https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121641
Bug ID: 121641
Summary: Rejects valid constexpr explicitly defaulted
constructor which is never a constant expression
(P2448R2)
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: janschultke at googlemail dot com
Target Milestone: ---
struct B { B(); };
struct D : B {
constexpr D() = default;
};
GCC rejects this code sample; Clang accepts.
https://godbolt.org/z/dM84Gjvoj
> <source>:4:15: error: explicitly defaulted function 'constexpr D::D()' cannot
> be declared 'constexpr' because the implicit declaration is not 'constexpr':
> 4 | constexpr D() = default;
> | ^
> <source>:1:12: note: defaulted constructor calls non-'constexpr' 'B::B()'
> 1 | struct B { B(); };
> | ^
> <source>:1:12: note: 'B::B()' declared here
> Compiler returned: 1
To my understanding, P2448R2 "Relaxing some constexpr restrictions" should make
this code valid because some restrictions in [dcl.constexpr] were lifted:
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2448r2.html#pnum_19
The status quo for C++23 seems to be that this code is perfectly fine, except
that you cannot use D() in a constant expression, but so is life.
