[clang][Itanium Mangle] Enable mangling of enclosing class for member… (

#110503)

…-like friend function templates only if ` -fclang-abi-compat>19`.

clang/docs/ReleaseNotes.rst

@@ -104,6 +104,7 @@ ABI Changes in This Version
 
 - Fixed Microsoft name mangling of placeholder, auto and decltype(auto), return types for MSVC 1920+. This change resolves incompatibilities with code compiled by MSVC 1920+ but will introduce incompatibilities with code compiled by earlier versions of Clang unless such code is built with the compiler option -fms-compatibility-version=19.14 to imitate the MSVC 1914 mangling behavior.
 - Fixed the Itanium mangling of the construction vtable name. This change will introduce incompatibilities with code compiled by Clang 19 and earlier versions, unless the -fclang-abi-compat=19 option is used. (#GH108015)
+- Mangle member-like friend function templates as members of the enclosing class. (#GH110247, #GH110503)
 
 AST Dumping Potentially Breaking Changes
 ----------------------------------------
@@ -466,7 +467,6 @@ Bug Fixes to C++ Support
   diagnosing a failed cast caused indirectly by a failed implicit conversion to the type of the constructor parameter.
 - Fixed an assertion failure by adjusting integral to boolean vector conversions (#GH108326)
 - Fixed an issue deducing non-type template arguments of reference type. (#GH73460)
-- Mangle friend function templates with a constraint that depends on a template parameter from an enclosing template as members of the enclosing class. (#GH110247)
 - Fixed an issue in constraint evaluation, where type constraints on the lambda expression
   containing outer unexpanded parameters were not correctly expanded. (#GH101754)
 - Fixed a bug in constraint expression comparison where the ``sizeof...`` expression was not handled properly

clang/include/clang/Basic/LangOptions.h

@@ -241,8 +241,10 @@ class LangOptionsBase {
 
     /// Attempt to be ABI-compatible with code generated by Clang 19.0.x.
     /// This causes clang to:
-    ///   - Incorrectly mangles the 'base type' substitutions of the CXX
+    ///   - Incorrectly mangle the 'base type' substitutions of the CXX
     ///   construction vtable because it hasn't added 'type' as a substitution.
+    ///   - Skip mangling enclosing class templates of member-like friend
+    ///   function templates.
     Ver19,
 
     /// Conform to the underlying platform's C and C++ ABIs as closely

clang/lib/AST/ItaniumMangle.cpp

@@ -693,7 +693,10 @@ ItaniumMangleContextImpl::getEffectiveDeclContext(const Decl *D) {
     if (VD->isExternC())
       return getASTContext().getTranslationUnitDecl();
 
-  if (const auto *FD = D->getAsFunction()) {
+  if (const auto *FD = getASTContext().getLangOpts().getClangABICompat() >
+                               LangOptions::ClangABI::Ver19
+                           ? D->getAsFunction()
+                           : dyn_cast<FunctionDecl>(D)) {
     if (FD->isExternC())
       return getASTContext().getTranslationUnitDecl();
     // Member-like constrained friends are mangled as if they were members of

clang/test/CodeGenCXX/mangle-concept.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -verify -std=c++20 -emit-llvm -triple %itanium_abi_triple -o - %s -fclang-abi-compat=latest | FileCheck %s
+// RUN: %clang_cc1 -verify -std=c++20 -emit-llvm -triple %itanium_abi_triple -o - %s -fclang-abi-compat=19 | FileCheck %s --check-prefix=CLANG19
 // RUN: %clang_cc1 -verify -std=c++20 -emit-llvm -triple %itanium_abi_triple -o - %s -fclang-abi-compat=17 | FileCheck %s --check-prefix=CLANG17
 // expected-no-diagnostics
 
@@ -59,18 +60,21 @@ namespace test2 {
     // CLANG17: call {{.*}}@_ZN5test21fEz(
     f(ai);
     // CHECK: call {{.*}}@_ZN5test21AIiEF1gIvEEvzQaa4TrueIT_E4TrueITL0__E(
+    // CLANG19: call {{.*}}@_ZN5test2F1gIvEEvzQaa4TrueIT_E4TrueITL0__E(
     // CLANG17: call {{.*}}@_ZN5test21gIvEEvz(
     g(ai);
     // CHECK: call {{.*}}@_ZN5test21hIvEEvzQ4TrueITL0__E(
     // CLANG17: call {{.*}}@_ZN5test21hIvEEvz(
     h(ai);
     // CHECK: call {{.*}}@_ZN5test21AIiEF1iIvQaa4TrueIT_E4TrueITL0__EEEvz(
+    // CLANG19: call {{.*}}@_ZN5test2F1iIvQaa4TrueIT_E4TrueITL0__EEEvz(
     // CLANG17: call {{.*}}@_ZN5test21iIvEEvz(
     i(ai);
     // CHECK: call {{.*}}@_ZN5test21jIvQ4TrueITL0__EEEvz(
     // CLANG17: call {{.*}}@_ZN5test21jIvEEvz(
     j(ai);
     // CHECK: call {{.*}}@_ZN5test21AIiEF1kITk4TruevQ4TrueIT_EEEvz(
+    // CLANG19: call {{.*}}@_ZN5test2F1kITk4TruevQ4TrueIT_EEEvz(
     // CLANG17: call {{.*}}@_ZN5test21kIvEEvz(
     k(ai);
     // CHECK: call {{.*}}@_ZN5test21lITk4TruevEEvz(