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

["andysem at mail dot ru" <gcc-bugzilla@gcc.gnu.org>]

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

--- Comment #13 from andysem at mail dot ru ---
(In reply to Jonathan Wakely from comment #10)
> (In reply to Fedor Chelnokov from comment #7)
> > This struct definition:
> > ```
> > struct A {
> >     struct B {
> >         int i = 0;
> >         B() {}
> 
> This declares default constructor, meaning that the type is default
> constructible. Period.
> 
> By declaring B(); you assert that B is default constructible.
> 
> >     };
> >     A(B = {});
> 
> This is valid, because the user-provided default constructor for B is all
> the compiler needs to see to know that B={} is valid.
> 
> > };
> > ```
> > is accepted by GCC, but another one ({} replaced with = default) is not:
> > ```
> > struct C {
> >     struct D {
> >         int i = 0;
> >         D() = default;
> 
> This declares a default constructor that might be defined implicitly by the
> compiler, **or** it might get deleted if the member definitions of D would
> make the implicit default constructor ill-formed. This is obviously very
> different from the case where you declare B(); There is no assertion that D
> is default constructible, the compiler has to deduce whether or not that's
> true. That depends on the default member initializer for D::i. The
> initializer (which is just '0' here) is a "complete class context" which
> means it is not processed until the class D is complete (this allows you to
> use other members, or e.g. sizeof(D) as the initializer).
> 
> A nested class like C::D is not complete until its enclosing class is
> complete. This means the initializer for C::D::i is compiled after C is
> complete. This means whether C::D is default constructible is not known
> until C is complete.
> 
> 
> >     };
> >     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.

> > };
> > ```
> > Demo: https://gcc.godbolt.org/z/WTPdTn1Yf
> > 
> > Could you please explain why? I though that both must be same accepted or
> > same rejected.
> 
> I hope the explanation above helps.

Thank you for the explanation.

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