https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124431
Nathaniel Shead <nshead at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
CC| |nshead at gcc dot gnu.org
Summary|Possibly incorrect 'error: |[16 Regression] Possibly
|accessing uninitialised |incorrect 'error: accessing
|array element' in C++20 |uninitialised array
|module |element' in C++20 module
Last reconfirmed| |2026-03-10
Status|UNCONFIRMED |NEW
Keywords| |needs-bisection,
| |rejects-valid
--- Comment #2 from Nathaniel Shead <nshead at gcc dot gnu.org> ---
Confirmed. Bit of a strange case; here's as reduced as I can get it for now:
// a.cpp
export module A;
export template <int N> struct mat {
int data[1] = { 123 };
constexpr const int& operator[](int x) const { return data[x]; };
};
// b.cpp
export module B;
export import A;
struct VisualTextModel {
mat<4> viewmatrix;
};
// c.cpp
export module C;
export import A;
mat<4> from_triangle_frame;
// d.cpp
export module D;
import C;
import B;
export constexpr mat<4> rotate_about_y() {
mat<4> r;
r[0] * 1;
return r;
}
// e.cpp
import D;
constexpr auto rotn_y = rotate_about_y();
$ g++ -fmodules -S [abcde].cpp
In module D, imported at e.cpp:1:
e.cpp:2:39: in ‘constexpr’ expansion of ‘rotate_about_y@D()’
2 | constexpr auto rotn_y = rotate_about_y();
| ~~~~~~~~~~~~~~^~
d.cpp:6:8: error: accessing uninitialized member ‘mat@A<4>::data’
6 | r[0] * 1;
| ^~~
Swapping the order of importing C and B in module D fixes the issue. First
quick check looks the NSDMI is getting dropped somewhere.
GCC15 at least can correctly compile the reduced version, so I'll mark as a
regression.
