More diags

src/Draco.Compiler/Api/Diagnostics/Diagnostic_Builder.cs

@@ -69,7 +69,7 @@ public Builder WithFormatArgs(params object?[] args)
             return this;
         }
 
-        public Builder WithLocation(Location location)
+        public Builder WithLocation(Location? location)
         {
             this.Location = location;
             return this;

src/Draco.Compiler/Internal/FlowAnalysis/DecisionTree.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
@@ -269,6 +270,15 @@ private SpecializationComparer()
         _ => null,
     };
 
+    // TODO: Doc
+    private static BoundPattern? ConstructUnhandledPattern(INode node)
+    {
+        if (node.NotCovered is null) return null;
+
+        // TODO: Wrap in parent
+        return node.NotCovered;
+    }
+
     /// <summary>
     /// The root node of this tree.
     /// </summary>
@@ -291,7 +301,12 @@ private SpecializationComparer()
     /// <summary>
     /// An example of an unhandled pattern, if any.
     /// </summary>
-    public BoundPattern? UnhandledExample => throw new NotImplementedException();
+    public BoundPattern? UnhandledExample => GraphTraversal
+        .DepthFirst(this.Root, n => n.Children.Select(c => c.Value))
+        .Where(n => n.IsFail)
+        .Select(ConstructUnhandledPattern)
+        .OfType<BoundPattern>()
+        .FirstOrDefault();
 
     private readonly IntrinsicSymbols intrinsicSymbols;
     private readonly HashSet<TAction> usedActions = new();

src/Draco.Compiler/Internal/FlowAnalysis/MatchExhaustiveness.cs

@@ -48,11 +48,19 @@ public override void VisitMatchExpression(BoundMatchExpression node)
 
         if (!decisionTree.IsExhaustive)
         {
-            // TODO: Add example
             // Report
-            this.diagnostics.Add(Diagnostic.Create(
-                template: FlowAnalysisErrors.NonExhaustiveMatchExpression,
-                location: node.Syntax?.Location));
+            var diagBuilder = Diagnostic.CreateBuilder()
+                .WithTemplate(FlowAnalysisErrors.NonExhaustiveMatchExpression)
+                .WithLocation(node.Syntax?.Location);
+            var example = decisionTree.UnhandledExample;
+            if (example is not null)
+            {
+                diagBuilder.WithRelatedInformation(DiagnosticRelatedInformation.Create(
+                    location: node.Syntax?.Location,
+                    format: "for example, the pattern {0} is not handled",
+                    formatArgs: DecisionTree.ToDisplayString(example)));
+            }
+            this.diagnostics.Add(diagBuilder.Build());
         }
 
         foreach (var (covers, redundant) in decisionTree.Redundancies)