[C++20][Modules] Name is not visible in instantiation of an imported function template

[cpplearner]

clang++ --version:

clang version 17.0.4
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: D:\LLVM-17.0.4\bin

mod1.cpp:

export module mod1;

export template<class T>
T mod1_f(T x) {
    return x;
}

mod2.cpp:

export module mod2;
import mod1;

export template<class U>
U mod2_g(U y) {
    return mod1_f(y);
}

mod3.cpp:

export module mod3;
import mod2;

export int mod3_h(int p) {
    return mod2_g(p);
}

clang++ -std=c++20 -x c++-module -c -fprebuilt-module-path=. mod1.cpp -fmodule-output=mod1.pcm
clang++ -std=c++20 -x c++-module -c -fprebuilt-module-path=. mod2.cpp -fmodule-output=mod2.pcm
clang++ -std=c++20 -x c++-module -c -fprebuilt-module-path=. mod3.cpp -fmodule-output=mod3.pcm:

In file included from mod3.cpp:2:
D:\test\test-module\mod2.cpp:6:12: error: call to function 'mod1_f' that is neither visible in the template definition nor found by argument-dependent lookup
    6 |     return mod1_f(y);
      |            ^
mod3.cpp:5:12: note: in instantiation of function template specialization 'mod2_g<int>' requested here
    5 |     return mod2_g(p);
      |            ^
D:\test\test-module\mod1.cpp:4:3: note: 'mod1_f' should be declared prior to the call site
    4 | T mod1_f(T x) {
      |   ^
1 error generated.

I think the name mod1_f should be visible within mod2, because mod1 is imported there.

Adding import mod1; to mod3 makes the error disappear, but IMO it shouldn't matter whether mod3 imports mod1, since mod1_f is not directly referenced within mod3.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: S. B. Tam (cpplearner)

Details

`clang++ --version`:

clang version 17.0.4
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: D:\LLVM-17.0.4\bin

mod1.cpp:

export module mod1;

export template<class T>
T mod1_f(T x) {
    return x;
}

mod2.cpp:

export module mod2;
import mod1;

export template<class U>
U mod2_g(U y) {
    return mod1_f(y);
}

mod3.cpp:

export module mod3;
import mod2;

export int mod3_h(int p) {
    return mod2_g(p);
}

clang++ -std=c++20 -x c++-module -c -fprebuilt-module-path=. mod1.cpp -fmodule-output=mod1.pcm
clang++ -std=c++20 -x c++-module -c -fprebuilt-module-path=. mod2.cpp -fmodule-output=mod2.pcm
clang++ -std=c++20 -x c++-module -c -fprebuilt-module-path=. mod3.cpp -fmodule-output=mod3.pcm:

In file included from mod3.cpp:2:
D:\test\test-module\mod2.cpp:6:12: error: call to function 'mod1_f' that is neither visible in the template definition nor found by argument-dependent lookup
    6 |     return mod1_f(y);
      |            ^
mod3.cpp:5:12: note: in instantiation of function template specialization 'mod2_g<int>' requested here
    5 |     return mod2_g(p);
      |            ^
D:\test\test-module\mod1.cpp:4:3: note: 'mod1_f' should be declared prior to the call site
    4 | T mod1_f(T x) {
      |   ^
1 error generated.

I think the name mod1_f should be visible within mod2, because mod1 is imported there.

Adding import mod1; to mod3 makes the error disappear, but IMO it shouldn't matter whether mod3 imports mod1, since mod1_f is not directly referenced within mod3.

[ChuanqiXu9]

This is a surprise to me. Since I feel the issue is fixed already. I've tested in trunk and everything looks fine. I don't remember there is any related change between clang17 and trunk for this.

@cpplearner would you like to test it again on trunk?

[cpplearner]

It seems to work on trunk.

Curiously, clang++ 17 also seems to work if the target is not windows. If I pass --target=x86_64-linux to clang++, then it can successfully compile these modules.

[ChuanqiXu9]

Then it should be #69431. And I'll close the issue since it is fixed in trunk.