ValueTuple should derive from ValueType or an error.

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

@@ -898,7 +898,8 @@ private void CheckTupleUnderlying(NamedTypeSymbol namedTypeSymbol, SyntaxNode sy
             // but if it does happen we should make it a failure.
             // NOTE: declaredBase could be null for interfaces
             var declaredBase = namedTypeSymbol.BaseTypeNoUseSiteDiagnostics;
-            if (declaredBase?.SpecialType != SpecialType.System_ValueType && declaredBase?.IsErrorType() != true)
+            if (declaredBase == null ||
+                (declaredBase.SpecialType != SpecialType.System_ValueType && !declaredBase.IsErrorType()))
             {
                 // Try to decrease noise by not complaining about the same type over and over again.
                 if (_reportedErrorTypesMap.Add(namedTypeSymbol))

src/Compilers/VisualBasic/Portable/Emit/SymbolTranslator.vb

@@ -13,7 +13,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Emit
         ' TODO: Need to estimate amount of elements for this map and pass that value to the constructor.
         Protected ReadOnly m_AssemblyOrModuleSymbolToModuleRefMap As New ConcurrentDictionary(Of Symbol, Microsoft.Cci.IModuleReference)()
         Private ReadOnly _genericInstanceMap As New ConcurrentDictionary(Of Symbol, Object)()
-        Private ReadOnly _reportedErrorTypesMap As New ConcurrentSet(Of ErrorTypeSymbol)()
+        Private ReadOnly _reportedErrorTypesMap As New ConcurrentSet(Of TypeSymbol)()
 
         Private ReadOnly _embeddedTypesManagerOpt As NoPia.EmbeddedTypesManager
         Public Overrides ReadOnly Property EmbeddedTypesManagerOpt As NoPia.EmbeddedTypesManager
@@ -119,6 +119,8 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Emit
             ElseIf namedTypeSymbol.IsTupleType Then
                 Debug.Assert(Not needDeclaration)
                 namedTypeSymbol = namedTypeSymbol.TupleUnderlyingType
+
+                CheckTupleUnderlying(namedTypeSymbol, syntaxNodeOpt, diagnostics)
             End If
 
             ' Substitute error types with a special singleton object.
@@ -201,6 +203,24 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Emit
             Return namedTypeSymbol
         End Function
 
+        Private Sub CheckTupleUnderlying(namedTypeSymbol As NamedTypeSymbol, syntaxNodeOpt As SyntaxNode, diagnostics As DiagnosticBag)
+            ' check that underlying type of a ValueTuple is indeed a value type (or error)
+            ' this should never happen, in theory,
+            ' but if it does happen we should make it a failure.
+            ' NOTE: declaredBase could be null for interfaces
+            Dim declaredBase = namedTypeSymbol.BaseTypeNoUseSiteDiagnostics
+            If declaredBase Is Nothing OrElse
+                    (declaredBase.SpecialType <> SpecialType.System_ValueType AndAlso Not declaredBase.IsErrorType()) Then
+                ' Try to decrease noise by not complaining about the same type over and over again.
+                If (_reportedErrorTypesMap.Add(namedTypeSymbol)) Then
+                    diagnostics.Add(New VBDiagnostic(
+                                    ErrorFactory.ErrorInfo(ERRID.ERR_PredefinedValueTupleTypeMustBeStruct, namedTypeSymbol.MetadataName),
+                                    If(syntaxNodeOpt Is Nothing, NoLocation.Singleton, syntaxNodeOpt.GetLocation())))
+
+                End If
+            End If
+        End Sub
+
         Friend Overloads Function Translate([param] As TypeParameterSymbol) As Microsoft.Cci.IGenericParameterReference
             Debug.Assert(param Is param.OriginalDefinition)
             Return [param]

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

@@ -1707,6 +1707,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         ERR_ExplicitTupleElementNamesAttribute = 37269
         ERR_TupleLiteralDisallowsTypeChar = 37270
         ERR_NewWithTupleTypeSyntax = 37271
+        ERR_PredefinedValueTupleTypeMustBeStruct = 37272
 
         ' Available 37270
         ERR_DuplicateProcDefWithDifferentTupleNames2 = 37271

src/Compilers/VisualBasic/Portable/Symbols/Tuples/TupleTypeSymbol.vb

@@ -68,18 +68,6 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
             End Get
         End Property
 
-        Public Overrides ReadOnly Property IsReferenceType As Boolean
-            Get
-                Return Not Me._underlyingType.IsErrorType() AndAlso Me._underlyingType.IsReferenceType
-            End Get
-        End Property
-
-        Public Overrides ReadOnly Property IsValueType As Boolean
-            Get
-                Return Me._underlyingType.IsErrorType() OrElse Me._underlyingType.IsValueType
-            End Get
-        End Property
-
         Public Overrides ReadOnly Property IsImplicitlyDeclared As Boolean
             Get
                 Return False
@@ -122,7 +110,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
 
         Public Overrides ReadOnly Property TypeKind As TypeKind
             Get
-                Return If(Me._underlyingType.TypeKind = TypeKind.Class, TypeKind.Class, TypeKind.Struct)
+                ' From the language perspective tuple is a value type
+                ' composed of its underlying elements
+                Return TypeKind.Struct
             End Get
         End Property
 

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

@@ -9117,6 +9117,15 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
             End Get
         End Property
         
+        '''<summary>
+        '''  Looks up a localized string similar to Predefined type &apos;{0}&apos; must be a structure..
+        '''</summary>
+        Friend ReadOnly Property ERR_PredefinedValueTupleTypeMustBeStruct() As String
+            Get
+                Return ResourceManager.GetString("ERR_PredefinedValueTupleTypeMustBeStruct", resourceCulture)
+            End Get
+        End Property
+        
         '''<summary>
         '''  Looks up a localized string similar to SecurityAction value &apos;{0}&apos; is invalid for PrincipalPermission attribute..
         '''</summary>

src/Compilers/VisualBasic/Portable/VBResources.resx

@@ -3169,6 +3169,9 @@
   <data name="ERR_NewWithTupleTypeSyntax" xml:space="preserve">
     <value>'New' cannot be used with tuple type. Use a tuple literal expression instead.</value>
   </data>
+  <data name="ERR_PredefinedValueTupleTypeMustBeStruct" xml:space="preserve">
+    <value>Predefined type '{0}' must be a structure.</value>
+  </data>
   <data name="ERR_ExtensionMethodUncallable1" xml:space="preserve">
     <value>Extension method '{0}' has type constraints that can never be satisfied.</value>
   </data>

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

@@ -51,6 +51,19 @@ namespace System.Runtime.CompilerServices
 End Namespace
 
 "
+
+        ReadOnly s_tupleattributes As String = "
+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_trivial3uple As String = "
 Namespace System
     Public Structure ValueTuple(Of T1, T2, T3)
@@ -1089,7 +1102,7 @@ False
   IL_0035:  ldfld      "System.ValueTuple(Of Integer, Integer, Integer, Integer, Integer, Integer, Integer, (Integer, Integer, Integer, Integer)).Item5 As Integer"
   IL_003a:  call       "Sub System.Console.WriteLine(Integer)"
   IL_003f:  call       "Sub C.TestArray()"
-  IL_0044:  call       "Sub C.Testullable()"
+  IL_0044:  call       "Sub C.TestNullable()"
   IL_0049:  ret
 }
 ]]>)
@@ -15549,6 +15562,117 @@ BC30402: 'evtTest3' cannot implement event 'evtTest2' on interface 'I1' because
 </errors>)
         End Sub
 
+        <WorkItem(11689, "https://github.com/dotnet/roslyn/issues/11689")>
+        <Fact>
+        Public Sub ValueTupleNotStruct0()
+
+            Dim comp = CreateCompilationWithMscorlibAndVBRuntime(
+<compilation name="Tuples">
+    <file name="a.vb">
+class C
+    Shared Sub Main()
+        Dim x as (a As Integer, b As String)
+        x.Item1 = 1
+        x.b = "2"
+ 
+        ' by the language rules tuple x is definitely assigned
+        ' since all its elements are definitely assigned
+        System.Console.WriteLine(x)
+    end sub
+end class
+
+namespace System
+    public class ValueTuple(Of T1, T2)
+        public Item1 as T1
+        public Item2 as T2
+
+        public Sub New(item1 as T1 , item2 as T2 )
+            Me.Item1 = item1
+            Me.Item2 = item2
+        end sub
+    End class
+end Namespace
+
+    <%= s_tupleattributes %>
+    </file>
+</compilation>,
+options:=TestOptions.DebugExe)
+
+            comp.AssertTheseEmitDiagnostics(
+<errors>
+BC37272: Predefined type 'ValueTuple`2' must be a structure.
+        Dim x as (a As Integer, b As String)
+            ~
+</errors>)
+
+        End Sub
+
+        <WorkItem(11689, "https://github.com/dotnet/roslyn/issues/11689")>
+        <Fact>
+        Public Sub ValueTupleNotStruct1()
+
+            Dim comp = CreateCompilationWithMscorlibAndVBRuntime(
+            <compilation name="Tuples">
+                <file name="a.vb">
+class C
+    Shared Sub Main()
+        Dim x = (1,2,3,4,5,6,7,8,9)
+ 
+        System.Console.WriteLine(x)
+    end sub
+end class
+
+namespace System
+    public class ValueTuple(Of T1, T2)
+        public Item1 as T1
+        public Item2 as T2
+
+        public Sub New(item1 as T1 , item2 as T2 )
+            Me.Item1 = item1
+            Me.Item2 = item2
+        end sub
+    End class
+
+    public class ValueTuple(Of T1, T2, T3, T4, T5, T6, T7, TRest)
+        public Item1 As T1
+        public Item2 As T2
+        public Item3 As T3
+        public Item4 As T4
+        public Item5 As T5
+        public Item6 As T6
+        public Item7 As T7
+        public Rest As TRest
+
+        public Sub New(item1 As T1, item2 As T2, item3 As T3, item4 As T4, item5 As T5, item6 As T6, item7 As T7, rest As TRest)
+            Item1 = item1
+            Item2 = item2
+            Item3 = item3
+            Item4 = item4
+            Item5 = item5
+            Item6 = item6
+            Item7 = item7
+            Rest = rest
+        end Sub
+
+    End Class
+end Namespace
+
+    <%= s_tupleattributes %>
+                </file>
+            </compilation>,
+            options:=TestOptions.DebugExe)
+
+            comp.AssertTheseEmitDiagnostics(
+<errors>
+BC37272: Predefined type 'ValueTuple`2' must be a structure.
+        Dim x = (1,2,3,4,5,6,7,8,9)
+            ~
+BC37272: Predefined type 'ValueTuple`8' must be a structure.
+        Dim x = (1,2,3,4,5,6,7,8,9)
+            ~
+</errors>)
+        End Sub
+
         <Fact>
         Public Sub ConversionToBase()
             Dim compilation1 = CompilationUtils.CreateCompilationWithMscorlib(
@@ -15582,6 +15706,163 @@ BC33030: Conversion operators cannot convert from a base type.
 </errors>)
         End Sub
 
+        <WorkItem(11689, "https://github.com/dotnet/roslyn/issues/11689")>
+        <Fact>
+        Public Sub ValueTupleNotStruct2()
+
+            Dim comp = CreateCompilationWithMscorlibAndVBRuntime(
+<compilation name="Tuples">
+    <file name="a.vb">
+class C
+    Shared Sub Main()
+    end sub
+
+    Shared Sub Test2(arg as (a As Integer, b As Integer))
+    End Sub
+end class
+
+namespace System
+    public class ValueTuple(Of T1, T2)
+        public Item1 as T1
+        public Item2 as T2
+
+        public Sub New(item1 as T1 , item2 as T2 )
+            Me.Item1 = item1
+            Me.Item2 = item2
+        end sub
+    End class
+end Namespace
+
+    <%= s_tupleattributes %>
+    </file>
+</compilation>,
+options:=TestOptions.DebugExe)
+
+            comp.AssertTheseEmitDiagnostics(
+<errors>
+BC37272: Predefined type 'ValueTuple`2' must be a structure.
+</errors>)
+
+        End Sub
+
+        <WorkItem(11689, "https://github.com/dotnet/roslyn/issues/11689")>
+        <Fact>
+        Public Sub ValueTupleNotStruct2i()
+
+            Dim comp = CreateCompilationWithMscorlibAndVBRuntime(
+<compilation name="Tuples">
+    <file name="a.vb">
+class C
+    Shared Sub Main()
+    end sub
+
+    Shared Sub Test2(arg as (a As Integer, b As Integer))
+    End Sub
+end class
+
+namespace System
+    public interface ValueTuple(Of T1, T2)
+    End Interface
+end Namespace
+
+    <%= s_tupleattributes %>
+    </file>
+</compilation>,
+options:=TestOptions.DebugExe)
+
+            comp.AssertTheseEmitDiagnostics(
+<errors>
+BC37272: Predefined type 'ValueTuple`2' must be a structure.
+</errors>)
+
+        End Sub
+
+
+        <WorkItem(11689, "https://github.com/dotnet/roslyn/issues/11689")>
+        <Fact>
+        Public Sub ValueTupleNotStruct3()
+
+            Dim comp = CreateCompilationWithMscorlibAndVBRuntime(
+<compilation name="Tuples">
+    <file name="a.vb">
+class C
+    Shared Sub Main()
+        Dim x as (a As Integer, b As String)() = Nothing
+ 
+        ' by the language rules tuple x is definitely assigned
+        ' since all its elements are definitely assigned
+        System.Console.WriteLine(x)
+    end sub
+end class
+
+namespace System
+    public class ValueTuple(Of T1, T2)
+        public Item1 as T1
+        public Item2 as T2
+
+        public Sub New(item1 as T1 , item2 as T2 )
+            Me.Item1 = item1
+            Me.Item2 = item2
+        end sub
+    End class
+end Namespace
+
+    <%= s_tupleattributes %>
+    </file>
+</compilation>,
+options:=TestOptions.DebugExe)
+
+            comp.AssertTheseEmitDiagnostics(
+<errors>
+BC37272: Predefined type 'ValueTuple`2' must be a structure.
+        Dim x as (a As Integer, b As String)() = Nothing
+            ~
+</errors>)
+
+        End Sub
+
+        <WorkItem(11689, "https://github.com/dotnet/roslyn/issues/11689")>
+        <Fact>
+        Public Sub ValueTupleNotStruct4()
+
+            Dim comp = CreateCompilationWithMscorlibAndVBRuntime(
+<compilation name="Tuples">
+    <file name="a.vb">
+class C
+    Shared Sub Main()
+
+    end sub
+    
+    Shared Function Test2()as (a As Integer, b As Integer)
+    End Function
+end class
+
+namespace System
+    public class ValueTuple(Of T1, T2)
+        public Item1 as T1
+        public Item2 as T2
+
+        public Sub New(item1 as T1 , item2 as T2 )
+            Me.Item1 = item1
+            Me.Item2 = item2
+        end sub
+    End class
+end Namespace
+
+    <%= s_tupleattributes %>
+    </file>
+</compilation>,
+options:=TestOptions.DebugExe)
+
+            comp.AssertTheseEmitDiagnostics(
+<errors>
+BC37272: Predefined type 'ValueTuple`2' must be a structure.
+    Shared Function Test2()as (a As Integer, b As Integer)
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+</errors>)
+
+        End Sub
+
         <Fact>
         Public Sub IsBaseOf_WithoutCustomModifiers()
             ' The IL is from this code, but with modifiers