[Proposal] Allow generic structs to match the unmanaged constraint

[SamPruden]

void Foo<T>() where T : unmanaged => throw new NotImplementedException();

// This errors because it doesn't know that (int, int) is unmanaged.
Foo<(int, int)>();

I would like the above to work.

I am specifically focused on tuples in my use case, but more generally I would propose that unmanaged should be checked after resolving generics. Currently it seems that all generic structs are rejected by unmanaged. I propose that a generic struct should only be rejected when, after generic resolution, it has a field of a managed type.

struct IntWrapper { public int Value; }
struct GenericWrapper<T> { public T Value; }

IntWrapper can be used with the unmanaged constraint.

GenericWrapper<int> cannot. I propose that it should be able to be.

[ufcpp]

Dup of #1504
This requires CLR change

[SamPruden]

@ufcpp Oops, missed that. Thanks, I'll close this down then. It's a shame that it's blocked, but not very surprising.