Allow nameof to access instance members.

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

@@ -1956,18 +1956,19 @@ private BoundExpression SynthesizeReceiver(SyntaxNode node, Symbol member, Bindi
                 (currentType.IsInterface && (declaringType.IsObjectType() || currentType.AllInterfacesNoUseSiteDiagnostics.Contains(declaringType))))
             {
                 bool hasErrors = false;
-                if (EnclosingNameofArgument != node)
+                if (!IsInsideNameof || (EnclosingNameofArgument != node && !IsFeatureAvailable(node, MessageID.IDS_FeatureReducedMemberAccessChecksInNameof)))
                 {
+                    var diagnosticsTemp = IsInsideNameof ? BindingDiagnosticBag.Discarded : diagnostics;
                     if (InFieldInitializer && !currentType.IsScriptClass)
                     {
                         //can't access "this" in field initializers
-                        Error(diagnostics, ErrorCode.ERR_FieldInitRefNonstatic, node, member);
+                        Error(diagnosticsTemp, ErrorCode.ERR_FieldInitRefNonstatic, node, member);
                         hasErrors = true;
                     }
                     else if (InConstructorInitializer || InAttributeArgument)
                     {
                         //can't access "this" in constructor initializers or attribute arguments
-                        Error(diagnostics, ErrorCode.ERR_ObjectRequired, node, member);
+                        Error(diagnosticsTemp, ErrorCode.ERR_ObjectRequired, node, member);
                         hasErrors = true;
                     }
                     else
@@ -1980,12 +1981,17 @@ private BoundExpression SynthesizeReceiver(SyntaxNode node, Symbol member, Bindi
                         if (!locationIsInstanceMember)
                         {
                             // error CS0120: An object reference is required for the non-static field, method, or property '{0}'
-                            Error(diagnostics, ErrorCode.ERR_ObjectRequired, node, member);
+                            Error(diagnosticsTemp, ErrorCode.ERR_ObjectRequired, node, member);
                             hasErrors = true;
                         }
                     }
 
-                    hasErrors = hasErrors || IsRefOrOutThisParameterCaptured(node, diagnostics);
+                    hasErrors = hasErrors || IsRefOrOutThisParameterCaptured(node, diagnosticsTemp);
+
+                    if (hasErrors && IsInsideNameof)
+                    {
+                        CheckFeatureAvailability(node, MessageID.IDS_FeatureReducedMemberAccessChecksInNameof, diagnostics);
+                    }
                 }
 
                 return ThisReference(node, currentType, hasErrors, wasCompilerGenerated: true);
@@ -7255,10 +7261,17 @@ private bool CheckInstanceOrStatic(
             {
                 if (instanceReceiver == true)
                 {
-                    ErrorCode errorCode = this.Flags.Includes(BinderFlags.ObjectInitializerMember) ?
-                        ErrorCode.ERR_StaticMemberInObjectInitializer :
-                        ErrorCode.ERR_ObjectProhibited;
-                    Error(diagnostics, errorCode, node, symbol);
+                    if (!IsInsideNameof)
+                    {
+                        ErrorCode errorCode = this.Flags.Includes(BinderFlags.ObjectInitializerMember) ?
+                            ErrorCode.ERR_StaticMemberInObjectInitializer :
+                            ErrorCode.ERR_ObjectProhibited;
+                        Error(diagnostics, errorCode, node, symbol);
+                    }
+                    else if (CheckFeatureAvailability(node, MessageID.IDS_FeatureReducedMemberAccessChecksInNameof, diagnostics))
+                    {
+                        return false;
+                    }
                     resultKind = LookupResultKind.StaticInstanceMismatch;
                     return true;
                 }

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

@@ -2499,6 +2499,11 @@ protected AssemblySymbol GetForwardedToAssembly(string name, int arity, ref Name
         }
 
 #nullable enable
+        internal static bool IsFeatureAvailable(SyntaxNode syntax, MessageID feature)
+        {
+            return ((CSharpParseOptions)syntax.SyntaxTree.Options).IsFeatureEnabled(feature);
+        }
+
         internal static bool CheckFeatureAvailability(SyntaxNode syntax, MessageID feature, BindingDiagnosticBag diagnostics, Location? location = null)
         {
             return CheckFeatureAvailability(syntax, feature, diagnostics.DiagnosticBag, location);

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6676,6 +6676,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_UnreadRecordParameter_Title" xml:space="preserve">
     <value>Parameter is unread. Did you forget to use it to initialize the property with that name?</value>
   </data>
+  <data name="IDS_FeatureReducedMemberAccessChecksInNameof" xml:space="preserve">
+    <value>reduced member access checks in 'nameof'</value>
+  </data>
   <data name="ERR_RecordAmbigCtor" xml:space="preserve">
     <value>The primary constructor conflicts with the synthesized copy constructor.</value>
   </data>

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

@@ -241,8 +241,8 @@ internal enum MessageID
         IDS_FeatureNewLinesInInterpolations = MessageBase + 12813,
         IDS_FeatureListPattern = MessageBase + 12814,
         IDS_ParameterNullChecking = MessageBase + 12815,
-
         IDS_FeatureCacheStaticMethodGroupConversion = MessageBase + 12816,
+        IDS_FeatureReducedMemberAccessChecksInNameof = MessageBase + 12817,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -358,6 +358,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureListPattern: // semantic check
                 case MessageID.IDS_FeatureCacheStaticMethodGroupConversion: // lowering check
                 case MessageID.IDS_ParameterNullChecking: // syntax check
+                case MessageID.IDS_FeatureReducedMemberAccessChecksInNameof:
                     return LanguageVersion.Preview;
 
                 // C# 10.0 features.