[Bug c++/88165] error: default member initializer for 'A::B::m' required before the end of its enclosing class

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88165

--- Comment #14 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to andysem from comment #13)
> (In reply to Jonathan Wakely from comment #10)
> > >     C(D = {});
> > 
> > This requires checking whether C::D is default constructible, but we are
> > still in the body of C, so C is not complete, which means C::D is not
> > complete, which means we don't know if C::D is default constructible.
> 
> Does it? I thought the default arguments were only evaluated at the point
> where they are actually used, i.e. in this case - when `C` constructor is
> invoked with no arguments.

It's surprising, I agree. Whether the default argument is valid determines
whether this constructor is considered a default constructor, which determines
whether the compiler needs to implicitly declare a separate default constructor
I think that's why the default argument is needed earlier than you expect.

> If this is how the standard specifies compiler behavior, I wonder if this
> should be considered as a defect (DR). 

I closed this as a dup of PR 96645 which has a DR number right there in the
title. Like I said, see there for more details.

> There is nothing in
> `std::numeric_limits<double>::max()` that depends on the definition of `C`,
> so there is no reason to prevent it from compiling.

Are you sure? What if C::std is declared? Then the initializer means something
very different. Again, see PR 96645 for further discussion of when the
initializer can be compiled (and a possible patch to do it differently).