MemberNameSameAsType

Author: Julien Couvreur

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -8218,4 +8218,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_InterpolatedStringHandlerArgumentDisallowed" xml:space="preserve">
     <value>Interpolated string handler arguments are not allowed in this context.</value>
   </data>
+  <data name="ERR_MemberNameSameAsExtendedType" xml:space="preserve">
+    <value>'{0}': extension member names cannot be the same as their extended type</value>
+  </data>
 </root>

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

@@ -2420,6 +2420,7 @@ internal enum ErrorCode
         ERR_InstanceOperatorExtensionWrongReceiverType = 9323,
         ERR_ExpressionTreeContainsExtensionBasedConditionalLogicalOperator = 9324,
         ERR_InterpolatedStringHandlerArgumentDisallowed = 9325,
+        ERR_MemberNameSameAsExtendedType = 9326,
 
         // Note: you will need to do the following after adding errors:
         //  1) Update ErrorFacts.IsBuildOnlyDiagnostic (src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs)

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

@@ -2532,6 +2532,7 @@ or ErrorCode.ERR_InstanceOperatorStructExtensionWrongReceiverRefKind
                 or ErrorCode.ERR_InstanceOperatorExtensionWrongReceiverType
                 or ErrorCode.ERR_ExpressionTreeContainsExtensionBasedConditionalLogicalOperator
                 or ErrorCode.ERR_InterpolatedStringHandlerArgumentDisallowed
+                or ErrorCode.ERR_MemberNameSameAsExtendedType
                     => false,
             };
 #pragma warning restore CS8524 // The switch expression does not handle some values of its input type (it is not exhaustive) involving an unnamed enum value.

src/Compilers/CSharp/Portable/Symbols/Source/SourceMemberContainerSymbol.cs

@@ -1391,18 +1391,45 @@ private void CheckMemberNameDistinctFromType(Symbol member, BindingDiagnosticBag
             {
                 case TypeKind.Class:
                 case TypeKind.Struct:
-                    if (member.Name == this.Name)
-                    {
-                        diagnostics.Add(ErrorCode.ERR_MemberNameSameAsType, member.GetFirstLocation(), this.Name);
-                    }
+                    checkContainingTypeName(member, this.Name, diagnostics);
                     break;
                 case TypeKind.Interface:
                     if (member.IsStatic)
                     {
-                        goto case TypeKind.Class;
+                        checkContainingTypeName(member, this.Name, diagnostics);
                     }
+
+                    break;
+                case TypeKind.Extension:
+                    // Since implementation methods have the same name as the extension method, we don't need to report the problem twice
+                    if (member.Kind != SymbolKind.Method && this.ContainingType is { } containingType)
+                    {
+                        checkContainingTypeName(member, containingType.Name, diagnostics);
+                    }
+
+                    if (this.ExtensionParameter is { Type: NamedTypeSymbol { Name: var extendedTypeName } })
+                    {
+                        checkExtendedTypeName(member, extendedTypeName, diagnostics);
+                    }
+
                     break;
             }
+
+            static void checkContainingTypeName(Symbol member, string typeName, BindingDiagnosticBag diagnostics)
+            {
+                if (member.Name == typeName)
+                {
+                    diagnostics.Add(ErrorCode.ERR_MemberNameSameAsType, member.GetFirstLocation(), typeName);
+                }
+            }
+
+            static void checkExtendedTypeName(Symbol member, string typeName, BindingDiagnosticBag diagnostics)
+            {
+                if (member.Name == typeName)
+                {
+                    diagnostics.Add(ErrorCode.ERR_MemberNameSameAsExtendedType, member.GetFirstLocation(), typeName);
+                }
+            }
         }
 
         internal override bool HasDeclaredRequiredMembers
@@ -1812,7 +1839,7 @@ protected void AfterMembersChecks(BindingDiagnosticBag diagnostics)
                 CheckExtensionMembers(this.GetMembers(), diagnostics);
             }
 
-            CheckMemberNamesDistinctFromType(diagnostics); // Tracked by https://github.com/dotnet/roslyn/issues/76130 : Should this check "see through" extensions?
+            CheckMemberNamesDistinctFromType(diagnostics);
             CheckMemberNameConflictsAndUnmatchedOperators(diagnostics);
             CheckRecordMemberNames(diagnostics);
             CheckSpecialMemberErrors(diagnostics);