Improve method type argument inference around nullable tuples.

Fixes #14152.

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenTupleTest.cs

@@ -8453,10 +8453,16 @@ static void Main()
     {
         // nested tuple literals set lower bounds
         Test1((a: 1, b: (a: 1, b: 2)), (a: 1, b: (object)1));
+        Test1(Nullable((a: 1, b: (a: 1, b: 2))), (a: 1, b: (object)1));
         Test1((a: 1, b: (a: 1, b: 2)), (a: 1, b: (c: 1, d: 2)));
         Test1((a: 1, b: (a: 1, b: 2)), (a: 1, b: (1, 2)));
         Test1((a: 1, b: (a: 1, b: 2)), (a: 1, b: (a: 1, b: 2)));
-}
+    }
+
+    static T? Nullable<T>(T x) where T : struct
+    {
+        return x;
+    }
 
     static void Test1<T, U>((T, U)? x, (T, U) y)
     {
@@ -8469,12 +8475,45 @@ static void Main()
                 additionalRefs: s_valueTupleRefs,
                 parseOptions: TestOptions.Regular, expectedOutput: @"
 System.Object
+System.Object
 System.ValueTuple`2[System.Int32,System.Int32]
 System.ValueTuple`2[System.Int32,System.Int32]
 System.ValueTuple`2[System.Int32,System.Int32]
 ");
         }
 
+        [Fact]
+        public void Inference13_Err()
+        {
+            var source = @"
+class C
+{
+    static void Main()
+    {
+        Test1(Nullable((a: 1, b: (a: 1, b: 2))), (a: 1, b: (object)1));
+    }
+
+    static T? Nullable<T>(T x) where T : struct
+    {
+        return x;
+    }
+
+    static void Test1<T, U>((T, U) x, (T, U) y)
+    {
+        System.Console.WriteLine(typeof(U));
+    }
+}
+";
+
+            var comp = CreateCompilationWithMscorlib(source,
+                references: s_valueTupleRefs);
+            comp.VerifyDiagnostics(
+                // (6,15): error CS1503: Argument 1: cannot convert from '(int a, (int a, int b) b)?' to '(int, object)'
+                //         Test1(Nullable((a: 1, b: (a: 1, b: 2))), (a: 1, b: (object)1));
+                Diagnostic(ErrorCode.ERR_BadArgType, "Nullable((a: 1, b: (a: 1, b: 2)))").WithArguments("1", "(int a, (int a, int b) b)?", "(int, object)").WithLocation(6, 15)
+                );
+        }
+
         [Fact]
         public void Inference14()
         {

src/Compilers/VisualBasic/Portable/Semantics/TypeInference/TypeArgumentInference.vb

@@ -1408,11 +1408,13 @@ HandleAsAGeneralExpression:
 
 
                 Dim parameterElementTypes As ImmutableArray(Of TypeSymbol) = Nothing
-                If parameterType.TryGetElementTypesIfTupleOrCompatible(parameterElementTypes) Then
+                Dim argumentElementTypes As ImmutableArray(Of TypeSymbol) = Nothing
 
-                    Dim argumentElementTypes As ImmutableArray(Of TypeSymbol) = Nothing
-                    If Not argumentType.TryGetElementTypesIfTupleOrCompatible(argumentElementTypes) OrElse
-                            parameterElementTypes.Length <> argumentElementTypes.Length Then
+                If parameterType.GetNullableUnderlyingTypeOrSelf().TryGetElementTypesIfTupleOrCompatible(parameterElementTypes) AndAlso
+                   If(parameterType.IsNullableType(), argumentType.GetNullableUnderlyingTypeOrSelf(), argumentType).
+                       TryGetElementTypesIfTupleOrCompatible(argumentElementTypes) Then
+
+                    If parameterElementTypes.Length <> argumentElementTypes.Length Then
                         Return False
                     End If
 

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

@@ -12966,7 +12966,8 @@ System.ValueTuple`2[System.Int32,System.Int32]
 
         End Sub
 
-        <Fact(Skip:="https://github.com/dotnet/roslyn/issues/14152")>
+        <Fact>
+        <WorkItem(14152, "https://github.com/dotnet/roslyn/issues/14152")>
         Public Sub Inference13()
 
             Dim comp = CreateCompilationWithMscorlibAndVBRuntime(
@@ -12976,14 +12977,24 @@ Imports System
 Public Class C
     Shared Sub Main()
         Test1((a:=1, b:=(a:=1, b:=2)), (a:=1, b:=DirectCast(1, Object)))
+        Test1(Nullable((a:=1, b:=(a:=1, b:=2))), (a:=1, b:=DirectCast(1, Object)))
+        Test2(Nullable((a:=1, b:=(a:=1, b:=2))), (a:=1, b:=DirectCast(1, Object)))
         Test1((a:=1, b:=(a:=1, b:=2)), (a:=1, b:=(c:=1, d:=2)))
         Test1((a:=1, b:=(a:=1, b:=2)), (a:=1, b:=(1, 2)))
         Test1((a:=1, b:=(a:=1, b:=2)), (a:=1, b:=(a:=1, b:=2)))
     End Sub
 
+    Shared Function Nullable(Of T as structure)(x as T) as T?
+        return x
+    End Function
+
     Shared Sub Test1(Of T, U)(x As (T, U)?, y As (T, U))
         Console.WriteLine(GetType(U))
     End Sub
+
+    Shared Sub Test2(Of T, U)(x As (T, U), y As (T, U))
+        Console.WriteLine(GetType(U))
+    End Sub
 End Class
     </file>
 </compilation>,
@@ -12991,6 +13002,8 @@ options:=TestOptions.ReleaseExe, additionalRefs:=s_valueTupleRefs)
 
             CompileAndVerify(comp, expectedOutput:="
 System.Object
+System.Object
+System.Object
 System.ValueTuple`2[System.Int32,System.Int32]
 System.ValueTuple`2[System.Int32,System.Int32]
 System.ValueTuple`2[System.Int32,System.Int32]