VB symbolic API incorrect for tuple types.

[CyrusNajmabadi]

Given the following code:

imports System

class Test(of T)
    sub Blah(of T)(t as T)
        dim t2 = (a:=t, b:=4) ' this tuple
    end sub
end class

The VB semantic model till produce a type symbol for that tuple like so:

However, strangely, that type symbol will have itself as its 'ConstructedFrom'. i.e.:

This is due to this logic here in tupletypesymbol.vb

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

Lines 220 to 224 in c5e8e95

	Public Overrides ReadOnly Property ConstructedFrom As NamedTypeSymbol
	Get
	Return Me
	End Get
	End Property

This does not match C# behavior. In C#, if we use the corresponding code:

using System;

class Test<T>
{
    void Blah<T>(T t)
    {
        var t2 = (a: t, b: 4); // this tuple
    }
}

We can see the appropriate 'ConstructedFrom' symbol:

[CyrusNajmabadi]

Tagging @gafter @jcouv . Not sure what's up with VB here, but it's super weird. Is there a way in VB to tell if a tuple symbol is 'constructed' and what it was constructed from?

[CyrusNajmabadi]

Oh. It's even weirder. In VB .OriginalDefinition returns the constructed type as well.

[gafter]

I believe @jcouv intends to implement #39370 for VB, which would fix these issues.

[jcouv]

@CyrusNajmabadi Just to let you know: at the moment, issue #20648 for VB isn't prioritized.
If the current VB API behavior blocks your work (ie. there is no way to achieve the desired language-independent abstraction) then please let me know. We'll discuss prioritization with Jared.