[Clang][Module] Miss definition of inline function in implementation partition units

[24bit-xjkp]

Information

clang version 20.0.0git (https://github.com/llvm/llvm-project.git 73c72f2)
Target: x86_64-unknown-windows-gnu
Thread model: posix
InstalledDir: D:/Tools/clang/bin
OS: Windows11 24H2

Here is my code:

// a.cpp
import B;
int main()
{
    g<void>();
}
// b.cppm
export module B;
import :C;
export template <typename T> inline void g() noexcept
{
    return f<T>();
}
// c.cppm
module B:C;
template<typename> inline void f() noexcept {}

I can compile these code with gcc and msvc, but when I compile with clang:

mkdir build
clang++ -std=c++26 c.cppm --precompile -o build/c.pcm
clang++ -std=c++26 b.cppm "-fmodule-file=B:C=build/c.pcm" --precompile -o build/b.pcm
clang++ -std=c++26 a.cpp "-fmodule-file=B=build/b.pcm" "-fmodule-file=B:C=build/c.pcm"

Then I receive errors below:

In module 'B' imported from a.cpp:1:
C:/Users/2283/Desktop/test/c.cppm:4:13: warning: inline function 'f<void>' is not defined [-Wundefined-inline]
    4 | inline void f() noexcept
      |             ^
C:/Users/2283/Desktop/test/b.cppm:7:12: note: used here
    7 |     return f<T>();
      |            ^
1 warning generated.
ld.lld: error: undefined symbol: void f@B<void>()
>>> referenced by C:/Users/2283/AppData/Local/Temp/a-7d0622.o:(void g@B<void>())
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

Clang doesn't generate code correctly for the inline function f<void> therefore lld can't find the symbol f@B<void>() while linking objects.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: 小鸡快跑 (24bit-xjkp)

## Information

clang version 20.0.0git (https://github.com/llvm/llvm-project.git 73c72f2)
Target: x86_64-unknown-windows-gnu
Thread model: posix
InstalledDir: D:/Tools/clang/bin
OS: Windows11 24H2

Here is my code:

// a.cpp
import B;
int main()
{
    g<void>();
}
// b.cppm
export module B;
import :C;
export template <typename T> inline void g() noexcept
{
    return f<T>();
}
// c.cppm
module B:C;
template<typename> inline void f() noexcept {}

I can compile these code with gcc and msvc, but when I compile with clang:

mkdir build
clang++ -std=c++26 c.cppm --precompile -o build/c.pcm
clang++ -std=c++26 b.cppm "-fmodule-file=B:C=build/c.pcm" --precompile -o build/b.pcm
clang++ -std=c++26 a.cpp "-fmodule-file=B=build/b.pcm" "-fmodule-file=B:C=build/c.pcm"

Then I receive errors below:

In module 'B' imported from a.cpp:1:
C:/Users/2283/Desktop/test/c.cppm:4:13: warning: inline function 'f<void>' is not defined [-Wundefined-inline]
    4 | inline void f() noexcept
      |             ^
C:/Users/2283/Desktop/test/b.cppm:7:12: note: used here
    7 |     return f<T>();
      |            ^
1 warning generated.
ld.lld: error: undefined symbol: void f@<!-- -->B&lt;void&gt;()
&gt;&gt;&gt; referenced by C:/Users/2283/AppData/Local/Temp/a-7d0622.o:(void g@<!-- -->B&lt;void&gt;())
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

Clang doesn't generate code correctly for the inline function f<void> therefore lld can't find the symbol f@<!-- -->B&lt;void&gt;() while linking objects.

[ChuanqiXu9]

BTW, the last command you give should be clang++ -std=c++26 a.cpp "-fmodule-file=B=build/b.pcm" "-fmodule-file=B:C=build/c.pcm" build/c.pcm build/b.pcm -o build/a.out. But it doesn't matter. The error is still there.

For the issue itself, the compiler's behavior may be good. According to [module.reach]:

A translation unit U is necessarily reachable from a point P if U is a module interface unit on which the translation unit containing P has an interface dependency, or the translation unit containing P imports U, in either case prior to P

All translation units that are necessarily reachable are reachable. Additional translation units on which the point within the program has an interface dependency may be considered reachable, but it is unspecified which are and under what circumstances.

Here the module unit c.cppm is not necessarily reachable to a.cpp since:
(1) c.cppm is an implementation partition unit. And,
(2) a.cpp doesn't import c.cppm.

So whether or not c.cppm is reachable to a.cpp or not is unspecified. And clang's decision is, only the necessary reachable TU are reachable. So the compiler's behavior is not bad at least.

---

My personal suggestion for implementation partition unit is, envision them like the headers which can only be included by sources. In another word, only import the implementation partition unit in the implementation unit (note that implementation unit is different from implementation partition unit). Otherwise, users need to understand what they are doing.