Creating named tuple type synonyms in using not allways allowed

[AlexeiScherbakov]

Version Used:
Microsoft Visual Studio Community 2019
Version 16.11.2
.NET Core SDK 5.0.400
Steps to Reproduce:

Create 2 files with using synonyms definition

This is working

using TreeKey = System.ValueTuple<string, string, string, string, string, string>;
using TreeKey2 = System.ValueTuple<(string a, string b, string c, string d, string e, string f)>;

This is not

using TreeKey = System.ValueTuple<string, string, string, string, string, string>;
using TreeKey2 = System.ValueTuple<(string a, string b, string c, string d, string e, string f)>;
using TreeKey3 = (string a, string b, string c, string d, string e, string f);

Expected Behavior:
Tuple synonym TreeKey3 must be allowed, because TreeKey2 synonym is allowed

Actual Behavior:
Tuple synonym TreeKey3 don't compile. Tuple synonym TreeKey2 which contains same(!) named tuple inside can be compiled. TreeKey & TreeKey3 are equivalent, but TreeKey3 contains syntax sugar.

[CyrusNajmabadi]

Language feature request tracked here: dotnet/csharplang#4284