Inference failure for tuple type vs tuple underlying type (regression)

[gafter]

The following correct program compiles successfully in VS2015, but fails to compile in VS2017:

using System;
public class C
{
    public static void Main()
    {
        // var b = (1, 2, 3, 4, 5, 6, 7, 8);
        var b = new ValueTuple<int, int, int, int, int, int, int, ValueTuple<int>>(1, 2, 3, 4, 5, 6, 7, new ValueTuple<int>(8));
        M1(b);
    }
    public static void M1<T1, T2, T3, T4, T5, T6, T7, TRest>(ValueTuple<T1, T2, T3, T4, T5, T6, T7, TRest> a) where TRest : struct { Console.Write(1); }
}

with the following error

Error CS0411: The type arguments for method 'C.M1<T1, T2, T3, T4, T5, T6, T7, TRest>(ValueTuple<T1, T2, T3, T4, T5, T6, T7, TRest>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

[gafter]

A regression test is being added for this (skipped) in #20587.

[gafter]

I did not test whether or not this bug is present for VB.

[gafter]

This bug is present for VB as well. The following program compiles in earlier versions of VS, but not in VS 2017.

Public Class C
    Public Shared Sub Main()
        Dim b = New ValueTuple(Of Integer, Integer, Integer, Integer, Integer, Integer, Integer, ValueTuple(Of Integer))()
        M1(b)
    End Sub
    Public Shared Sub M1(Of T1, T2, T3, T4, T5, T6, T7, TRest As Structure)(x As ValueTuple(Of T1, T2, T3, T4, T5, T6, T7, TRest))
    End Sub
End Class

[gafter]

This would likely be fixed as a side-effect of #20648