Use of unannotated unconstrained type parameter in nullable diabled code

[gafter]

The issue is the treatment of a reference to a (unconstrainted) type parameter that was both declared and referenced in a disabled context (without an annotation). Such a reference should designate an oblivious type and should be treated accordingly. That does not appear to be the treatment today. That is the issue in M1 below. There should be no problem with the default expression because it is being converted to an oblivious type.

See also #30214

#nullable disable
class A<T1, T2, T3> where T2 : class where T3 : object
{
    T1 F1;
    T2 F2;
    T3 F3;
    B F4;

#nullable enable
    void M1()
    {
        F1 = default; // unexpected warning
        F2 = default; // unexpected warning
        F3 = default; // unexpected warning
        F4 = default;
    }
}

class B {}