Disallow sealed in structs

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

@@ -853,6 +853,11 @@ private void CheckModifiers(Location location, DiagnosticBag diagnostics)
                 // '{0}' cannot be sealed because it is not an override
                 diagnostics.Add(ErrorCode.ERR_SealedNonOverride, location, this);
             }
+            else if (IsSealed && ContainingType.TypeKind == TypeKind.Struct)
+            {
+                // The modifier '{0}' is not valid for this item
+                diagnostics.Add(ErrorCode.ERR_BadMemberFlag, location, SyntaxFacts.GetText(SyntaxKind.SealedKeyword));
+            }
             else if (!ContainingType.IsInterfaceType() && _lazyReturnType.IsStatic)
             {
                 // '{0}': static types cannot be used as return types

src/Compilers/CSharp/Test/Symbol/Symbols/SymbolErrorTests.cs

@@ -1688,6 +1688,7 @@ struct Foo
     public virtual int this[int x] { get { return 1;} set {;} }
     // use long for to prevent signature clash
     public abstract int this[long x] { get; set; }
+    public sealed override string ToString() => null;
 }
 ";
             CreateCompilationWithMscorlib(text).VerifyDiagnostics(
@@ -1714,7 +1715,10 @@ struct Foo
                 Diagnostic(ErrorCode.ERR_BadMemberFlag, "Bar5").WithArguments("abstract").WithLocation(8, 47),
                 // (9,46): error CS0106: The modifier 'virtual' is not valid for this item
                 //     public virtual event System.EventHandler Bar6;
-                Diagnostic(ErrorCode.ERR_BadMemberFlag, "Bar6").WithArguments("virtual").WithLocation(9, 46));
+                Diagnostic(ErrorCode.ERR_BadMemberFlag, "Bar6").WithArguments("virtual").WithLocation(9, 46),
+                // (15,35): error CS0106: The modifier 'sealed' is not valid for this item
+                //      public sealed override string ToString() => null;
+                Diagnostic(ErrorCode.ERR_BadMemberFlag, "ToString").WithArguments("sealed").WithLocation(15, 35));
         }
 
         [Fact]