Extensions: allow extern and disallow in nameof

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

@@ -2306,21 +2306,29 @@ private BoundExpression BindNameofOperatorInternal(InvocationExpressionSyntax no
             var boundArgument = BindExpression(argument, diagnostics);
 
             bool syntaxIsOk = CheckSyntaxForNameofArgument(argument, out string name, boundArgument.HasAnyErrors ? BindingDiagnosticBag.Discarded : diagnostics);
-            if (!boundArgument.HasAnyErrors && syntaxIsOk && boundArgument.Kind == BoundKind.MethodGroup)
+            if (boundArgument is BoundMethodGroup methodGroup)
             {
-                var methodGroup = (BoundMethodGroup)boundArgument;
-                if (!methodGroup.TypeArgumentsOpt.IsDefaultOrEmpty)
+                if (!boundArgument.HasAnyErrors && syntaxIsOk)
                 {
-                    // method group with type parameters not allowed
-                    diagnostics.Add(ErrorCode.ERR_NameofMethodGroupWithTypeParameters, argument.Location);
+                    if (!methodGroup.TypeArgumentsOpt.IsDefaultOrEmpty)
+                    {
+                        // method group with type parameters not allowed
+                        diagnostics.Add(ErrorCode.ERR_NameofMethodGroupWithTypeParameters, argument.Location);
+                    }
+                    else
+                    {
+                        EnsureNameofExpressionSymbols(methodGroup, diagnostics);
+                    }
                 }
-                else
+            }
+            else if (boundArgument is BoundPropertyAccess propertyAccess)
+            {
+                if (propertyAccess.PropertySymbol.GetIsNewExtensionMember())
                 {
-                    EnsureNameofExpressionSymbols(methodGroup, diagnostics);
+                    diagnostics.Add(ErrorCode.ERR_NameofExtensionMember, boundArgument.Syntax);
                 }
             }
-
-            if (boundArgument is BoundNamespaceExpression nsExpr)
+            else if (boundArgument is BoundNamespaceExpression nsExpr)
             {
                 diagnostics.AddAssembliesUsedByNamespaceReference(nsExpr.NamespaceSymbol);
             }

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -8188,4 +8188,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_MisplacedExtension" xml:space="preserve">
     <value>An extension member syntax is disallowed in nested position within an extension member syntax</value>
   </data>
+  <data name="ERR_NameofExtensionMember" xml:space="preserve">
+    <value>Extension member are not allowed as an argument to 'nameof'.</value>
+  </data>
 </root>

src/Compilers/CSharp/Portable/Emitter/Model/MethodSymbolAdapter.cs

@@ -395,7 +395,7 @@ bool Cci.IMethodDefinition.IsExternal
             {
                 CheckDefinitionInvariant();
 
-                return AdaptedMethodSymbol.IsExternal;
+                return !AdaptedMethodSymbol.ContainingType.IsExtension && AdaptedMethodSymbol.IsExternal;
             }
         }
 
@@ -422,7 +422,7 @@ bool Cci.IMethodDefinition.IsPlatformInvoke
             get
             {
                 CheckDefinitionInvariant();
-                return AdaptedMethodSymbol.GetDllImportData() != null;
+                return !AdaptedMethodSymbol.ContainingType.IsExtension && AdaptedMethodSymbol.GetDllImportData() != null;
             }
         }
 #nullable disable
@@ -431,7 +431,7 @@ Cci.IPlatformInvokeInformation Cci.IMethodDefinition.PlatformInvokeData
             get
             {
                 CheckDefinitionInvariant();
-                return AdaptedMethodSymbol.GetDllImportData();
+                return AdaptedMethodSymbol.ContainingType.IsExtension ? null : AdaptedMethodSymbol.GetDllImportData();
             }
         }
 

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

@@ -2409,6 +2409,7 @@ internal enum ErrorCode
         ERR_PPShebangInProjectBasedProgram = 9314,
 
         ERR_TooManyUserStrings_RestartRequired = 9315,
+        ERR_NameofExtensionMember = 9316,
 
         // 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

@@ -2522,6 +2522,7 @@ or ErrorCode.ERR_CloseUnimplementedInterfaceMemberOperatorMismatch
                 or ErrorCode.ERR_OperatorMismatchOnOverride
                 or ErrorCode.ERR_BadCompoundAssignmentOpArgs
                 or ErrorCode.ERR_PPShebangInProjectBasedProgram
+                or ErrorCode.ERR_NameofExtensionMember
                     => 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/Extensions/SourceExtensionImplementationMethodSymbol.cs

@@ -22,8 +22,6 @@ public SourceExtensionImplementationMethodSymbol(MethodSymbol sourceMethod)
         {
             Debug.Assert(sourceMethod.GetIsNewExtensionMember());
             Debug.Assert(sourceMethod.IsStatic || sourceMethod.ContainingType.ExtensionParameter is not null);
-            Debug.Assert(!sourceMethod.IsExtern);
-            Debug.Assert(!sourceMethod.IsExternal);
 
             // Tracked by https://github.com/dotnet/roslyn/issues/78963 : Are we creating type parameters with the right emit behavior? Attributes, etc.
             //            Also, they should be IsImplicitlyDeclared
@@ -59,9 +57,9 @@ internal override int ParameterCount
 
         internal sealed override bool IsAccessCheckedOnOverride => false;
 
-        public sealed override bool IsExtern => false;
-        public sealed override DllImportData? GetDllImportData() => null;
-        internal sealed override bool IsExternal => false;
+        public sealed override bool IsExtern => _originalMethod.IsExtern;
+        public sealed override DllImportData? GetDllImportData() => _originalMethod.GetDllImportData();
+        internal sealed override bool IsExternal => _originalMethod.IsExternal;
 
         internal sealed override bool IsDeclaredReadOnly => false;
 

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

@@ -754,12 +754,7 @@ private static (DeclarationModifiers mods, bool hasExplicitAccessMod) MakeModifi
                 allowedModifiers |= DeclarationModifiers.Static;
             }
 
-            allowedModifiers |= DeclarationModifiers.Async;
-
-            if (!isExtension)
-            {
-                allowedModifiers |= DeclarationModifiers.Extern;
-            }
+            allowedModifiers |= DeclarationModifiers.Async | DeclarationModifiers.Extern;
 
             if (containingType.IsStructType())
             {

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

@@ -449,10 +449,7 @@ private static (DeclarationModifiers modifiers, bool hasExplicitAccessMod) MakeM
                 allowedModifiers |= DeclarationModifiers.ReadOnly;
             }
 
-            if (!isExtension)
-            {
-                allowedModifiers |= DeclarationModifiers.Extern;
-            }
+            allowedModifiers |= DeclarationModifiers.Extern;
 
             bool hasExplicitAccessMod;
             var mods = ModifierUtils.MakeAndCheckNonTypeMemberModifiers(isOrdinaryMethod: false, isForInterfaceMember: isInterface,