Fix warning on conversion of anon func with implicit parameters (#59522)

Author: jcouv

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

@@ -34,7 +34,6 @@ public QueryUnboundLambdaState(Binder binder, RangeVariableMap rangeVariableMap,
             public override bool ParameterIsDiscard(int index) { return false; }
             public override bool ParameterIsNullChecked(int index) { return false; }
             public override SyntaxList<AttributeListSyntax> ParameterAttributes(int index) => default;
-            public override bool HasNames { get { return true; } }
             public override bool HasSignature { get { return true; } }
 
             public override bool HasExplicitReturnType(out RefKind refKind, out TypeWithAnnotations returnType)

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

@@ -335,7 +335,7 @@ private UnboundLambda BindAnonymousFunction(AnonymousFunctionExpressionSyntax sy
             // are reported now.
             ModifierUtils.ToDeclarationModifiers(syntax.Modifiers, diagnostics.DiagnosticBag ?? new DiagnosticBag());
 
-            if (data.HasNames)
+            if (data.HasSignature)
             {
                 var binder = new LocalScopeBinder(this);
                 bool allowShadowingNames = binder.Compilation.IsFeatureEnabled(MessageID.IDS_FeatureNameShadowingInNestedFunctions);

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

@@ -526,7 +526,6 @@ internal UnboundLambdaState WithCaching(bool includeCache)
         public abstract bool HasExplicitlyTypedParameterList { get; }
         public abstract int ParameterCount { get; }
         public abstract bool IsAsync { get; }
-        public abstract bool HasNames { get; }
         public abstract bool IsStatic { get; }
         public abstract Location ParameterLocation(int index);
         public abstract TypeWithAnnotations ParameterTypeWithAnnotations(int index);
@@ -1357,7 +1356,6 @@ internal PlainUnboundLambdaState(
             _isStatic = isStatic;
         }
 
-        public override bool HasNames { get { return !_parameterNames.IsDefault; } }
 
         public override bool HasSignature { get { return !_parameterNames.IsDefault; } }
 

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.HasSignature)
+                {
+                    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 DelegateConversionWithImplicitParameters()
+        {
+            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();
+        }
     }
 }