Pre-processed C++ module generated code lead to compilation error when using clang++-22 #151985
Description
Summary
clang++-22 outputs a compilation error when compiling a pre-processed c++ modules file (using -frewrite-includes) which generated code starts with # 1 "<built-in>" while clang++-22 expects module;, module ...; or export module ...;.
Note: I couldnt test with clangc++-21 yet because of #151221 , will test it later.
Context
$ clang++-22 --version
Ubuntu clang version 22.0.0 (++20250804082146+0cfe9f7287f7-1~exp1~20250804082205.1075)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-22/bin
$ clang++-20 --version
Ubuntu clang version 20.1.8 (++20250708082440+6fb913d3e2ec-1~exp1~20250708202457.136)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-20/bin
How To Reproduce
Reproducer: https://github.com/Klaim/reprocase-clang-22-modules
Requires clang++-22 to be available in the environment.
Clone the reproducer then run repro.sh for the failing version using clang++-22, repro-works.sh for the succeeding version which is the same commands but wiht clang++-20.
From scratch:
- Create a simple module file, here
module.mxx:
export module mymodule;
export namespace mymodule
{
void func() { }
}mkdir output/- Then run these compilation commands in sequence:
# step 1
clang++-22 -std=c++26 -D__cpp_modules=201907L -finput-charset=UTF-8 -w -x c++-module -MQ ^ -MD -E -frewrite-includes -MF - -o ./output/mymodule.pcm.ii ./mymodule.mxx
# step 2
clang++-22 -std=c++26 -D__cpp_modules=201907L -finput-charset=UTF-8 -E -x c++-module ./output/mymodule.pcm.ii
# step 3
clang++-22 -std=c++26 -D__cpp_modules=201907L -Wno-unqualified-std-cast-call -fdiagnostics-color -finput-charset=UTF-8 -Xclang -fmodules-embed-all-files -fmodule-output=./output/mymodule.pcm -o ./output/mymodule.pcm.o -c -x c++-module ./output/mymodule.pcm.ii
Observed
With clang++-22:
$ ./repro.sh
step 1
^: mymodule.mxx
step 2
# 1 "./output/mymodule.pcm.ii"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 495 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "./output/mymodule.pcm.ii" 2
# 1 "<built-in>"
# 1 "./mymodule.mxx"
export module mymodule;
export namespace mymodule
{
void func() { }
}
step 3
./mymodule.mxx:1:8: error: module declaration must occur at the start of the translation unit
1 | export module mymodule;
| ^
./output/mymodule.pcm.ii:1:1: note: add 'module;' to the start of the file to introduce a global module fragment
1 | # 1 "<built-in>"
| ^
1 error generated.
Expected
No error.
For example repro-works.sh which only changes the compiler to clang++-20 works as expected, no error.
$ ./repro-works.sh
step 1
^: mymodule.mxx
step 2
# 1 "./output/mymodule.pcm.ii"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 495 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "./output/mymodule.pcm.ii" 2
# 1 "<built-in>"
# 1 "./mymodule.mxx"
export module mymodule;
export namespace mymodule
{
void func() { }
}
step 3
More Context
I initially hit this issue when attempting to build a modules-only (except some dependencies) project using build2 which uses these build steps.
Depending on the module, if there was a import std; then the error would first appear for std.cppm.