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From: "bbi5291 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107945] static constexpr incomplete (depedent) data member
 of a class template and in-member initialized incorrectly accepted
Date: Wed, 03 Apr 2024 01:28:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107945

Brian Bi <bbi5291 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bbi5291 at gmail dot com

--- Comment #5 from Brian Bi <bbi5291 at gmail dot com> ---
In the first case, I think GCC's behaviour can be argued to be correct.
[temp.inst]/3.1 says that when you implicitly instantiate a class template,=
 you
only instantiate the declarations of static data members, not their
definitions. Which means that until you do something that provokes the
instantiation of the *definition* of `C<incomplete>::t`, the compiler must
treat it as a variable of type `const incomplete` that is
declared-but-not-defined, which means that you haven't yet actually attempt=
ed
to define a variable with incomplete type, which means the program is
well-formed.

In the most recent example given by Jason Liam we can reduce it to:

template <class T>
struct S {
    constexpr static S s =3D {};
};
int main() {
    auto s =3D S<int>::s;
}

This may be related to bug 70820, which was closed as INVALID. I think perh=
aps
that decision should be revisited; while I can see the argument for the abo=
ve
code being well-formed, I think the interpretation of the standard being ta=
ken
in this case is wrong.=