Generic extension method with in this parameter fails to compile

[controlflow]

Version Used:

master branch @ feb 5

Steps to Reproduce:

Compile the following code:

public static class C {
  public static ref readonly T E<T>(this ref T x) where T : struct => ref x;
}

Expected Behavior:

Successful compilation, the same as for ref extension version:

public static class C {
  public static ref readonly T E<T>(this ref T x) where T : struct => ref x;
}

Actual Behavior:

error CS8338: The first parameter of an 'in' extension method 'E' must be a value type

[sharwell]

This seems very similar to #24601.

[controlflow]

Thanks, @sharwell ! What a confusing corner case, I was not realized any kind of member access would copy the struct in generic code since compiler can't tell if T is readonly struct or not...