Same method signature confusion

问题

Assuming C# 8.0 and reference type nullability, why are these signatures considered equal:

void Test<T>(IEnumerable<T> enumerable) where T : class
void Test<T>(IEnumerable<T> enumerable) where T : struct

but these are not?

void Test<T>(IEnumerable<T?> enumerable) where T : class
void Test<T>(IEnumerable<T?> enumerable) where T : struct

The only thing different is the nullability of enumerable elements.

来源：https://stackoverflow.com/questions/60115233/same-method-signature-confusion