https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118784
Bug ID: 118784
Summary: [modules] internal compiler error when consuming {fmt}
within a C++20 module
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: thomas.khyn_gccbugzilla at m4x dot org
Target Milestone: ---
Hello,
I have run into an internal compiler error when consuming {fmt}
(https://github.com/fmtlib/fmg) within a C++20 module.
My setup:
- running on WSL (Ubuntu 24.04) - uname -r = 5.15.167.4-microsoft-standard-WSL2
- cmake 3.31.5
- gcc 15.0.1 20250205, build options `--prefix=/opt/gcc-dev
--enable-languages=c,c++ --disable-multilib`
- ninja 1.11.1
- directory with:
- g...@github.com:fmtlib/fmt.git with master branch (although checking out
earlier versions leads to the same result) checked out in an fmt subfolder
- the following files CMakeLists.txt, main.cpp and fmt.cppm
CMakeLists.txt:
```cmake
cmake_minimum_required(VERSION 3.31)
project(fmt_test)
set(CMAKE_CXX_STANDARD 23)
add_subdirectory(fmt)
add_executable(main main.cpp)
target_sources(main PRIVATE FILE_SET CXX_MODULES FILES fmt.cppm)
target_link_libraries(main PRIVATE fmt::fmt)
```
main.cpp
```c++
import fmt;
int main() {
auto hw = fmt::format("{}, {}!", "Hello", "World");
return static_cast<int>(hw.size());
}
```
fmt.cppm
```c++
module;
#include <fmt/format.h>
export module fmt;
export namespace fmt {
using fmt::format;
}
```
# Issue 1: FMT_MODULE set to OFF
Steps to reproduce:
```
mkdir build && cd build
cmake .. -G Ninja -DCMAKE_BUILD_TYPE=Debug
cmake --build .
```
Expected:
The build completes (as it does in release mode or with Clang 21).
Result:
```
[5/9] Building CXX object CMakeFiles/main.dir/fmt.cppm.o
FAILED: CMakeFiles/main.dir/fmt.cppm.o CMakeFiles/main.dir/fmt.gcm
/usr/bin/c++ -I/mnt/d/dev/sandbox/fmt_test/fmt/include -Wall -Wextra -g
-std=gnu++23 -MD -MT CMakeFiles/main.dir/fmt.cppm.o -MF
CMakeFiles/main.dir/fmt.cppm.o.d -fmodules-ts
-fmodule-mapper=CMakeFiles/main.dir/fmt.cppm.o.modmap -MD -fdeps-format=p1689r5
-x c++ -o CMakeFiles/main.dir/fmt.cppm.o -c
/mnt/d/dev/sandbox/fmt_test/fmt.cppm
/mnt/d/dev/sandbox/fmt_test/fmt.cppm:9:8: internal compiler error: in
core_vals, at cp/module.cc:6465
9 | export module fmt;
| ^~~~~~
0x29e60df internal_error(char const*, ...)
../../gcc/gcc/diagnostic-global-context.cc:517
0xb04991 fancy_abort(char const*, int, char const*)
../../gcc/gcc/diagnostic.cc:1722
0x86310a core_vals
../../gcc/gcc/cp/module.cc:6465
0x86310a tree_node_vals
../../gcc/gcc/cp/module.cc:7500
0xc8a0cd tree_value
../../gcc/gcc/cp/module.cc:9484
0xc8a0cd tree_node
../../gcc/gcc/cp/module.cc:9787
0xc8ce12 core_vals
../../gcc/gcc/cp/module.cc:6370
0xc8ce12 tree_node_vals
../../gcc/gcc/cp/module.cc:7500
0xc87fe2 decl_value
../../gcc/gcc/cp/module.cc:8121
0xc90bf1 depset::hash::find_dependencies(module_state*)
../../gcc/gcc/cp/module.cc:14443
0xc91a56 module_state::write_begin(elf_out*, cpp_reader*, module_state_config&,
unsigned int&)
../../gcc/gcc/cp/module.cc:19323
0xc933c3 finish_module_processing(cpp_reader*)
../../gcc/gcc/cp/module.cc:21840
0xc132ea c_parse_final_cleanups()
../../gcc/gcc/cp/decl2.cc:5724
0xe944c0 c_common_parse_file()
../../gcc/gcc/c-family/c-opts.cc:1408
Please submit a full bug report, with preprocessed source (by using
-freport-bug).
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.
[6/9] Building CXX object fmt/CMakeFiles/fmt.dir/src/format.cc.o
ninja: build stopped: subcommand failed.
```
Note: /usr/bin/c++ is mapped to development g++ on this WSL instance.
I am attaching the pre-processed source after re-running the command that
triggers the internal error with -freport-bug option:
/usr/bin/c++ -I/mnt/d/dev/sandbox/fmt_test/fmt/include -Wall -Wextra -g
-std=gnu++23 -MD -MT CMakeFiles/main.dir/fmt.cppm.o -MF
CMakeFiles/main.dir/fmt.cppm.o.d -fmodules-ts
-fmodule-mapper=CMakeFiles/main.dir/fmt.cppm.o.modmap -MD -fdeps-format=p1689r5
-x c++ -o CMakeFiles/main.dir/fmt.cppm.o -c
/mnt/d/dev/sandbox/fmt_test/fmt.cppm -freport-bug
