https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119154
Bug ID: 119154
Summary: GCC 15.0 std module with <string> compiled with -O3
breaks when loaded with -O0 (not -O1, -O2 and -Ofast)
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: igor.machado at gmail dot com
Target Milestone: ---
I stumbled on another strange gcc 15 bug when doing some basic CXX module
testing (just recently reported some -Ofast with -fopenmp bug, but this is
different now).
When I build the std module with <string> and -O3, it works:
main.cpp
```
#include <string>
import std;
int main(){return 0;}
```
Just build with:
g++-15 -std=c++23 -O3 -fmodules -fsearch-include-path bits/std.cc main.cpp
Then, gcm.cache/std.gcm is created.
Finally, I can rebuild without the 'std' module and reuse from cache, and it
works fine with -O3:
g++-15 -std=c++23 -O3 -fmodules main.cpp
Strangely, when I rebuild with -O0, it breaks:
```
$ g++-15 -std=c++23 -O0 -fmodules main.cpp -freport-bug
main.cpp:3:24: internal compiler error: in import_export_decl, at
cp/decl2.cc:3526
3 | int main() { return 0; }
| ^
0x718bb9c2a1c9 __libc_start_call_main
../sysdeps/nptl/libc_start_call_main.h:58
0x718bb9c2a28a __libc_start_main_impl
../csu/libc-start.c:360
Please submit a full bug report, with preprocessed source.
Please include the complete backtrace with any bug report.
See <file:///usr/share/doc/gcc-15/README.Bugs> for instructions.
The bug is not reproducible, so it is likely a hardware or OS problem.
```
If I rebuild with -O1, -O2, -O3 or even -Ofast, it runs fine... only -O0 is
problematic.
Strangely, the header <string> is not even being used... but if I remove it,
then the bug is gone.
My system is Ubuntu 24.04 with gcc-15:
$ g++-15 --version
g++-15 (Ubuntu 15-20250213-1ubuntu1) 15.0.1 20250213 (experimental) [master
r15-7502-g26baa2c09b3]
Sorry for the inconvenience, and good luck!
