Lambdas cannot be used inside C++20 modules because they share the “definition with same mangled name”

[ruoka]

How to reproduce:

// module.cpp 
export module test;
export namespace test
{
    auto foo = [] {return 1;};
    auto bar = [] {return 'a';};
}

// main.cpp 
import test;
int main()
{
    test::foo();
    test::bar();
}

$ clang++ -std=c++20 -fprebuilt-module-path=. -Xclang -emit-module-interface -c module.cpp -o test.pcm
$ clang++ -std=c++20 -fprebuilt-module-path=. main.cpp test.pcm -o main

In file included from main.cpp:1:
./module.cpp:6:16: error: definition with same mangled name '_ZW4testENK4testUt_clEv' as another definition
auto bar = [] {return 'a';};
^
./module.cpp:5:16: note: previous definition is here
auto foo = [] {return 1;};
^

[ChuanqiXu9]

It is curious that it would be OK if the lambdas assigning to foo and bar receive an argument.

[ruoka]

Seems to work if the other lambdas use different arguments, but fails again if the same argument is used:

//modules.cpp
export module test;
export namespace test
{
    auto foo = [] (int) {return 1;};
    auto bar = [] (double) {return 2;};
    auto bar2 = [] (double) {return 3;};
}

// main.cpp
import test;
int main()
{
    test::foo(1);
    test::bar(1.0);
    test::bar(2.0);
}

module.cpp:6:17: error: definition with same mangled name '_ZW4testENK4testUt_clEd' as another definition
auto bar2 = [] (double) {return 1;};
^
/module.cpp:5:16: note: previous definition is here
auto bar = [] (double) {return 0;};

[llvmbot]

@llvm/issue-subscribers-clang-modules

[llvmbot]

@llvm/issue-subscribers-c-20

[hugoam]

Just mentioning here that this issue might be tied to #53232 which I opened last week. A fix might probably want to make sure it fixes both scenarios.

[ChuanqiXu9]

@hugoam Thanks for telling this. I've sent https://reviews.llvm.org/D118018 to try to address this. And I've checked the patch could solve #53232 locally.

[ChuanqiXu9]

Assigned to @urnathan. He knows much more about mangling for modules.