Disallow modifiers in local_using_declaration

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7902,4 +7902,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_ParamsCollectionMissingConstructor" xml:space="preserve">
     <value>Non-array params collection type must have an applicable constructor that can be called with no arguments.</value>
   </data>
+  <data name="ERR_NoModifiersOnUsing" xml:space="preserve">
+    <value>Modifiers cannot be placed on using declarations</value>
+  </data>
 </root>

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

@@ -2300,6 +2300,8 @@ internal enum ErrorCode
         ERR_ParamsCollectionExtensionAddMethod = 9227,
         ERR_ParamsCollectionMissingConstructor = 9228,
 
+        ERR_NoModifiersOnUsing = 9229,
+
         #endregion
 
         // Note: you will need to do the following after adding warnings:

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

@@ -2430,6 +2430,7 @@ internal static bool IsBuildOnlyDiagnostic(ErrorCode code)
                 case ErrorCode.ERR_ParamsCollectionExpressionTree:
                 case ErrorCode.ERR_ParamsCollectionExtensionAddMethod:
                 case ErrorCode.ERR_ParamsCollectionMissingConstructor:
+                case ErrorCode.ERR_NoModifiersOnUsing:
                     return false;
                 default:
                     // NOTE: All error codes must be explicitly handled in this switch statement

src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

@@ -9575,7 +9575,7 @@ private StatementSyntax ParseLocalDeclarationStatement(SyntaxList<AttributeListS
             }
 
             var mods = _pool.Allocate();
-            this.ParseDeclarationModifiers(mods);
+            this.ParseDeclarationModifiers(mods, isUsingDeclaration: usingKeyword is not null);
 
             var variables = _pool.AllocateSeparated<VariableDeclaratorSyntax>();
             try
@@ -9623,13 +9623,17 @@ private StatementSyntax ParseLocalDeclarationStatement(SyntaxList<AttributeListS
                     type = _syntaxFactory.ScopedType(scopedKeyword, type);
                 }
 
-                for (int i = 0; i < mods.Count; i++)
+                // We've already reported all modifiers for local_using_declaration as errors
+                if (usingKeyword is null)
                 {
-                    var mod = (SyntaxToken)mods[i];
-
-                    if (IsAdditionalLocalFunctionModifier(mod.ContextualKind))
+                    for (int i = 0; i < mods.Count; i++)
                     {
-                        mods[i] = this.AddError(mod, ErrorCode.ERR_BadMemberFlag, mod.Text);
+                        var mod = (SyntaxToken)mods[i];
+
+                        if (IsAdditionalLocalFunctionModifier(mod.ContextualKind))
+                        {
+                            mods[i] = this.AddError(mod, ErrorCode.ERR_BadMemberFlag, mod.Text);
+                        }
                     }
                 }
 
@@ -9824,7 +9828,7 @@ private bool IsEndOfDeclarationClause()
             }
         }
 
-        private void ParseDeclarationModifiers(SyntaxListBuilder list)
+        private void ParseDeclarationModifiers(SyntaxListBuilder list, bool isUsingDeclaration)
         {
             SyntaxKind k;
             while (IsDeclarationModifier(k = this.CurrentToken.ContextualKind) || IsAdditionalLocalFunctionModifier(k))
@@ -9845,7 +9849,11 @@ private void ParseDeclarationModifiers(SyntaxListBuilder list)
                     mod = this.EatToken();
                 }
 
-                if (k is SyntaxKind.ReadOnlyKeyword or SyntaxKind.VolatileKeyword)
+                if (isUsingDeclaration)
+                {
+                    mod = this.AddError(mod, ErrorCode.ERR_NoModifiersOnUsing);
+                }
+                else if (k is SyntaxKind.ReadOnlyKeyword or SyntaxKind.VolatileKeyword)
                 {
                     mod = this.AddError(mod, ErrorCode.ERR_BadMemberFlag, mod.Text);
                 }