Stage2 validate extern types

[Vexu]

Question: in stage1 pointers to zero bit types are zero bit types themselves and are therefore not allowed in extern types but with #6706 they now behave like other pointers. This means that *void is now allowed in extern types even though it's not the same as *anyopaque that the user likely meant. Is this OK? Relevant test case is optional_pointer_to_void_in_extern_struct.zig.

[andrewrk]

I think it's OK. In fact, I was wondering if we should take it a step further and remove anyopaque now that *void functions almost identically. This would remove one strange new thing that Zig introduces from the language.

[Vexu]

I was wondering if we should take it a step further and remove anyopaque now that *void functions almost identically

It would be nice but I think it would also lead in to new strange things that don't currently exist. For example in stage2 dereferencing a runtime known *anyopaque gives the error values of type 'anyopaque' must be comptime known, but operand value is runtime known but dereferencing *void is allowed.

[andrewrk]

Good point.