Merge pull request #14463 from OmarTawfik/fix-8556-nre-on-method-group-cast

Fix printing ambiguous method group types on invalid casts

src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs

@@ -1784,7 +1784,10 @@ private BoundExpression BindCastCore(ExpressionSyntax node, BoundExpression oper
                 ImmutableArray<MethodSymbol> originalUserDefinedConversions = conversion.OriginalUserDefinedConversions;
                 if (originalUserDefinedConversions.Length > 1)
                 {
-                    diagnostics.Add(ErrorCode.ERR_AmbigUDConv, syntax.Location, originalUserDefinedConversions[0], originalUserDefinedConversions[1], operand.Type, targetType);
+                    // Method groups always have null types
+                    var operandType = (operand.Kind == BoundKind.MethodGroup) ? (object)MessageID.IDS_SK_METHOD.Localize() : operand.Type;
+
+                    diagnostics.Add(ErrorCode.ERR_AmbigUDConv, syntax.Location, originalUserDefinedConversions[0], originalUserDefinedConversions[1], operandType, targetType);
                 }
                 else
                 {

src/Compilers/CSharp/Test/Semantic/Semantics/MethodBodyModelTests.cs

@@ -956,5 +956,43 @@ public Inner(int xyzzy)
                 Diagnostic(ErrorCode.ERR_ObjectRequired, "f1").WithArguments("Outer.f1").WithLocation(10, 21)
                 );
         }
+
+        [Fact, WorkItem(8556, "https://github.com/dotnet/roslyn/issues/8556")]
+        public void CastAmbiguousMethodGroupTypeProducesCorrectErrorMessage()
+        {
+            var text = @"
+using System;
+
+public delegate void Foo ();
+
+class D
+{
+    public static implicit operator D (Action d)
+    {
+        return new D ();
+    }
+
+    public static explicit operator D (Foo d)
+    {
+        return new D ();
+    }
+}
+
+class Program
+{
+    static void Main()
+    {
+        D d = (D) Main;
+    }
+}
+";
+            var tree = Parse(text);
+            var comp = CreateCompilationWithMscorlib(tree);
+            comp.GetMethodBodyDiagnostics().Verify(
+                // (23,15): error CS0457: Ambiguous user defined conversions 'D.explicit operator D(Foo)' and 'D.implicit operator D(Action)' when converting from 'method' to 'D'
+                //          D d = (D) Main;
+                Diagnostic(ErrorCode.ERR_AmbigUDConv, "(D) Main").WithArguments("D.explicit operator D(Foo)", "D.implicit operator D(System.Action)", "method", "D").WithLocation(23, 15)
+                );
+        }
     }
 }