Fix ILLink analyzer assertion for delegates with default parameters

src/tools/illink/src/ILLink.RoslynAnalyzer/DataFlow/LocalDataFlowAnalysis.cs

@@ -80,6 +80,12 @@ public bool InterproceduralAnalyze()
                 return succeeded;
             }
 
+            // Delegate types with default parameter values produce operation blocks whose
+            // OwningSymbol is the delegate INamedTypeSymbol. These blocks contain only
+            // simple constant initializers with no interesting dataflow, so skip them.
+            if (Context.OwningSymbol is INamedTypeSymbol { TypeKind: TypeKind.Delegate })
+                return succeeded;
+
             Debug.Assert(Context.OwningSymbol is not IMethodSymbol methodSymbol ||
                 methodSymbol.MethodKind is not (MethodKind.LambdaMethod or MethodKind.LocalFunction));
             var startMethod = new MethodBodyValue(Context.OwningSymbol, Context.GetControlFlowGraph(OperationBlock));

src/tools/illink/test/Mono.Linker.Tests.Cases/DataFlow/AnnotatedMembersAccessedViaReflection.cs

@@ -890,6 +890,11 @@ class DelegateCreation
 
             static event Action<Type> Event;
 
+            // Delegate with a default parameter value.
+            // The default parameter generates an operation block whose OwningSymbol
+            // is the delegate type (INamedTypeSymbol), not a method.
+            delegate void DelegateWithDefaultParameter(Type type = null);
+
             static void TestField()
             {
                 var d = new UnannotatedDelegate(field);