[C++20] [Modules] ADL does not work properly for dependent expressions

[davidstone]

Given the following two valid files:

module;

namespace n {

struct s { };

void operator+(s, int) {
}

} // namespace n

export module a;

export template<typename T>
void a(T x) {
	n::s() + x;
}

module;

export module b;

import a;

void b() {
	a(0);
}

When compiled with

clang++ -std=c++20 -x c++-module a.cpp --precompile -o a.pcm
clang++ -std=c++20 -x c++-module b.cpp --precompile -o b.pcm -fmodule-file=a.pcm

causes Clang to fail with

In file included from b.cpp:1:
a.cpp:16:9: error: invalid operands to binary expression ('n::s' and 'int')
        n::s() + x;
        ~~~~~~ ^ ~
b.cpp:8:2: note: in instantiation of function template specialization 'a<int>' requested here
        a(0);
        ^
1 error generated.

In practice, this comes up with code like std::string() + some_custom_to_string(my_type). Calling code needs to #include <string> in their module, even though they know nothing about std::string in the called function.

[llvmbot]

@llvm/issue-subscribers-clang-modules

[ChuanqiXu9]

This looks like a bug indeed. BTW, as a workaround, you can export the operator +() in practice. For example:

module;

namespace n {

struct s { };

void operator+(s, int) {
}

} // namespace n

export module a;

export template<typename T>
void a(T x) {
	n::s() + x;
}

namespace n {
     export using n::operator+;
}

[davidstone]

Here's a second test case that might be the same problem, might be a different problem. In this example, the dependent expression occurs slightly differently. It looks like this needs the global module fragment to trigger:

export module a;

export template<typename T>
void a(T x) {
	+x;
}

module;

struct s {
};
void operator+(s) {
}

export module b;

import a;

export template<typename T>
void b() {
	a(s());
}

export module c;

import b;

void c() {
	b<int>();
}

when compiled with

clang++ -std=c++20 -x c++-module a.cpp --precompile -o a.pcm
clang++ -std=c++20 -x c++-module b.cpp --precompile -o b.pcm -fmodule-file=a.pcm
clang++ -std=c++20 -x c++-module c.cpp --precompile -o c.pcm -fmodule-file=b.pcm

fails with

In module 'b':
a.cpp:5:2: error: invalid argument type 's' to unary expression
        +x;
        ^~
b.cpp:14:2: note: in instantiation of function template specialization 'a<s>' requested here
        a(s());
        ^
c.cpp:6:2: note: in instantiation of function template specialization 'b<int>' requested here
        b<int>();
        ^
1 error generated.

This is reduced from comparing std::string::iterator.

[ChuanqiXu9]

Update: the corresponding of the wording is http://eel.is/c++draft/module.context#3. We didn't implement modules' Instantiation context before. So here is the problem.