[Bug c++/125491] New: GCC accepts a constexpr variable whose user-defined copy constructor leaves a subobject uninitialized

[qurong at ios dot ac.cn via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125491

            Bug ID: 125491
           Summary: GCC accepts a constexpr variable whose user-defined
                    copy constructor leaves a subobject uninitialized
           Product: gcc
           Version: 15.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: qurong at ios dot ac.cn
  Target Milestone: ---

GCC 15.2.0 accepts the following C++20 program, while Clang 22 rejects it:

struct B {
  int x;
  bool failed;

  constexpr B() : failed(false) {}
  constexpr B(const B& o) : failed(o.failed) {}
};

constexpr B id(const B& b) {
  return b;
}

constexpr B q = id(B());


GCC: gcc15 15.2.0
-std=c++20: accepts
-std=c++20 -pedantic-errors: accepts

Clang: clang++-22 22.0.0
-std=c++20: rejects
-std=c++20 -pedantic-errors: rejects

Clang diagnostic:
error: constexpr variable 'q' must be initialized by a constant expression
note: subobject 'x' is not initialized