require the attribute unconditionally

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

@@ -455,19 +455,18 @@ private TypeSymbol BindTupleType(TupleTypeSyntax syntax, DiagnosticBag diagnosti
 
             if (hasExplicitNames)
             {
-                // If the tuple type with names is bound in a declaration
-                // context then we must have the TupleElementNamesAttribute to emit
-                if (syntax.IsTypeInContextWhichNeedsTupleNamesAttribute())
-                {
-                    // Report diagnostics if System.String doesn't exist
-                    this.GetSpecialType(SpecialType.System_String, diagnostics, syntax);
+                // If the tuple type with names is bound we must have the TupleElementNamesAttribute to emit
+                // it is typically there though, if we have ValueTuple at all
+                // and we need System.String as well
 
-                    if (!Compilation.HasTupleNamesAttributes)
-                    {
-                        var info = new CSDiagnosticInfo(ErrorCode.ERR_TupleElementNamesAttributeMissing,
-                            AttributeDescription.TupleElementNamesAttribute.FullName);
-                        Error(diagnostics, info, syntax);
-                    }
+                // Report diagnostics if System.String doesn't exist
+                this.GetSpecialType(SpecialType.System_String, diagnostics, syntax);
+
+                if (!Compilation.HasTupleNamesAttributes)
+                {
+                    var info = new CSDiagnosticInfo(ErrorCode.ERR_TupleElementNamesAttributeMissing,
+                        AttributeDescription.TupleElementNamesAttribute.FullName);
+                    Error(diagnostics, info, syntax);
                 }
             }
 

src/Compilers/CSharp/Portable/Syntax/SyntaxExtensions.cs

@@ -214,12 +214,6 @@ internal static bool IsTypeInContextWhichNeedsDynamicAttribute(this IdentifierNa
             return SyntaxFacts.IsInTypeOnlyContext(typeNode) && IsInContextWhichNeedsDynamicAttribute(typeNode);
         }
 
-        internal static bool IsTypeInContextWhichNeedsTupleNamesAttribute(this TupleTypeSyntax syntax)
-        {
-            Debug.Assert(syntax != null);
-            return SyntaxFacts.IsInTypeOnlyContext(syntax) && IsInContextWhichNeedsTupleNamesAttribute(syntax);
-        }
-
         internal static SyntaxNode SkipParens(this SyntaxNode expression)
         {
             while (expression != null && expression.Kind() == SyntaxKind.ParenthesizedExpression)
@@ -262,45 +256,6 @@ private static bool IsInContextWhichNeedsDynamicAttribute(CSharpSyntaxNode node)
             }
         }
 
-        private static bool IsInContextWhichNeedsTupleNamesAttribute(CSharpSyntaxNode node)
-        {
-            Debug.Assert(node != null);
-
-            var current = node;
-            do
-            {
-                switch (current.Kind())
-                {
-                    case SyntaxKind.Parameter:
-                    case SyntaxKind.FieldDeclaration:
-                    case SyntaxKind.MethodDeclaration:
-                    case SyntaxKind.IndexerDeclaration:
-                    case SyntaxKind.OperatorDeclaration:
-                    case SyntaxKind.ConversionOperatorDeclaration:
-                    case SyntaxKind.PropertyDeclaration:
-                    case SyntaxKind.DelegateDeclaration:
-                    case SyntaxKind.EventDeclaration:
-                    case SyntaxKind.EventFieldDeclaration:
-                    case SyntaxKind.BaseList:
-                    case SyntaxKind.SimpleBaseType:
-                    case SyntaxKind.TypeParameterConstraintClause:
-                        return true;
-
-                    case SyntaxKind.Block:
-                    case SyntaxKind.VariableDeclarator:
-                    case SyntaxKind.Attribute:
-                    case SyntaxKind.EqualsValueClause:
-                        return false;
-
-                    default:
-                        break;
-                }
-                current = current.Parent;
-            } while (current != null);
-
-            return false;
-        }
-
         public static IndexerDeclarationSyntax Update(
             this IndexerDeclarationSyntax syntax,
             SyntaxList<AttributeListSyntax> attributeLists,

src/Compilers/CSharp/Test/Emit/Attributes/AttributeTests_Tuples.cs

@@ -160,9 +160,11 @@ static void Main()
             var ref0 = comp.EmitToImageReference();
             comp = CreateCompilation(source1,
                 references: s_attributeRefs.Concat(new[] { ref0 }));
-            comp.VerifyDiagnostics();
-            // Make sure we emit without errors when System.String is missing.
-            CompileAndVerify(comp, verify: false);
+            comp.VerifyDiagnostics(
+                // (6,11): error CS0518: Predefined type 'System.String' is not defined or imported
+                //         D<(int x, int y)> d = o => { };
+                Diagnostic(ErrorCode.ERR_PredefinedTypeNotFound, "(int x, int y)").WithArguments("System.String").WithLocation(6, 11)
+                );
         }
 
         [Fact]

src/Compilers/CSharp/Test/Semantic/Semantics/SemanticErrorTests.cs

@@ -23120,7 +23120,20 @@ struct ValueTuple<T1, T2>
         public T2 Item2;
         public ValueTuple(T1 item1, T2 item2) { Item1 = item1; Item2 = item2; }
     }
-}";
+}
+
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// Indicates that the use of <see cref=""System.ValueTuple""/> on a member is meant to be treated as a tuple with element names.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue | AttributeTargets.Class | AttributeTargets.Struct )]
+    public sealed class TupleElementNamesAttribute : Attribute
+    {
+        public TupleElementNamesAttribute(string[] transformNames) { }
+    }
+}
+";
             var compilation = CreateCompilationWithMscorlibAndSystemCore(source, options: TestOptions.DebugExe);
             compilation.VerifyDiagnostics(
                 // (10,59): error CS8198: An expression tree may not contain an out argument variable declaration.

src/Compilers/Core/Portable/WellKnownMembers.cs

@@ -3273,7 +3273,6 @@ internal static bool IsSynthesizedAttributeOptional(WellKnownMember attributeMem
                 case WellKnownMember.System_STAThreadAttribute__ctor:
                 case WellKnownMember.System_Runtime_CompilerServices_AsyncStateMachineAttribute__ctor:
                 case WellKnownMember.System_Runtime_CompilerServices_IteratorStateMachineAttribute__ctor:
-                case WellKnownMember.System_Runtime_CompilerServices_TupleElementNamesAttribute__ctorTransformNames:
                     return true;
 
                 default:

src/Compilers/VisualBasic/Portable/Binding/Binder_Symbols.vb

@@ -695,6 +695,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
 
                 ' set of names already used
                 Dim uniqueFieldNames = New HashSet(Of String)(IdentifierComparison.Comparer)
+                Dim hasExplicitNames = False
 
                 For i As Integer = 0 To numElements - 1
                     Dim argumentSyntax = syntax.Elements(i)
@@ -713,6 +714,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
                         name = nameSyntax.Identifier.ValueText
 
                         ' validate name if we have one
+                        hasExplicitNames = True
                         Binder.CheckTupleMemberName(name, i, nameSyntax, diagnostics, uniqueFieldNames)
                         locations.Add(nameSyntax.GetLocation)
                     Else
@@ -722,6 +724,19 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
                     Binder.CollectTupleFieldMemberNames(name, i + 1, numElements, elementNames)
                 Next
 
+                If hasExplicitNames Then
+                    ' If the tuple type with names is bound then we must have the TupleElementNamesAttribute to emit
+                    ' it is typically there though, if we have ValueTuple at all
+                    ' and we need System.String as well
+
+                    ' Report diagnostics if System.String doesn't exist
+                    binder.GetSpecialType(SpecialType.System_String, syntax, diagnostics)
+
+                    If Not binder.Compilation.HasTupleNamesAttributes Then
+                        Binder.ReportDiagnostic(diagnostics, syntax, ERRID.ERR_TupleElementNamesAttributeMissing, AttributeDescription.TupleElementNamesAttribute.FullName)
+                    End If
+                End If
+
                 Dim typesArray As ImmutableArray(Of TypeSymbol) = types.ToImmutableAndFree()
                 Dim locationsArray As ImmutableArray(Of Location) = locations.ToImmutableAndFree()
 

src/Compilers/VisualBasic/Portable/Errors/Errors.vb

@@ -1702,6 +1702,10 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
 
         ERR_ValueTupleTypeRefResolutionError = 37267
 
+        ERR_TupleElementNamesAttributeMissing = 37268
+        ERR_ExplicitTupleElementNames = 37269
+
+
         '// WARNINGS BEGIN HERE
         WRN_UseOfObsoleteSymbol2 = 40000
         WRN_MustOverloadBase4 = 40003

src/Compilers/VisualBasic/Portable/VBResources.Designer.vb

@@ -4457,6 +4457,15 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
             End Get
         End Property
         
+        '''<summary>
+        '''  Looks up a localized string similar to Cannot reference &apos;System.Runtime.CompilerServices.TupleElementNamesAttribute&apos; explicitly. Use the tuple syntax to define tuple names..
+        '''</summary>
+        Friend ReadOnly Property ERR_ExplicitTupleElementNames() As String
+            Get
+                Return ResourceManager.GetString("ERR_ExplicitTupleElementNames", resourceCulture)
+            End Get
+        End Property
+        
         '''<summary>
         '''  Looks up a localized string similar to Type &apos;{0}&apos; exported from module &apos;{1}&apos; conflicts with type declared in primary module of this assembly..
         '''</summary>
@@ -10388,6 +10397,15 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
             End Get
         End Property
         
+        '''<summary>
+        '''  Looks up a localized string similar to Cannot define a class or member that utilizes tuples because the compiler required type &apos;{0}&apos; cannot be found. Are you missing a reference?.
+        '''</summary>
+        Friend ReadOnly Property ERR_TupleElementNamesAttributeMissing() As String
+            Get
+                Return ResourceManager.GetString("ERR_TupleElementNamesAttributeMissing", resourceCulture)
+            End Get
+        End Property
+        
         '''<summary>
         '''  Looks up a localized string similar to Tuple member name &apos;{0}&apos; is only allowed at position {1}..
         '''</summary>

src/Compilers/VisualBasic/Portable/VBResources.resx

@@ -5391,6 +5391,12 @@
   <data name="ERR_TupleTooFewElements" xml:space="preserve">
     <value>Tuple must contain at least two elements.</value>
   </data>
+  <data name="ERR_TupleElementNamesAttributeMissing" xml:space="preserve">
+    <value>Cannot define a class or member that utilizes tuples because the compiler required type '{0}' cannot be found. Are you missing a reference?</value>
+  </data>
+  <data name="ERR_ExplicitTupleElementNames" xml:space="preserve">
+    <value>Cannot reference 'System.Runtime.CompilerServices.TupleElementNamesAttribute' explicitly. Use the tuple syntax to define tuple names.</value>
+  </data>
   <data name="ERR_RefReturningCallInExpressionTree" xml:space="preserve">
     <value>An expression tree may not contain a call to a method or property that returns by reference.</value>
   </data>

src/Compilers/VisualBasic/Test/Emit/CodeGen/CodeGenTuples.vb

@@ -35,6 +35,15 @@ Namespace System
         End Sub
     End Structure
 End Namespace
+
+namespace System.Runtime.CompilerServices
+    <AttributeUsage(AttributeTargets.Field Or AttributeTargets.Parameter Or AttributeTargets.Property Or AttributeTargets.ReturnValue Or AttributeTargets.Class Or AttributeTargets.Struct )>
+    public class TupleElementNamesAttribute : Inherits Attribute
+        public Sub New(transformNames As String())
+	    End Sub
+    End Class
+End Namespace
+
 "
         ReadOnly s_trivialRemainingTuples As String = "
 Namespace System
@@ -64,6 +73,7 @@ Namespace System
     End Structure
 End Namespace
 "
+
         <Fact>
         Public Sub TupleTypeBinding()
 
@@ -143,6 +153,9 @@ BC37267: Predefined type 'ValueTuple(Of ,)' is not defined or imported.
 BC37267: Predefined type 'ValueTuple(Of ,)' is not defined or imported.
         Dim t1 as (A As Integer, B As Integer)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC37268: Cannot define a class or member that utilizes tuples because the compiler required type 'System.Runtime.CompilerServices.TupleElementNamesAttribute' cannot be found. Are you missing a reference?
+        Dim t1 as (A As Integer, B As Integer)
+                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 </errors>)
 
         End Sub
@@ -710,6 +723,14 @@ Namespace System
     End Structure
 End Namespace
 
+namespace System.Runtime.CompilerServices
+    &lt;AttributeUsage(AttributeTargets.Field Or AttributeTargets.Parameter Or AttributeTargets.Property Or AttributeTargets.ReturnValue Or AttributeTargets.Class Or AttributeTargets.Struct )>
+    public class TupleElementNamesAttribute : Inherits Attribute
+        public Sub New(transformNames As String())
+	    End Sub
+    End Class
+End Namespace
+
     </file>
 </compilation>, expectedOutput:=<![CDATA[
 42
@@ -966,6 +987,14 @@ Namespace System
     End Structure
 End Namespace
 
+namespace System.Runtime.CompilerServices
+    &lt;AttributeUsage(AttributeTargets.Field Or AttributeTargets.Parameter Or AttributeTargets.Property Or AttributeTargets.ReturnValue Or AttributeTargets.Class Or AttributeTargets.Struct )>
+    public class TupleElementNamesAttribute : Inherits Attribute
+        public Sub New(transformNames As String())
+	    End Sub
+    End Class
+End Namespace
+
     </file>
 </compilation>, useLatestFramework:=True, expectedOutput:="42
 123
@@ -4250,6 +4279,9 @@ BC37267: Predefined type 'ValueTuple(Of ,)' is not defined or imported.
 BC37267: Predefined type 'ValueTuple(Of ,)' is not defined or imported.
     Public Shared Function M(Of T1, T2)() As (first As T1, second As T2)
                                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC37268: Cannot define a class or member that utilizes tuples because the compiler required type 'System.Runtime.CompilerServices.TupleElementNamesAttribute' cannot be found. Are you missing a reference?
+    Public Shared Function M(Of T1, T2)() As (first As T1, second As T2)
+                                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 BC37267: Predefined type 'ValueTuple(Of ,)' is not defined or imported.
         return (Nothing, Nothing)
                ~~~~~~~~~~~~~~~~~~