clang rejects reusing names in unexported variables between modules

[davidstone]

Given the following three translation units:

export module a;

constexpr int x = 0;

export void a() {
	(void)x;
}

export module b;

constexpr int x = 0;

export void b() {
	(void)x;
}

import a;
import b;

And compiling with

clang++ -std=c++23 -x c++-module -fmodule-output=a.pcm --precompile -c a.cpp
clang++ -std=c++23 -x c++-module -fmodule-output=b.pcm --precompile -c b.cpp
clang++ -std=c++23 -fmodule-file=a=a.pcm -fmodule-file=b=b.pcm -c c.cpp

causes clang to fail with

In file included from c.cpp:1:
a.cpp:3:15: error: declaration 'x' attached to named module 'a' can't be attached to other modules
    3 | constexpr int x = 0;
      |               ^
b.cpp:3:15: note: also found
    3 | constexpr int x = 0;
      |               ^
1 error generated.

I believe this code should be accepted and have the same behavior as if x were declared static in both module a and module b. x has module linkage in both module a and module b. Because of that, and based on https://eel.is/c++draft/basic.link#8, the two declarations of x are not declaring the same entity and thus the error is incorrect.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: David Stone (davidstone)

Given the following three translation units:

export module a;

constexpr int x = 0;

export void a() {
	(void)x;
}

export module b;

constexpr int x = 0;

export void b() {
	(void)x;
}

import a;
import b;

And compiling with

clang++ -std=c++23 -x c++-module -fmodule-output=a.pcm --precompile -c a.cpp
clang++ -std=c++23 -x c++-module -fmodule-output=b.pcm --precompile -c b.cpp
clang++ -std=c++23 -fmodule-file=a=a.pcm -fmodule-file=b=b.pcm -c c.cpp

causes clang to fail with

In file included from c.cpp:1:
a.cpp:3:15: error: declaration 'x' attached to named module 'a' can't be attached to other modules
    3 | constexpr int x = 0;
      |               ^
b.cpp:3:15: note: also found
    3 | constexpr int x = 0;
      |               ^
1 error generated.

I believe this code should be accepted and have the same behavior as if x were declared static in both module a and module b. x has module linkage in both module a and module b. Because of that, and based on https://eel.is/c++draft/basic.link#8, the two declarations of x are not declaring the same entity and thus the error is incorrect.

[ChuanqiXu9]

This may be duplicated with #90154.

And I don't have an expectation that how and when this problem will be solved properly. It is somewhat fundamental.

The workaround suggestion may be: 1. use namespaces. 2. Use static variable with Reduced BMI (https://clang.llvm.org/docs/StandardCPlusPlusModules.html#reduced-bmi).