, Error, Fatal
+  enum Level : uint8_t { Ignored, Note, Remark, Warning, Error, Fatal };
+
+  // Diagnostic classes.
+  enum Class {
+    CLASS_INVALID = 0x00,
+    CLASS_NOTE = 0x01,
+    CLASS_REMARK = 0x02,
+    CLASS_WARNING = 0x03,
+    CLASS_EXTENSION = 0x04,
+    CLASS_ERROR = 0x05
+  };
+
+  static bool IsCustomDiag(diag::kind Diag) {
+    return Diag >= diag::DIAG_UPPER_LIMIT;
+  }
+
+  class CustomDiagDesc {
+    LLVM_PREFERRED_TYPE(diag::Severity)
+    unsigned DefaultSeverity : 3;
+    LLVM_PREFERRED_TYPE(Class)
+    unsigned DiagClass : 3;
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned ShowInSystemHeader : 1;
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned ShowInSystemMacro : 1;
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned HasGroup : 1;
+    diag::Group Group;
+    std::string Description;
+
+    auto get_as_tuple() const {
+      return std::tuple(DefaultSeverity, DiagClass, ShowInSystemHeader,
+                        ShowInSystemMacro, HasGroup, Group,
+                        std::string_view{Description});
+    }
+
+  public:
+    CustomDiagDesc(diag::Severity DefaultSeverity, std::string Description,
+                   unsigned Class = CLASS_WARNING,
+                   bool ShowInSystemHeader = false,
+                   bool ShowInSystemMacro = false,
+                   std::optional<diag::Group> Group = std::nullopt)
+        : DefaultSeverity(static_cast<unsigned>(DefaultSeverity)),
+          DiagClass(Class), ShowInSystemHeader(ShowInSystemHeader),
+          ShowInSystemMacro(ShowInSystemMacro), HasGroup(Group != std::nullopt),
+          Group(Group.value_or(diag::Group{})),
+          Description(std::move(Description)) {}
+
+    std::optional<diag::Group> GetGroup() const {
+      if (HasGroup)
+        return Group;
+      return std::nullopt;
+    }
+
+    diag::Severity GetDefaultSeverity() const {
+      return static_cast<diag::Severity>(DefaultSeverity);
+    }
+
+    Class GetClass() const { return static_cast<Class>(DiagClass); }
+    std::string_view GetDescription() const { return Description; }
+    bool ShouldShowInSystemHeader() const { return ShowInSystemHeader; }
+
+    friend bool operator==(const CustomDiagDesc &lhs,
+                           const CustomDiagDesc &rhs) {
+      return lhs.get_as_tuple() == rhs.get_as_tuple();
+    }
+
+    friend bool operator<(const CustomDiagDesc &lhs,
+                          const CustomDiagDesc &rhs) {
+      return lhs.get_as_tuple() < rhs.get_as_tuple();
+    }
+  };
+
+  struct GroupInfo {
+    LLVM_PREFERRED_TYPE(diag::Severity)
+    unsigned Severity : 3;
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned HasNoWarningAsError : 1;
   };
 
 private:
   /// Information for uniquing and looking up custom diags.
   std::unique_ptr<diag::CustomDiagInfo> CustomDiagInfo;
+  std::unique_ptr<GroupInfo[]> GroupInfos = []() {
+    auto GIs = std::make_unique<GroupInfo[]>(
+        static_cast<size_t>(diag::Group::NUM_GROUPS));
+    for (size_t i = 0; i != static_cast<size_t>(diag::Group::NUM_GROUPS); ++i)
+      GIs[i] = {{}, false};
+    return GIs;
+  }();
 
 public:
   DiagnosticIDs();
@@ -200,7 +284,34 @@ class DiagnosticIDs : public RefCountedBase<DiagnosticIDs> {
   // FIXME: Replace this function with a create-only facilty like
   // createCustomDiagIDFromFormatString() to enforce safe usage. At the time of
   // writing, nearly all callers of this function were invalid.
-  unsigned getCustomDiagID(Level L, StringRef FormatString);
+  unsigned getCustomDiagID(CustomDiagDesc Diag);
+
+  // TODO: Deprecate this once all uses are removed from LLVM
+  // [[deprecated("Use a CustomDiagDesc instead of a Level")]]
+  unsigned getCustomDiagID(Level Level, StringRef Message) {
+    return getCustomDiagID([&]() -> CustomDiagDesc {
+      switch (Level) {
+      case DiagnosticIDs::Level::Ignored:
+        return {diag::Severity::Ignored, std::string(Message), CLASS_WARNING,
+                /*ShowInSystemHeader*/ true};
+      case DiagnosticIDs::Level::Note:
+        return {diag::Severity::Fatal, std::string(Message), CLASS_NOTE,
+                /*ShowInSystemHeader*/ true};
+      case DiagnosticIDs::Level::Remark:
+        return {diag::Severity::Remark, std::string(Message), CLASS_REMARK,
+                /*ShowInSystemHeader*/ true};
+      case DiagnosticIDs::Level::Warning:
+        return {diag::Severity::Warning, std::string(Message), CLASS_WARNING,
+                /*ShowInSystemHeader*/ true};
+      case DiagnosticIDs::Level::Error:
+        return {diag::Severity::Error, std::string(Message), CLASS_ERROR,
+                /*ShowInSystemHeader*/ true};
+      case DiagnosticIDs::Level::Fatal:
+        return {diag::Severity::Fatal, std::string(Message), CLASS_ERROR,
+                /*ShowInSystemHeader*/ true};
+      }
+    }());
+  }
 
   //===--------------------------------------------------------------------===//
   // Diagnostic classification and reporting interfaces.
@@ -212,35 +323,36 @@ class DiagnosticIDs : public RefCountedBase<DiagnosticIDs> {
   /// Return true if the unmapped diagnostic levelof the specified
   /// diagnostic ID is a Warning or Extension.
   ///
-  /// This only works on builtin diagnostics, not custom ones, and is not
-  /// legal to call on NOTEs.
-  static bool isBuiltinWarningOrExtension(unsigned DiagID);
+  /// This is not legal to call on NOTEs.
+  bool isWarningOrExtension(unsigned DiagID) const;
 
   /// Return true if the specified diagnostic is mapped to errors by
   /// default.
-  static bool isDefaultMappingAsError(unsigned DiagID);
+  bool isDefaultMappingAsError(unsigned DiagID) const;
 
   /// Get the default mapping for this diagnostic.
-  static DiagnosticMapping getDefaultMapping(unsigned DiagID);
+  DiagnosticMapping getDefaultMapping(unsigned DiagID) const;
+
+  void initCustomDiagMapping(DiagnosticMapping &, unsigned DiagID);
 
-  /// Determine whether the given built-in diagnostic ID is a Note.
-  static bool isBuiltinNote(unsigned DiagID);
+  /// Determine whether the given diagnostic ID is a Note.
+  bool isNote(unsigned DiagID) const;
 
-  /// Determine whether the given built-in diagnostic ID is for an
+  /// Determine whether the given diagnostic ID is for an
   /// extension of some sort.
-  static bool isBuiltinExtensionDiag(unsigned DiagID) {
+  bool isExtensionDiag(unsigned DiagID) const {
     bool ignored;
-    return isBuiltinExtensionDiag(DiagID, ignored);
+    return isExtensionDiag(DiagID, ignored);
   }
 
-  /// Determine whether the given built-in diagnostic ID is for an
+  /// Determine whether the given diagnostic ID is for an
   /// extension of some sort, and whether it is enabled by default.
   ///
   /// This also returns EnabledByDefault, which is set to indicate whether the
   /// diagnostic is ignored by default (in which case -pedantic enables it) or
   /// treated as a warning/error by default.
   ///
-  static bool isBuiltinExtensionDiag(unsigned DiagID, bool &EnabledByDefault);
+  bool isExtensionDiag(unsigned DiagID, bool &EnabledByDefault) const;
 
   /// Given a group ID, returns the flag that toggles the group.
   /// For example, for Group::DeprecatedDeclarations, returns
@@ -250,19 +362,22 @@ class DiagnosticIDs : public RefCountedBase<DiagnosticIDs> {
   /// Given a diagnostic group ID, return its documentation.
   static StringRef getWarningOptionDocumentation(diag::Group GroupID);
 
+  void setGroupSeverity(StringRef Group, diag::Severity);
+  void setGroupNoWarningsAsError(StringRef Group, bool);
+
   /// Given a group ID, returns the flag that toggles the group.
   /// For example, for "deprecated-declarations", returns
   /// Group::DeprecatedDeclarations.
   static std::optional<diag::Group> getGroupForWarningOption(StringRef);
 
   /// Return the lowest-level group that contains the specified diagnostic.
-  static std::optional<diag::Group> getGroupForDiag(unsigned DiagID);
+  std::optional<diag::Group> getGroupForDiag(unsigned DiagID) const;
 
   /// Return the lowest-level warning option that enables the specified
   /// diagnostic.
   ///
   /// If there is no -Wfoo flag that controls the diagnostic, this returns null.
-  static StringRef getWarningOptionForDiag(unsigned DiagID);
+  StringRef getWarningOptionForDiag(unsigned DiagID);
 
   /// Return the category number that a specified \p DiagID belongs to,
   /// or 0 if no category.
@@ -363,6 +478,8 @@ class DiagnosticIDs : public RefCountedBase<DiagnosticIDs> {
   getDiagnosticSeverity(unsigned DiagID, SourceLocation Loc,
                         const DiagnosticsEngine &Diag) const LLVM_READONLY;
 
+  Class getDiagClass(unsigned DiagID) const;
+
   /// Used to report a diagnostic that is finally fully formed.
   ///
   /// \returns \c true if the diagnostic was emitted, \c false if it was
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index e506811b0347456..d42558d2223aae0 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2933,9 +2933,15 @@ def ext_constexpr_function_never_constant_expr : ExtWarn<
   "constant expression">, InGroup<DiagGroup<"invalid-constexpr">>, DefaultError;
 def err_attr_cond_never_constant_expr : Error<
   "%0 attribute expression never produces a constant expression">;
+def err_diagnose_if_unknown_warning : Error<"unknown warning group '%0'">;
 def err_diagnose_if_invalid_diagnostic_type : Error<
   "invalid diagnostic type for 'diagnose_if'; use \"error\" or \"warning\" "
   "instead">;
+def err_diagnose_if_unknown_option : Error<"unknown diagnostic option">;
+def err_diagnose_if_expected_equals : Error<
+  "expected '=' after diagnostic option">;
+def err_diagnose_if_unexpected_value : Error<
+  "unexpected value; use 'true' or 'false'">;
 def err_constexpr_body_no_return : Error<
   "no return statement in %select{constexpr|consteval}0 function">;
 def err_constexpr_return_missing_expr : Error<
diff --git a/clang/lib/Basic/Diagnostic.cpp b/clang/lib/Basic/Diagnostic.cpp
index 66776daa5e14937..ecff80a5090630f 100644
--- a/clang/lib/Basic/Diagnostic.cpp
+++ b/clang/lib/Basic/Diagnostic.cpp
@@ -138,7 +138,7 @@ void DiagnosticsEngine::Reset(bool soft /*=false*/) {
 
     // Create a DiagState and DiagStatePoint representing diagnostic changes
     // through command-line.
-    DiagStates.emplace_back();
+    DiagStates.emplace_back(*Diags);
     DiagStatesByLoc.appendFirst(&DiagStates.back());
   }
 }
@@ -166,8 +166,11 @@ DiagnosticsEngine::DiagState::getOrAddMapping(diag::kind Diag) {
       DiagMap.insert(std::make_pair(Diag, DiagnosticMapping()));
 
   // Initialize the entry if we added it.
-  if (Result.second)
-    Result.first->second = DiagnosticIDs::getDefaultMapping(Diag);
+  if (Result.second) {
+    Result.first->second = DiagIDs.getDefaultMapping(Diag);
+    if (DiagnosticIDs::IsCustomDiag(Diag))
+      DiagIDs.initCustomDiagMapping(Result.first->second, Diag);
+  }
 
   return Result.first->second;
 }
@@ -309,7 +312,8 @@ void DiagnosticsEngine::DiagStateMap::dump(SourceManager &SrcMgr,
 
       for (auto &Mapping : *Transition.State) {
         StringRef Option =
-            DiagnosticIDs::getWarningOptionForDiag(Mapping.first);
+            SrcMgr.getDiagnostics().Diags->getWarningOptionForDiag(
+                Mapping.first);
         if (!DiagName.empty() && DiagName != Option)
           continue;
 
@@ -353,9 +357,7 @@ void DiagnosticsEngine::PushDiagStatePoint(DiagState *State,
 
 void DiagnosticsEngine::setSeverity(diag::kind Diag, diag::Severity Map,
                                     SourceLocation L) {
-  assert(Diag < diag::DIAG_UPPER_LIMIT &&
-         "Can only map builtin diagnostics");
-  assert((Diags->isBuiltinWarningOrExtension(Diag) ||
+  assert((Diags->isWarningOrExtension(Diag) ||
           (Map == diag::Severity::Fatal || Map == diag::Severity::Error)) &&
          "Cannot map errors into warnings!");
   assert((L.isInvalid() || SourceMgr) && "No SourceMgr for valid location");
@@ -407,6 +409,8 @@ bool DiagnosticsEngine::setSeverityForGroup(diag::Flavor Flavor,
   if (Diags->getDiagnosticsInGroup(Flavor, Group, GroupDiags))
     return true;
 
+  Diags->setGroupSeverity(Group, Map);
+
   // Set the mapping.
   for (diag::kind Diag : GroupDiags)
     setSeverity(Diag, Map, Loc);
@@ -429,6 +433,7 @@ bool DiagnosticsEngine::setDiagnosticGroupWarningAsError(StringRef Group,
   if (Enabled)
     return setSeverityForGroup(diag::Flavor::WarningOrError, Group,
                                diag::Severity::Error);
+  Diags->setGroupSeverity(Group, diag::Severity::Warning);
 
   // Otherwise, we want to set the diagnostic mapping's "no Werror" bit, and
   // potentially downgrade anything already mapped to be a warning.
@@ -460,6 +465,7 @@ bool DiagnosticsEngine::setDiagnosticGroupErrorAsFatal(StringRef Group,
   if (Enabled)
     return setSeverityForGroup(diag::Flavor::WarningOrError, Group,
                                diag::Severity::Fatal);
+  Diags->setGroupSeverity(Group, diag::Severity::Error);
 
   // Otherwise, we want to set the diagnostic mapping's "no Wfatal-errors" bit,
   // and potentially downgrade anything already mapped to be a fatal error.
@@ -492,7 +498,7 @@ void DiagnosticsEngine::setSeverityForAll(diag::Flavor Flavor,
 
   // Set the mapping.
   for (diag::kind Diag : AllDiags)
-    if (Diags->isBuiltinWarningOrExtension(Diag))
+    if (Diags->isWarningOrExtension(Diag))
       setSeverity(Diag, Map, Loc);
 }
 
diff --git a/clang/lib/Basic/DiagnosticIDs.cpp b/clang/lib/Basic/DiagnosticIDs.cpp
index cd42573968b2129..cae6642bd4bd3e0 100644
--- a/clang/lib/Basic/DiagnosticIDs.cpp
+++ b/clang/lib/Basic/DiagnosticIDs.cpp
@@ -102,13 +102,12 @@ const uint32_t StaticDiagInfoDescriptionOffsets[] = {
 #undef DIAG
 };
 
-// Diagnostic classes.
 enum DiagnosticClass {
-  CLASS_NOTE       = 0x01,
-  CLASS_REMARK     = 0x02,
-  CLASS_WARNING    = 0x03,
-  CLASS_EXTENSION  = 0x04,
-  CLASS_ERROR      = 0x05
+  CLASS_NOTE = DiagnosticIDs::CLASS_NOTE,
+  CLASS_REMARK = DiagnosticIDs::CLASS_REMARK,
+  CLASS_WARNING = DiagnosticIDs::CLASS_WARNING,
+  CLASS_EXTENSION = DiagnosticIDs::CLASS_EXTENSION,
+  CLASS_ERROR = DiagnosticIDs::CLASS_ERROR,
 };
 
 struct StaticDiagInfoRec {
@@ -269,11 +268,60 @@ CATEGORY(INSTALLAPI, REFACTORING)
   return Found;
 }
 
-DiagnosticMapping DiagnosticIDs::getDefaultMapping(unsigned DiagID) {
+//===----------------------------------------------------------------------===//
+// Custom Diagnostic information
+//===----------------------------------------------------------------------===//
+
+namespace clang {
+namespace diag {
+using CustomDiagDesc = DiagnosticIDs::CustomDiagDesc;
+class CustomDiagInfo {
+  std::vector<CustomDiagDesc> DiagInfo;
+  std::map<CustomDiagDesc, unsigned> DiagIDs;
+  std::map<diag::Group, std::vector<unsigned>> GroupToDiags;
+
+public:
+  /// getDescription - Return the description of the specified custom
+  /// diagnostic.
+  const CustomDiagDesc &getDescription(unsigned DiagID) const {
+    assert(DiagID - DIAG_UPPER_LIMIT < DiagInfo.size() &&
+           "Invalid diagnostic ID");
+    return DiagInfo[DiagID - DIAG_UPPER_LIMIT];
+  }
+
+  unsigned getOrCreateDiagID(DiagnosticIDs::CustomDiagDesc D) {
+    // Check to see if it already exists.
+    std::map<CustomDiagDesc, unsigned>::iterator I = DiagIDs.lower_bound(D);
+    if (I != DiagIDs.end() && I->first == D)
+      return I->second;
+
+    // If not, assign a new ID.
+    unsigned ID = DiagInfo.size() + DIAG_UPPER_LIMIT;
+    DiagIDs.insert(std::make_pair(D, ID));
+    DiagInfo.push_back(D);
+    if (auto Group = D.GetGroup())
+      GroupToDiags[*Group].emplace_back(ID);
+    return ID;
+  }
+
+  ArrayRef<unsigned> getDiagsInGroup(diag::Group G) const {
+    if (auto Diags = GroupToDiags.find(G); Diags != GroupToDiags.end())
+      return Diags->second;
+    return {};
+  }
+};
+
+} // namespace diag
+} // namespace clang
+
+DiagnosticMapping DiagnosticIDs::getDefaultMapping(unsigned DiagID) const {
   DiagnosticMapping Info = DiagnosticMapping::Make(
       diag::Severity::Fatal, /*IsUser=*/false, /*IsPragma=*/false);
 
-  if (const StaticDiagInfoRec *StaticInfo = GetDiagInfo(DiagID)) {
+  if (IsCustomDiag(DiagID)) {
+    Info.setSeverity(
+        CustomDiagInfo->getDescription(DiagID).GetDefaultSeverity());
+  } else if (const StaticDiagInfoRec *StaticInfo = GetDiagInfo(DiagID)) {
     Info.setSeverity((diag::Severity)StaticInfo->DefaultSeverity);
 
     if (StaticInfo->WarnNoWerror) {
@@ -286,6 +334,18 @@ DiagnosticMapping DiagnosticIDs::getDefaultMapping(unsigned DiagID) {
   return Info;
 }
 
+void DiagnosticIDs::initCustomDiagMapping(DiagnosticMapping &Mapping,
+                                          unsigned DiagID) {
+  assert(IsCustomDiag(DiagID));
+  const auto &Diag = CustomDiagInfo->getDescription(DiagID);
+  if (auto Group = Diag.GetGroup()) {
+    GroupInfo GroupInfo = GroupInfos[static_cast<size_t>(*Group)];
+    if (static_cast<diag::Severity>(GroupInfo.Severity) != diag::Severity())
+      Mapping.setSeverity(static_cast<diag::Severity>(GroupInfo.Severity));
+    Mapping.setNoWarningAsError(GroupInfo.HasNoWarningAsError);
+  }
+}
+
 /// getCategoryNumberForDiag - Return the category number that a specified
 /// DiagID belongs to, or 0 if no category.
 unsigned DiagnosticIDs::getCategoryNumberForDiag(unsigned DiagID) {
@@ -343,61 +403,6 @@ bool DiagnosticIDs::isDeferrable(unsigned DiagID) {
   return false;
 }
 
-/// getBuiltinDiagClass - Return the class field of the diagnostic.
-///
-static unsigned getBuiltinDiagClass(unsigned DiagID) {
-  if (const StaticDiagInfoRec *Info = GetDiagInfo(DiagID))
-    return Info->Class;
-  return ~0U;
-}
-
-//===----------------------------------------------------------------------===//
-// Custom Diagnostic information
-//===----------------------------------------------------------------------===//
-
-namespace clang {
-  namespace diag {
-    class CustomDiagInfo {
-      typedef std::pair<DiagnosticIDs::Level, std::string> DiagDesc;
-      std::vector<DiagDesc> DiagInfo;
-      std::map<DiagDesc, unsigned> DiagIDs;
-    public:
-
-      /// getDescription - Return the description of the specified custom
-      /// diagnostic.
-      StringRef getDescription(unsigned DiagID) const {
-        assert(DiagID - DIAG_UPPER_LIMIT < DiagInfo.size() &&
-               "Invalid diagnostic ID");
-        return DiagInfo[DiagID-DIAG_UPPER_LIMIT].second;
-      }
-
-      /// getLevel - Return the level of the specified custom diagnostic.
-      DiagnosticIDs::Level getLevel(unsigned DiagID) const {
-        assert(DiagID - DIAG_UPPER_LIMIT < DiagInfo.size() &&
-               "Invalid diagnostic ID");
-        return DiagInfo[DiagID-DIAG_UPPER_LIMIT].first;
-      }
-
-      unsigned getOrCreateDiagID(DiagnosticIDs::Level L, StringRef Message,
-                                 DiagnosticIDs &Diags) {
-        DiagDesc D(L, std::string(Message));
-        // Check to see if it already exists.
-        std::map<DiagDesc, unsigned>::iterator I = DiagIDs.lower_bound(D);
-        if (I != DiagIDs.end() && I->first == D)
-          return I->second;
-
-        // If not, assign a new ID.
-        unsigned ID = DiagInfo.size()+DIAG_UPPER_LIMIT;
-        DiagIDs.insert(std::make_pair(D, ID));
-        DiagInfo.push_back(D);
-        return ID;
-      }
-    };
-
-  } // end diag namespace
-} // end clang namespace
-
-
 //===----------------------------------------------------------------------===//
 // Common Diagnostic implementation
 //===----------------------------------------------------------------------===//
@@ -412,38 +417,32 @@ DiagnosticIDs::~DiagnosticIDs() {}
 ///
 /// \param FormatString A fixed diagnostic format string that will be hashed and
 /// mapped to a unique DiagID.
-unsigned DiagnosticIDs::getCustomDiagID(Level L, StringRef FormatString) {
+unsigned DiagnosticIDs::getCustomDiagID(CustomDiagDesc Diag) {
   if (!CustomDiagInfo)
     CustomDiagInfo.reset(new diag::CustomDiagInfo());
-  return CustomDiagInfo->getOrCreateDiagID(L, FormatString, *this);
+  return CustomDiagInfo->getOrCreateDiagID(Diag);
 }
 
-
-/// isBuiltinWarningOrExtension - Return true if the unmapped diagnostic
-/// level of the specified diagnostic ID is a Warning or Extension.
-/// This only works on builtin diagnostics, not custom ones, and is not legal to
-/// call on NOTEs.
-bool DiagnosticIDs::isBuiltinWarningOrExtension(unsigned DiagID) {
-  return DiagID < diag::DIAG_UPPER_LIMIT &&
-         getBuiltinDiagClass(DiagID) != CLASS_ERROR;
+bool DiagnosticIDs::isWarningOrExtension(unsigned DiagID) const {
+  return DiagID < diag::DIAG_UPPER_LIMIT
+             ? getDiagClass(DiagID) != CLASS_ERROR
+             : CustomDiagInfo->getDescription(DiagID).GetClass() != CLASS_ERROR;
 }
 
 /// Determine whether the given built-in diagnostic ID is a
 /// Note.
-bool DiagnosticIDs::isBuiltinNote(unsigned DiagID) {
-  return DiagID < diag::DIAG_UPPER_LIMIT &&
-    getBuiltinDiagClass(DiagID) == CLASS_NOTE;
+bool DiagnosticIDs::isNote(unsigned DiagID) const {
+  return DiagID < diag::DIAG_UPPER_LIMIT && getDiagClass(DiagID) == CLASS_NOTE;
 }
 
-/// isBuiltinExtensionDiag - Determine whether the given built-in diagnostic
+/// isExtensionDiag - Determine whether the given built-in diagnostic
 /// ID is for an extension of some sort.  This also returns EnabledByDefault,
 /// which is set to indicate whether the diagnostic is ignored by default (in
 /// which case -pedantic enables it) or treated as a warning/error by default.
 ///
-bool DiagnosticIDs::isBuiltinExtensionDiag(unsigned DiagID,
-                                        bool &EnabledByDefault) {
-  if (DiagID >= diag::DIAG_UPPER_LIMIT ||
-      getBuiltinDiagClass(DiagID) != CLASS_EXTENSION)
+bool DiagnosticIDs::isExtensionDiag(unsigned DiagID,
+                                    bool &EnabledByDefault) const {
+  if (IsCustomDiag(DiagID) || getDiagClass(DiagID) != CLASS_EXTENSION)
     return false;
 
   EnabledByDefault =
@@ -451,10 +450,7 @@ bool DiagnosticIDs::isBuiltinExtensionDiag(unsigned DiagID,
   return true;
 }
 
-bool DiagnosticIDs::isDefaultMappingAsError(unsigned DiagID) {
-  if (DiagID >= diag::DIAG_UPPER_LIMIT)
-    return false;
-
+bool DiagnosticIDs::isDefaultMappingAsError(unsigned DiagID) const {
   return getDefaultMapping(DiagID).getSeverity() >= diag::Severity::Error;
 }
 
@@ -464,7 +460,7 @@ StringRef DiagnosticIDs::getDescription(unsigned DiagID) const {
   if (const StaticDiagInfoRec *Info = GetDiagInfo(DiagID))
     return Info->getDescription();
   assert(CustomDiagInfo && "Invalid CustomDiagInfo");
-  return CustomDiagInfo->getDescription(DiagID);
+  return CustomDiagInfo->getDescription(DiagID).GetDescription();
 }
 
 static DiagnosticIDs::Level toLevel(diag::Severity SV) {
@@ -489,13 +485,7 @@ static DiagnosticIDs::Level toLevel(diag::Severity SV) {
 DiagnosticIDs::Level
 DiagnosticIDs::getDiagnosticLevel(unsigned DiagID, SourceLocation Loc,
                                   const DiagnosticsEngine &Diag) const {
-  // Handle custom diagnostics, which cannot be mapped.
-  if (DiagID >= diag::DIAG_UPPER_LIMIT) {
-    assert(CustomDiagInfo && "Invalid CustomDiagInfo");
-    return CustomDiagInfo->getLevel(DiagID);
-  }
-
-  unsigned DiagClass = getBuiltinDiagClass(DiagID);
+  unsigned DiagClass = getDiagClass(DiagID);
   if (DiagClass == CLASS_NOTE) return DiagnosticIDs::Note;
   return toLevel(getDiagnosticSeverity(DiagID, Loc, Diag));
 }
@@ -509,7 +499,8 @@ DiagnosticIDs::getDiagnosticLevel(unsigned DiagID, SourceLocation Loc,
 diag::Severity
 DiagnosticIDs::getDiagnosticSeverity(unsigned DiagID, SourceLocation Loc,
                                      const DiagnosticsEngine &Diag) const {
-  assert(getBuiltinDiagClass(DiagID) != CLASS_NOTE);
+  bool IsCustomDiag = DiagnosticIDs::IsCustomDiag(DiagID);
+  assert(getDiagClass(DiagID) != CLASS_NOTE);
 
   // Specific non-error diagnostics may be mapped to various levels from ignored
   // to error.  Errors can only be mapped to fatal.
@@ -517,7 +508,7 @@ DiagnosticIDs::getDiagnosticSeverity(unsigned DiagID, SourceLocation Loc,
 
   // Get the mapping information, or compute it lazily.
   DiagnosticsEngine::DiagState *State = Diag.GetDiagStateForLoc(Loc);
-  DiagnosticMapping &Mapping = State->getOrAddMapping((diag::kind)DiagID);
+  DiagnosticMapping Mapping = State->getOrAddMapping((diag::kind)DiagID);
 
   // TODO: Can a null severity really get here?
   if (Mapping.getSeverity() != diag::Severity())
@@ -525,14 +516,15 @@ DiagnosticIDs::getDiagnosticSeverity(unsigned DiagID, SourceLocation Loc,
 
   // Upgrade ignored diagnostics if -Weverything is enabled.
   if (State->EnableAllWarnings && Result == diag::Severity::Ignored &&
-      !Mapping.isUser() && getBuiltinDiagClass(DiagID) != CLASS_REMARK)
+      !Mapping.isUser() &&
+      (IsCustomDiag || getDiagClass(DiagID) != CLASS_REMARK))
     Result = diag::Severity::Warning;
 
   // Ignore -pedantic diagnostics inside __extension__ blocks.
   // (The diagnostics controlled by -pedantic are the extension diagnostics
   // that are not enabled by default.)
   bool EnabledByDefault = false;
-  bool IsExtensionDiag = isBuiltinExtensionDiag(DiagID, EnabledByDefault);
+  bool IsExtensionDiag = isExtensionDiag(DiagID, EnabledByDefault);
   if (Diag.AllExtensionsSilenced && IsExtensionDiag && !EnabledByDefault)
     return diag::Severity::Ignored;
 
@@ -550,10 +542,12 @@ DiagnosticIDs::getDiagnosticSeverity(unsigned DiagID, SourceLocation Loc,
   // as well as disabling all messages which are currently mapped to Warning
   // (whether by default or downgraded from Error via e.g. -Wno-error or #pragma
   // diagnostic.)
+  // FIXME: Should -w be ignored for custom warnings without a group?
   if (State->IgnoreAllWarnings) {
-    if (Result == diag::Severity::Warning ||
-        (Result >= diag::Severity::Error &&
-         !isDefaultMappingAsError((diag::kind)DiagID)))
+    if ((!IsCustomDiag || CustomDiagInfo->getDescription(DiagID).GetGroup()) &&
+        (Result == diag::Severity::Warning ||
+         (Result >= diag::Severity::Error &&
+          !isDefaultMappingAsError((diag::kind)DiagID))))
       return diag::Severity::Ignored;
   }
 
@@ -575,9 +569,10 @@ DiagnosticIDs::getDiagnosticSeverity(unsigned DiagID, SourceLocation Loc,
       Diag.CurDiagID != diag::fatal_too_many_errors && Diag.FatalsAsError)
     Result = diag::Severity::Error;
 
-  // Custom diagnostics always are emitted in system headers.
   bool ShowInSystemHeader =
-      !GetDiagInfo(DiagID) || GetDiagInfo(DiagID)->WarnShowInSystemHeader;
+      IsCustomDiag
+          ? CustomDiagInfo->getDescription(DiagID).ShouldShowInSystemHeader()
+          : !GetDiagInfo(DiagID) || GetDiagInfo(DiagID)->WarnShowInSystemHeader;
 
   // If we are in a system header, we ignore it. We look at the diagnostic class
   // because we also want to ignore extensions and warnings in -Werror and
@@ -597,6 +592,15 @@ DiagnosticIDs::getDiagnosticSeverity(unsigned DiagID, SourceLocation Loc,
   return Result;
 }
 
+DiagnosticIDs::Class DiagnosticIDs::getDiagClass(unsigned DiagID) const {
+  if (IsCustomDiag(DiagID))
+    return Class(CustomDiagInfo->getDescription(DiagID).GetClass());
+
+  if (const StaticDiagInfoRec *Info = GetDiagInfo(DiagID))
+    return Class(Info->Class);
+  return CLASS_INVALID;
+}
+
 #define GET_DIAG_ARRAYS
 #include "clang/Basic/DiagnosticGroups.inc"
 #undef GET_DIAG_ARRAYS
@@ -642,7 +646,12 @@ DiagnosticIDs::getGroupForWarningOption(StringRef Name) {
   return static_cast<diag::Group>(Found - OptionTable);
 }
 
-std::optional<diag::Group> DiagnosticIDs::getGroupForDiag(unsigned DiagID) {
+std::optional<diag::Group>
+DiagnosticIDs::getGroupForDiag(unsigned DiagID) const {
+  if (IsCustomDiag(DiagID)) {
+    assert(CustomDiagInfo);
+    return CustomDiagInfo->getDescription(DiagID).GetGroup();
+  }
   if (const StaticDiagInfoRec *Info = GetDiagInfo(DiagID))
     return static_cast<diag::Group>(Info->getOptionGroupIndex());
   return std::nullopt;
@@ -673,7 +682,8 @@ std::vector<std::string> DiagnosticIDs::getDiagnosticFlags() {
 /// were filtered out due to having the wrong flavor.
 static bool getDiagnosticsInGroup(diag::Flavor Flavor,
                                   const WarningOption *Group,
-                                  SmallVectorImpl<diag::kind> &Diags) {
+                                  SmallVectorImpl<diag::kind> &Diags,
+                                  diag::CustomDiagInfo *CustomDiagInfo) {
   // An empty group is considered to be a warning group: we have empty groups
   // for GCC compatibility, and GCC does not have remarks.
   if (!Group->Members && !Group->SubGroups)
@@ -692,9 +702,14 @@ static bool getDiagnosticsInGroup(diag::Flavor Flavor,
 
   // Add the members of the subgroups.
   const int16_t *SubGroups = DiagSubGroups + Group->SubGroups;
-  for (; *SubGroups != (int16_t)-1; ++SubGroups)
+  for (; *SubGroups != (int16_t)-1; ++SubGroups) {
+    if (CustomDiagInfo)
+      llvm::copy(
+          CustomDiagInfo->getDiagsInGroup(static_cast<diag::Group>(*SubGroups)),
+          std::back_inserter(Diags));
     NotFound &= getDiagnosticsInGroup(Flavor, &OptionTable[(short)*SubGroups],
-                                      Diags);
+                                      Diags, CustomDiagInfo);
+  }
 
   return NotFound;
 }
@@ -702,12 +717,49 @@ static bool getDiagnosticsInGroup(diag::Flavor Flavor,
 bool
 DiagnosticIDs::getDiagnosticsInGroup(diag::Flavor Flavor, StringRef Group,
                                      SmallVectorImpl<diag::kind> &Diags) const {
-  if (std::optional<diag::Group> G = getGroupForWarningOption(Group))
-    return ::getDiagnosticsInGroup(
-        Flavor, &OptionTable[static_cast<unsigned>(*G)], Diags);
+  if (std::optional<diag::Group> G = getGroupForWarningOption(Group)) {
+    if (CustomDiagInfo)
+      llvm::copy(CustomDiagInfo->getDiagsInGroup(*G),
+                 std::back_inserter(Diags));
+    return ::getDiagnosticsInGroup(Flavor,
+                                   &OptionTable[static_cast<unsigned>(*G)],
+                                   Diags, CustomDiagInfo.get());
+  }
   return true;
 }
 
+template <class Func>
+static void forEachSubGroupImpl(const WarningOption *Group, Func func) {
+  for (const int16_t *SubGroups = DiagSubGroups + Group->SubGroups;
+       *SubGroups != -1; ++SubGroups) {
+    func(static_cast<size_t>(*SubGroups));
+    forEachSubGroupImpl(&OptionTable[*SubGroups], std::move(func));
+  }
+}
+
+template <class Func>
+static void forEachSubGroup(diag::Group Group, Func func) {
+  const WarningOption *WarningOpt = &OptionTable[static_cast<size_t>(Group)];
+  func(static_cast<size_t>(Group));
+  ::forEachSubGroupImpl(WarningOpt, std::move(func));
+}
+
+void DiagnosticIDs::setGroupSeverity(StringRef Group, diag::Severity Sev) {
+  if (std::optional<diag::Group> G = getGroupForWarningOption(Group)) {
+    ::forEachSubGroup(*G, [&](size_t SubGroup) {
+      GroupInfos[SubGroup].Severity = static_cast<unsigned>(Sev);
+    });
+  }
+}
+
+void DiagnosticIDs::setGroupNoWarningsAsError(StringRef Group, bool Val) {
+  if (std::optional<diag::Group> G = getGroupForWarningOption(Group)) {
+    ::forEachSubGroup(*G, [&](size_t SubGroup) {
+      GroupInfos[static_cast<size_t>(*G)].HasNoWarningAsError = Val;
+    });
+  }
+}
+
 void DiagnosticIDs::getAllDiagnostics(diag::Flavor Flavor,
                                       std::vector<diag::kind> &Diags) {
   for (unsigned i = 0; i != StaticDiagInfoSize; ++i)
@@ -730,7 +782,7 @@ StringRef DiagnosticIDs::getNearestOption(diag::Flavor Flavor,
 
     // Don't suggest groups that are not of this kind.
     llvm::SmallVector<diag::kind, 8> Diags;
-    if (::getDiagnosticsInGroup(Flavor, &O, Diags) || Diags.empty())
+    if (::getDiagnosticsInGroup(Flavor, &O, Diags, nullptr) || Diags.empty())
       continue;
 
     if (Distance == BestDistance) {
@@ -843,14 +895,8 @@ void DiagnosticIDs::EmitDiag(DiagnosticsEngine &Diag, Level DiagLevel) const {
 }
 
 bool DiagnosticIDs::isUnrecoverable(unsigned DiagID) const {
-  if (DiagID >= diag::DIAG_UPPER_LIMIT) {
-    assert(CustomDiagInfo && "Invalid CustomDiagInfo");
-    // Custom diagnostics.
-    return CustomDiagInfo->getLevel(DiagID) >= DiagnosticIDs::Error;
-  }
-
   // Only errors may be unrecoverable.
-  if (getBuiltinDiagClass(DiagID) < CLASS_ERROR)
+  if (getDiagClass(DiagID) < CLASS_ERROR)
     return false;
 
   if (DiagID == diag::err_unavailable ||
diff --git a/clang/lib/Frontend/LogDiagnosticPrinter.cpp b/clang/lib/Frontend/LogDiagnosticPrinter.cpp
index 469d1c22633aa3c..4e963af837f01f6 100644
--- a/clang/lib/Frontend/LogDiagnosticPrinter.cpp
+++ b/clang/lib/Frontend/LogDiagnosticPrinter.cpp
@@ -129,7 +129,8 @@ void LogDiagnosticPrinter::HandleDiagnostic(DiagnosticsEngine::Level Level,
   DE.DiagnosticLevel = Level;
 
   DE.WarningOption =
-      std::string(DiagnosticIDs::getWarningOptionForDiag(DE.DiagnosticID));
+      std::string(Info.getDiags()->getDiagnosticIDs()->getWarningOptionForDiag(
+          DE.DiagnosticID));
 
   // Format the message.
   SmallString<100> MessageStr;
@@ -160,4 +161,3 @@ void LogDiagnosticPrinter::HandleDiagnostic(DiagnosticsEngine::Level Level,
   // Record the diagnostic entry.
   Entries.push_back(DE);
 }
-
diff --git a/clang/lib/Frontend/SerializedDiagnosticPrinter.cpp b/clang/lib/Frontend/SerializedDiagnosticPrinter.cpp
index 0887b5a504f05a3..d1db31763e50091 100644
--- a/clang/lib/Frontend/SerializedDiagnosticPrinter.cpp
+++ b/clang/lib/Frontend/SerializedDiagnosticPrinter.cpp
@@ -202,7 +202,7 @@ class SDiagsWriter : public DiagnosticConsumer {
 
   /// Emit the string information for diagnostic flags.
   unsigned getEmitDiagnosticFlag(DiagnosticsEngine::Level DiagLevel,
-                                 unsigned DiagID = 0);
+                                 const Diagnostic *Diag = nullptr);
 
   unsigned getEmitDiagnosticFlag(StringRef DiagName);
 
@@ -536,11 +536,13 @@ unsigned SDiagsWriter::getEmitCategory(unsigned int category) {
 }
 
 unsigned SDiagsWriter::getEmitDiagnosticFlag(DiagnosticsEngine::Level DiagLevel,
-                                             unsigned DiagID) {
-  if (DiagLevel == DiagnosticsEngine::Note)
+                                             const Diagnostic *Diag) {
+  if (!Diag || DiagLevel == DiagnosticsEngine::Note)
     return 0; // No flag for notes.
 
-  StringRef FlagName = DiagnosticIDs::getWarningOptionForDiag(DiagID);
+  StringRef FlagName =
+      Diag->getDiags()->getDiagnosticIDs()->getWarningOptionForDiag(
+          Diag->getID());
   return getEmitDiagnosticFlag(FlagName);
 }
 
@@ -655,7 +657,7 @@ void SDiagsWriter::EmitDiagnosticMessage(FullSourceLoc Loc, PresumedLoc PLoc,
     unsigned DiagID = DiagnosticIDs::getCategoryNumberForDiag(Info->getID());
     Record.push_back(getEmitCategory(DiagID));
     // Emit the diagnostic flag string lazily and get the mapped ID.
-    Record.push_back(getEmitDiagnosticFlag(Level, Info->getID()));
+    Record.push_back(getEmitDiagnosticFlag(Level, Info));
   } else {
     Record.push_back(getEmitCategory());
     Record.push_back(getEmitDiagnosticFlag(Level));
diff --git a/clang/lib/Frontend/TextDiagnosticPrinter.cpp b/clang/lib/Frontend/TextDiagnosticPrinter.cpp
index b2fb762537573ef..c2fea3d03f0c0fd 100644
--- a/clang/lib/Frontend/TextDiagnosticPrinter.cpp
+++ b/clang/lib/Frontend/TextDiagnosticPrinter.cpp
@@ -70,13 +70,17 @@ static void printDiagnosticOptions(raw_ostream &OS,
     // flag it as such. Note that diagnostics could also have been mapped by a
     // pragma, but we don't currently have a way to distinguish this.
     if (Level == DiagnosticsEngine::Error &&
-        DiagnosticIDs::isBuiltinWarningOrExtension(Info.getID()) &&
-        !DiagnosticIDs::isDefaultMappingAsError(Info.getID())) {
+        Info.getDiags()->getDiagnosticIDs()->isWarningOrExtension(
+            Info.getID()) &&
+        !Info.getDiags()->getDiagnosticIDs()->isDefaultMappingAsError(
+            Info.getID())) {
       OS << " [-Werror";
       Started = true;
     }
 
-    StringRef Opt = DiagnosticIDs::getWarningOptionForDiag(Info.getID());
+    StringRef Opt =
+        Info.getDiags()->getDiagnosticIDs()->getWarningOptionForDiag(
+            Info.getID());
     if (!Opt.empty()) {
       OS << (Started ? "," : " [")
          << (Level == DiagnosticsEngine::Remark ? "-R" : "-W") << Opt;
diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index 46ddd360870b4fc..d567de703903c99 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -1683,7 +1683,7 @@ void Sema::EmitCurrentDiagnostic(unsigned DiagID) {
   // that is different from the last template instantiation where
   // we emitted an error, print a template instantiation
   // backtrace.
-  if (!DiagnosticIDs::isBuiltinNote(DiagID))
+  if (!Diags.getDiagnosticIDs()->isNote(DiagID))
     PrintContextStack();
 }
 
@@ -1697,7 +1697,8 @@ bool Sema::hasUncompilableErrorOccurred() const {
   if (Loc == DeviceDeferredDiags.end())
     return false;
   for (auto PDAt : Loc->second) {
-    if (DiagnosticIDs::isDefaultMappingAsError(PDAt.second.getDiagID()))
+    if (Diags.getDiagnosticIDs()->isDefaultMappingAsError(
+            PDAt.second.getDiagID()))
       return true;
   }
   return false;
diff --git a/clang/lib/Sema/SemaCUDA.cpp b/clang/lib/Sema/SemaCUDA.cpp
index ec37c0df56c671d..fbb3de4b3e41655 100644
--- a/clang/lib/Sema/SemaCUDA.cpp
+++ b/clang/lib/Sema/SemaCUDA.cpp
@@ -835,7 +835,7 @@ SemaBase::SemaDiagnosticBuilder SemaCUDA::DiagIfDeviceCode(SourceLocation Loc,
       if (!getLangOpts().CUDAIsDevice)
         return SemaDiagnosticBuilder::K_Nop;
       if (SemaRef.IsLastErrorImmediate &&
-          getDiagnostics().getDiagnosticIDs()->isBuiltinNote(DiagID))
+          getDiagnostics().getDiagnosticIDs()->isNote(DiagID))
         return SemaDiagnosticBuilder::K_Immediate;
       return (SemaRef.getEmissionStatus(CurFunContext) ==
               Sema::FunctionEmissionStatus::Emitted)
@@ -866,7 +866,7 @@ Sema::SemaDiagnosticBuilder SemaCUDA::DiagIfHostCode(SourceLocation Loc,
       if (getLangOpts().CUDAIsDevice)
         return SemaDiagnosticBuilder::K_Nop;
       if (SemaRef.IsLastErrorImmediate &&
-          getDiagnostics().getDiagnosticIDs()->isBuiltinNote(DiagID))
+          getDiagnostics().getDiagnosticIDs()->isNote(DiagID))
         return SemaDiagnosticBuilder::K_Immediate;
       return (SemaRef.getEmissionStatus(CurFunContext) ==
               Sema::FunctionEmissionStatus::Emitted)
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index c9b9f3a0007daa6..14cc51cf89665a4 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -852,22 +852,38 @@ static void handleDiagnoseIfAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   if (!checkFunctionConditionAttr(S, D, AL, Cond, Msg))
     return;
 
-  StringRef DiagTypeStr;
-  if (!S.checkStringLiteralArgumentAttr(AL, 2, DiagTypeStr))
+  StringRef DefaultSevStr;
+  if (!S.checkStringLiteralArgumentAttr(AL, 2, DefaultSevStr))
     return;
 
-  DiagnoseIfAttr::DiagnosticType DiagType;
-  if (!DiagnoseIfAttr::ConvertStrToDiagnosticType(DiagTypeStr, DiagType)) {
+  DiagnoseIfAttr::DefaultSeverity DefaultSev;
+  if (!DiagnoseIfAttr::ConvertStrToDefaultSeverity(DefaultSevStr, DefaultSev)) {
     S.Diag(AL.getArgAsExpr(2)->getBeginLoc(),
            diag::err_diagnose_if_invalid_diagnostic_type);
     return;
   }
 
+  StringRef WarningGroup;
+  SmallVector<StringRef, 2> Options;
+  if (AL.getNumArgs() > 3) {
+    if (!S.checkStringLiteralArgumentAttr(AL, 3, WarningGroup))
+      return;
+    if (WarningGroup.empty() ||
+        !S.getDiagnostics().getDiagnosticIDs()->getGroupForWarningOption(
+            WarningGroup)) {
+      S.Diag(AL.getArgAsExpr(3)->getBeginLoc(),
+             diag::err_diagnose_if_unknown_warning)
+          << WarningGroup;
+      return;
+    }
+  }
+
   bool ArgDependent = false;
   if (const auto *FD = dyn_cast<FunctionDecl>(D))
     ArgDependent = ArgumentDependenceChecker(FD).referencesArgs(Cond);
   D->addAttr(::new (S.Context) DiagnoseIfAttr(
-      S.Context, AL, Cond, Msg, DiagType, ArgDependent, cast<NamedDecl>(D)));
+      S.Context, AL, Cond, Msg, DefaultSev, WarningGroup, ArgDependent,
+      cast<NamedDecl>(D)));
 }
 
 static void handleNoBuiltinAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index 0c1e054f7c30a49..fbeb6ee5b052f34 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -7300,8 +7300,10 @@ static bool diagnoseDiagnoseIfAttrsWith(Sema &S, const NamedDecl *ND,
     return false;
 
   auto WarningBegin = std::stable_partition(
-      Attrs.begin(), Attrs.end(),
-      [](const DiagnoseIfAttr *DIA) { return DIA->isError(); });
+      Attrs.begin(), Attrs.end(), [](const DiagnoseIfAttr *DIA) {
+        return DIA->getDefaultSeverity() == DiagnoseIfAttr::DS_error &&
+               DIA->getWarningGroup().empty();
+      });
 
   // Note that diagnose_if attributes are late-parsed, so they appear in the
   // correct order (unlike enable_if attributes).
@@ -7315,11 +7317,31 @@ static bool diagnoseDiagnoseIfAttrsWith(Sema &S, const NamedDecl *ND,
     return true;
   }
 
+  auto ToSeverity = [](DiagnoseIfAttr::DefaultSeverity Sev) {
+    switch (Sev) {
+    case DiagnoseIfAttr::DS_warning:
+      return diag::Severity::Warning;
+    case DiagnoseIfAttr::DS_error:
+      return diag::Severity::Error;
+    }
+  };
+
   for (const auto *DIA : llvm::make_range(WarningBegin, Attrs.end()))
     if (IsSuccessful(DIA)) {
-      S.Diag(Loc, diag::warn_diagnose_if_succeeded) << DIA->getMessage();
-      S.Diag(DIA->getLocation(), diag::note_from_diagnose_if)
-          << DIA->getParent() << DIA->getCond()->getSourceRange();
+      if (DIA->getWarningGroup().empty() &&
+          DIA->getDefaultSeverity() == DiagnoseIfAttr::DS_warning) {
+        S.Diag(Loc, diag::warn_diagnose_if_succeeded) << DIA->getMessage();
+        S.Diag(DIA->getLocation(), diag::note_from_diagnose_if)
+            << DIA->getParent() << DIA->getCond()->getSourceRange();
+      } else {
+        auto DiagGroup = S.Diags.getDiagnosticIDs()->getGroupForWarningOption(
+            DIA->getWarningGroup());
+        assert(DiagGroup);
+        auto DiagID = S.Diags.getDiagnosticIDs()->getCustomDiagID(
+            {ToSeverity(DIA->getDefaultSeverity()), "%0",
+             DiagnosticIDs::CLASS_WARNING, false, false, *DiagGroup});
+        S.Diag(Loc, DiagID) << DIA->getMessage();
+      }
     }
 
   return false;
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index bb311e384092804..e97a7d768b931bd 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -284,7 +284,8 @@ static void instantiateDependentDiagnoseIfAttr(
   if (Cond)
     New->addAttr(new (S.getASTContext()) DiagnoseIfAttr(
         S.getASTContext(), *DIA, Cond, DIA->getMessage(),
-        DIA->getDiagnosticType(), DIA->getArgDependent(), New));
+        DIA->getDefaultSeverity(), DIA->getWarningGroup(),
+        DIA->getArgDependent(), New));
 }
 
 // Constructs and adds to New a new instance of CUDALaunchBoundsAttr using
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index e5a1e20a265616a..4fae6ff02ea9a3e 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -6641,7 +6641,7 @@ void ASTReader::ReadPragmaDiagnosticMappings(DiagnosticsEngine &Diag) {
       // command line (-w, -Weverything, -Werror, ...) along with any explicit
       // -Wblah flags.
       unsigned Flags = Record[Idx++];
-      DiagState Initial;
+      DiagState Initial(*Diag.getDiagnosticIDs());
       Initial.SuppressSystemWarnings = Flags & 1; Flags >>= 1;
       Initial.ErrorsAsFatal = Flags & 1; Flags >>= 1;
       Initial.WarningsAsErrors = Flags & 1; Flags >>= 1;
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index c6289493fce1de6..008bf571f847dc0 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -3219,7 +3219,7 @@ void ASTWriter::WritePragmaDiagnosticMappings(const DiagnosticsEngine &Diag,
         // Skip default mappings. We have a mapping for every diagnostic ever
         // emitted, regardless of whether it was customized.
         if (!I.second.isPragma() &&
-            I.second == DiagnosticIDs::getDefaultMapping(I.first))
+            I.second == Diag.getDiagnosticIDs()->getDefaultMapping(I.first))
           continue;
         Mappings.push_back(I);
       }
diff --git a/clang/lib/StaticAnalyzer/Core/TextDiagnostics.cpp b/clang/lib/StaticAnalyzer/Core/TextDiagnostics.cpp
index 71268af22e2424f..7cdd545e61b3287 100644
--- a/clang/lib/StaticAnalyzer/Core/TextDiagnostics.cpp
+++ b/clang/lib/StaticAnalyzer/Core/TextDiagnostics.cpp
@@ -91,7 +91,6 @@ class TextDiagnostics : public PathDiagnosticConsumer {
                                     ? " [" + PD->getCheckerName() + "]"
                                     : "")
                                    .str();
-
       reportPiece(WarnID, PD->getLocation().asLocation(),
                   (PD->getShortDescription() + WarningMsg).str(),
                   PD->path.back()->getRanges(), PD->path.back()->getFixits());
diff --git a/clang/test/Sema/diagnose_if.c b/clang/test/Sema/diagnose_if.c
index 4df39916c031e30..e9b8497d5ca4e5d 100644
--- a/clang/test/Sema/diagnose_if.c
+++ b/clang/test/Sema/diagnose_if.c
@@ -2,10 +2,10 @@
 
 #define _diagnose_if(...) __attribute__((diagnose_if(__VA_ARGS__)))
 
-void failure1(void) _diagnose_if(); // expected-error{{exactly 3 arguments}}
-void failure2(void) _diagnose_if(0); // expected-error{{exactly 3 arguments}}
-void failure3(void) _diagnose_if(0, ""); // expected-error{{exactly 3 arguments}}
-void failure4(void) _diagnose_if(0, "", "error", 1); // expected-error{{exactly 3 arguments}}
+void failure1(void) _diagnose_if(); // expected-error{{at least 3 arguments}}
+void failure2(void) _diagnose_if(0); // expected-error{{at least 3 arguments}}
+void failure3(void) _diagnose_if(0, ""); // expected-error{{at least 3 arguments}}
+void failure4(void) _diagnose_if(0, "", "error", 1); // expected-error{{expected string literal as argument}}
 void failure5(void) _diagnose_if(0, 0, "error"); // expected-error{{expected string literal as argument of 'diagnose_if' attribute}}
 void failure6(void) _diagnose_if(0, "", "invalid"); // expected-error{{invalid diagnostic type for 'diagnose_if'; use "error" or "warning" instead}}
 void failure7(void) _diagnose_if(0, "", "ERROR"); // expected-error{{invalid diagnostic type}}
diff --git a/clang/test/SemaCXX/diagnose_if-warning-group.cpp b/clang/test/SemaCXX/diagnose_if-warning-group.cpp
new file mode 100644
index 000000000000000..a39c0c0c33c9ef0
--- /dev/null
+++ b/clang/test/SemaCXX/diagnose_if-warning-group.cpp
@@ -0,0 +1,63 @@
+// RUN: %clang_cc1 %s -verify=expected,wall -fno-builtin -Wno-pedantic -Werror=comment -Wno-error=abi -Wfatal-errors=assume -Wno-fatal-errors=assume -Wno-format
+// RUN: %clang_cc1 %s -verify=expected,wno-all,pedantic,format -fno-builtin -Wno-all -Werror=comment -Wno-error=abi -Werror=assume -Wformat
+
+#define diagnose_if(...) __attribute__((diagnose_if(__VA_ARGS__)))
+
+#ifndef EMTY_WARNING_GROUP
+void bougus_warning() diagnose_if(true, "oh no", "warning", "bogus warning") {} // expected-error {{unknown warning group 'bogus warning'}}
+
+void show_in_system_header() diagnose_if(true, "oh no", "warning", "assume", "Banane") {} // expected-error {{'diagnose_if' attribute takes no more than 4 arguments}}
+#endif // EMTY_WARNING_GROUP
+
+template <bool b>
+void diagnose_if_wcomma() diagnose_if(b, "oh no", "warning", "comma") {}
+
+template <bool b>
+void diagnose_if_wcomment() diagnose_if(b, "oh no", "warning", "comment") {}
+
+void empty_warning_group() diagnose_if(true, "oh no", "warning", "") {} // expected-error {{unknown warning group ''}}
+void empty_warning_group_error() diagnose_if(true, "oh no", "error", "") {} // expected-error {{unknown warning group ''}}
+
+void diagnose_if_wabi_default_error() diagnose_if(true, "ABI stuff", "error", "abi") {}
+void diagnose_assume() diagnose_if(true, "Assume diagnostic", "warning", "assume") {}
+
+void Wall() diagnose_if(true, "oh no", "warning", "all") {}
+void Wpedantic() diagnose_if(true, "oh no", "warning", "pedantic") {}
+void Wformat_extra_args() diagnose_if(true, "oh no", "warning", "format-extra-args") {}
+
+void call() {
+  diagnose_if_wcomma<true>(); // expected-warning {{oh no}}
+  diagnose_if_wcomma<false>();
+  diagnose_if_wcomment<true>(); // expected-error {{oh no}}
+  diagnose_if_wcomment<false>();
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wcomma"
+  diagnose_if_wcomma<true>();
+  diagnose_if_wcomment<true>(); // expected-error {{oh no}}
+#pragma clang diagnostic pop
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wcomment"
+  diagnose_if_wcomma<true>(); // expected-warning {{oh no}}
+  diagnose_if_wcomment<true>();
+#pragma clang diagnostic pop
+
+  diagnose_if_wcomma<true>(); // expected-warning {{oh no}}
+  diagnose_if_wcomment<true>(); // expected-error {{oh no}}
+
+  diagnose_if_wabi_default_error(); // expected-warning {{ABI stuff}}
+  diagnose_assume(); // expected-error {{Assume diagnostic}}
+
+  // Make sure that the -Wassume diagnostic isn't fatal
+  diagnose_if_wabi_default_error(); // expected-warning {{ABI stuff}}
+
+  Wall(); // wall-warning {{oh no}}
+  Wpedantic(); // pedantic-warning {{oh no}}
+  Wformat_extra_args(); // format-warning {{oh no}}
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wformat"
+  Wformat_extra_args();
+#pragma clang diagnostic pop
+}
diff --git a/clang/tools/diagtool/ListWarnings.cpp b/clang/tools/diagtool/ListWarnings.cpp
index a71f6e3a66c8eb2..9f9647126dd8a0f 100644
--- a/clang/tools/diagtool/ListWarnings.cpp
+++ b/clang/tools/diagtool/ListWarnings.cpp
@@ -53,13 +53,13 @@ int ListWarnings::run(unsigned int argc, char **argv, llvm::raw_ostream &out) {
   for (const DiagnosticRecord &DR : getBuiltinDiagnosticsByName()) {
     const unsigned diagID = DR.DiagID;
 
-    if (DiagnosticIDs::isBuiltinNote(diagID))
+    if (DiagnosticIDs{}.isNote(diagID))
       continue;
 
-    if (!DiagnosticIDs::isBuiltinWarningOrExtension(diagID))
+    if (!DiagnosticIDs{}.isWarningOrExtension(diagID))
       continue;
 
-    Entry entry(DR.getName(), DiagnosticIDs::getWarningOptionForDiag(diagID));
+    Entry entry(DR.getName(), DiagnosticIDs{}.getWarningOptionForDiag(diagID));
 
     if (entry.Flag.empty())
       Unflagged.push_back(entry);
@@ -97,4 +97,3 @@ int ListWarnings::run(unsigned int argc, char **argv, llvm::raw_ostream &out) {
 
   return 0;
 }
-
diff --git a/clang/tools/diagtool/ShowEnabledWarnings.cpp b/clang/tools/diagtool/ShowEnabledWarnings.cpp
index 66a295db054c353..caf67223921d4b3 100644
--- a/clang/tools/diagtool/ShowEnabledWarnings.cpp
+++ b/clang/tools/diagtool/ShowEnabledWarnings.cpp
@@ -117,10 +117,10 @@ int ShowEnabledWarnings::run(unsigned int argc, char **argv, raw_ostream &Out) {
   for (const DiagnosticRecord &DR : getBuiltinDiagnosticsByName()) {
     unsigned DiagID = DR.DiagID;
 
-    if (DiagnosticIDs::isBuiltinNote(DiagID))
+    if (DiagnosticIDs{}.isNote(DiagID))
       continue;
 
-    if (!DiagnosticIDs::isBuiltinWarningOrExtension(DiagID))
+    if (!DiagnosticIDs{}.isWarningOrExtension(DiagID))
       continue;
 
     DiagnosticsEngine::Level DiagLevel =
@@ -128,7 +128,7 @@ int ShowEnabledWarnings::run(unsigned int argc, char **argv, raw_ostream &Out) {
     if (DiagLevel == DiagnosticsEngine::Ignored)
       continue;
 
-    StringRef WarningOpt = DiagnosticIDs::getWarningOptionForDiag(DiagID);
+    StringRef WarningOpt = DiagnosticIDs{}.getWarningOptionForDiag(DiagID);
     Active.push_back(PrettyDiag(DR.getName(), WarningOpt, DiagLevel));
   }
 
diff --git a/clang/tools/libclang/CXStoredDiagnostic.cpp b/clang/tools/libclang/CXStoredDiagnostic.cpp
index 03018229549bd4e..6fb3050f5f8445e 100644
--- a/clang/tools/libclang/CXStoredDiagnostic.cpp
+++ b/clang/tools/libclang/CXStoredDiagnostic.cpp
@@ -51,7 +51,9 @@ CXString CXStoredDiagnostic::getSpelling() const {
 
 CXString CXStoredDiagnostic::getDiagnosticOption(CXString *Disable) const {
   unsigned ID = Diag.getID();
-  StringRef Option = DiagnosticIDs::getWarningOptionForDiag(ID);
+  if (DiagnosticIDs::IsCustomDiag(ID))
+    return cxstring::createEmpty();
+  StringRef Option = DiagnosticIDs{}.getWarningOptionForDiag(ID);
   if (!Option.empty()) {
     if (Disable)
       *Disable = cxstring::createDup((Twine("-Wno-") + Option).str());
diff --git a/flang/lib/Frontend/TextDiagnosticPrinter.cpp b/flang/lib/Frontend/TextDiagnosticPrinter.cpp
index 1e6414f38648e05..8b00fb69b3cefb4 100644
--- a/flang/lib/Frontend/TextDiagnosticPrinter.cpp
+++ b/flang/lib/Frontend/TextDiagnosticPrinter.cpp
@@ -38,8 +38,8 @@ TextDiagnosticPrinter::~TextDiagnosticPrinter() {}
 static void printRemarkOption(llvm::raw_ostream &os,
                               clang::DiagnosticsEngine::Level level,
                               const clang::Diagnostic &info) {
-  llvm::StringRef opt =
-      clang::DiagnosticIDs::getWarningOptionForDiag(info.getID());
+  llvm::StringRef opt = info.getDiags()->getDiagnosticIDs()
+      ->getWarningOptionForDiag(info.getID());
   if (!opt.empty()) {
     // We still need to check if the level is a Remark since, an unknown option
     // warning could be printed i.e. [-Wunknown-warning-option]