Fix nullable warning on conversion of anonymous function with implicit parameters

Author: Julien Couvreur

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/CSharpResources.resx

@@ -5668,6 +5668,12 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_NullabilityMismatchInParameterTypeOfTargetDelegate_Title" xml:space="preserve">
     <value>Nullability of reference types in type of parameter doesn't match the target delegate (possibly because of nullability attributes).</value>
   </data>
+  <data name="WRN_NullabilityMismatchInImplicitParameterTypeOfTargetDelegate" xml:space="preserve">
+    <value>Nullability of reference types in type of implicit parameter with index {0} of '{1}' doesn't match the target delegate '{2}' (possibly because of nullability attributes).</value>
+  </data>
+  <data name="WRN_NullabilityMismatchInImplicitParameterTypeOfTargetDelegate_Title" xml:space="preserve">
+    <value>Nullability of reference types in type of implicit parameter doesn't match the target delegate (possibly because of nullability attributes).</value>
+  </data>
   <data name="WRN_NullAsNonNullable" xml:space="preserve">
     <value>Cannot convert null literal to non-nullable reference type.</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1715,6 +1715,7 @@ internal enum ErrorCode
         ERR_NullableDirectiveTargetExpected = 8668,
         WRN_MissingNonNullTypesContextForAnnotationInGeneratedCode = 8669,
         WRN_NullReferenceInitializer = 8670,
+        WRN_NullabilityMismatchInImplicitParameterTypeOfTargetDelegate = 8671,
 
         ERR_MultipleAnalyzerConfigsInSameDir = 8700,
 

src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs

@@ -34,6 +34,7 @@ static ErrorFacts()
             nullableWarnings.Add(GetId(ErrorCode.WRN_NullabilityMismatchInArgumentForOutput));
             nullableWarnings.Add(GetId(ErrorCode.WRN_NullabilityMismatchInReturnTypeOfTargetDelegate));
             nullableWarnings.Add(GetId(ErrorCode.WRN_NullabilityMismatchInParameterTypeOfTargetDelegate));
+            nullableWarnings.Add(GetId(ErrorCode.WRN_NullabilityMismatchInImplicitParameterTypeOfTargetDelegate));
             nullableWarnings.Add(GetId(ErrorCode.WRN_NullAsNonNullable));
             nullableWarnings.Add(GetId(ErrorCode.WRN_NullableValueTypeMayBeNull));
             nullableWarnings.Add(GetId(ErrorCode.WRN_NullabilityMismatchInTypeParameterConstraint));
@@ -434,6 +435,7 @@ internal static int GetWarningLevel(ErrorCode code)
                 case ErrorCode.WRN_NullabilityMismatchInArgumentForOutput:
                 case ErrorCode.WRN_NullabilityMismatchInReturnTypeOfTargetDelegate:
                 case ErrorCode.WRN_NullabilityMismatchInParameterTypeOfTargetDelegate:
+                case ErrorCode.WRN_NullabilityMismatchInImplicitParameterTypeOfTargetDelegate:
                 case ErrorCode.WRN_NullAsNonNullable:
                 case ErrorCode.WRN_NullableValueTypeMayBeNull:
                 case ErrorCode.WRN_NullabilityMismatchInTypeParameterConstraint:

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

@@ -7570,10 +7570,20 @@ 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);
+                if (unboundLambda.HasParameterNames())
+                {
+                    ReportDiagnostic(ErrorCode.WRN_NullabilityMismatchInParameterTypeOfTargetDelegate, location,
+                        unboundLambda.ParameterName(parameterSymbol.Ordinal),
+                        unboundLambda.MessageID.Localize(),
+                        delegateType);
+                }
+                else
+                {
+                    ReportDiagnostic(ErrorCode.WRN_NullabilityMismatchInImplicitParameterTypeOfTargetDelegate, location,
+                        parameterSymbol.Ordinal,
+                        unboundLambda.MessageID.Localize(),
+                        delegateType);
+                }
             }
         }