Fix warning on conversion of anon func with implicit parameters

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

@@ -30,6 +30,7 @@ public QueryUnboundLambdaState(Binder binder, RangeVariableMap rangeVariableMap,
                 _bodyFactory = bodyFactory;
             }
 
+            public override bool HasParameterNames() { return true; }
             public override string ParameterName(int index) { return _parameters[index].Name; }
             public override bool ParameterIsDiscard(int index) { return false; }
             public override bool ParameterIsNullChecked(int index) { return false; }

src/Compilers/CSharp/Portable/BoundTree/UnboundLambda.cs

@@ -457,6 +457,7 @@ public TypeWithAnnotations InferReturnType(ConversionsBase conversions, NamedTyp
         public TypeWithAnnotations ParameterTypeWithAnnotations(int index) { return Data.ParameterTypeWithAnnotations(index); }
         public TypeSymbol ParameterType(int index) { return ParameterTypeWithAnnotations(index).Type; }
         public Location ParameterLocation(int index) { return Data.ParameterLocation(index); }
+        public bool HasParameterNames() { return Data.HasParameterNames(); }
         public string ParameterName(int index) { return Data.ParameterName(index); }
         public bool ParameterIsDiscard(int index) { return Data.ParameterIsDiscard(index); }
         public bool ParameterIsNullChecked(int index) { return Data.ParameterIsNullChecked(index); }
@@ -517,6 +518,7 @@ internal UnboundLambdaState WithCaching(bool includeCache)
         public UnboundLambda UnboundLambda => _unboundLambda;
 
         public abstract MessageID MessageID { get; }
+        public abstract bool HasParameterNames();
         public abstract string ParameterName(int index);
         public abstract bool ParameterIsDiscard(int index);
         public abstract bool ParameterIsNullChecked(int index);
@@ -1409,6 +1411,8 @@ public override SyntaxList<AttributeListSyntax> ParameterAttributes(int index)
             return _parameterAttributes.IsDefault ? default : _parameterAttributes[index];
         }
 
+        public override bool HasParameterNames() { return !_parameterNames.IsDefault; }
+
         public override string ParameterName(int index)
         {
             Debug.Assert(!_parameterNames.IsDefault && 0 <= index && index < _parameterNames.Length);

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -7570,10 +7570,14 @@ void reportBadDelegateReturn(BindingDiagnosticBag bag, MethodSymbol targetInvoke
 
             void reportBadDelegateParameter(BindingDiagnosticBag bag, MethodSymbol sourceInvokeMethod, MethodSymbol targetInvokeMethod, ParameterSymbol parameterSymbol, bool topLevel, Location location)
             {
-                ReportDiagnostic(ErrorCode.WRN_NullabilityMismatchInParameterTypeOfTargetDelegate, location,
-                    unboundLambda.ParameterName(parameterSymbol.Ordinal),
-                    unboundLambda.MessageID.Localize(),
-                    delegateType);
+                // For anonymous functions with implicit parameters, no need to report this since the parameters can't be referenced
+                if (unboundLambda.HasParameterNames())
+                {
+                    ReportDiagnostic(ErrorCode.WRN_NullabilityMismatchInParameterTypeOfTargetDelegate, location,
+                        unboundLambda.ParameterName(parameterSymbol.Ordinal),
+                        unboundLambda.MessageID.Localize(),
+                        delegateType);
+                }
             }
         }
 

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

@@ -155949,5 +155949,26 @@ void M()
                 Diagnostic(ErrorCode.WRN_NullabilityMismatchInReturnTypeOfTargetDelegate, "d2").WithArguments("ref readonly string? D<string?>.Invoke()", "D<string>").WithLocation(9, 28)
                 );
         }
+
+        [Fact, WorkItem(59336, "https://github.com/dotnet/roslyn/issues/59336")]
+        public void ParameterNameCrash()
+        {
+            var source = @"
+#nullable enable
+
+public delegate void MyDelegate([System.Diagnostics.CodeAnalysis.MaybeNull] object first, [System.Diagnostics.CodeAnalysis.MaybeNull] object second);
+
+class C
+{
+    public event MyDelegate? MyEvent;
+    void M()
+    {
+        MyEvent += delegate { };
+        MyEvent?.Invoke(new object(), new object());
+    }
+}";
+            var comp = CreateCompilation(new[] { source, MaybeNullAttributeDefinition });
+            comp.VerifyDiagnostics();
+        }
     }
 }