[Modules] "Cannot befriend target of using declaration"

[davidstone]

Given the following two translation units

module;

namespace n
{

template<typename>
struct a {
	template<typename T>
	friend void aa(a<T>);
};

template<typename T>
inline void aa(a<T>) {
}


} //namespace n

export module a;

namespace n {

export using n::a;
export using n::aa;

}

import a;

void b() {
	aa(n::a<int>());
}

And compiling with

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

Causes clang to error with

In file included from b.cpp:1:
a.cpp:9:14: error: cannot befriend target of using declaration
        friend void aa(a<T>);
                    ^
b.cpp:4:5: note: in instantiation of template class 'n::a<int>' requested here
        aa(n::a<int>());
           ^
a.cpp:13:13: note: target of using declaration
inline void aa(a<T>) {
            ^
a.cpp:24:17: note: using declaration
export using n::aa;
                ^
1 error generated.

I believe this code should be accepted, but I'm not 100% sure.

[llvmbot]

@llvm/issue-subscribers-clang-modules

[llvmbot]

@llvm/issue-subscribers-c-1

Author: David Stone (davidstone)

Given the following two translation units

module;

namespace n
{

template<typename>
struct a {
	template<typename T>
	friend void aa(a<T>);
};

template<typename T>
inline void aa(a<T>) {
}


} //namespace n

export module a;

namespace n {

export using n::a;
export using n::aa;

}

import a;

void b() {
	aa(n::a<int>());
}

And compiling with

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

Causes clang to error with

In file included from b.cpp:1:
a.cpp:9:14: error: cannot befriend target of using declaration
        friend void aa(a<T>);
                    ^
b.cpp:4:5: note: in instantiation of template class 'n::a<int>' requested here
        aa(n::a<int>());
           ^
a.cpp:13:13: note: target of using declaration
inline void aa(a<T>) {
            ^
a.cpp:24:17: note: using declaration
export using n::aa;
                ^
1 error generated.

I believe this code should be accepted, but I'm not 100% sure.

[ChuanqiXu9]

@davidstone @cor3ntin @AaronBallman @shafik I am not sure about the root reason for the error. (which seems not related to modules) So I tried to CC other guys.

What's the purpose of cannot befriend target of using declaration? I can't find it in https://eel.is/c++draft/namespace.udecl#14 and https://eel.is/c++draft/class.friend ? Is this an oversight?

[AaronBallman]

Those changes came in from c70fca60dab47 back in 2013. The explanation in https://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20130325/076977.html is that [dcl.meaning]p1 is the reason:
C++11 [dcl.meaning]p1:

... When the declarator-id is qualified, the declaration shall refer to a previously declared member of the class or namespace to which the qualifier refers (or, in the case of a namespace, of an element of the inline namespace set of that namespace (7.3.1)) or to a specialization thereof; the member shall not merely have been introduced by a using-declaration in the scope of the class or namespace nominated by the nested-name-specifier of the declarator-id. ...

Those words are no longer present, I think we hit this case instead: https://eel.is/c++draft/dcl.meaning#general-2.3. I didn't track down what changed the wording, but that wording is present in C++20, but not C++23. If I had to hazard a guess, I would guess the words changed thanks to "declarations and where to find them" (https://wg21.link/P1787) which @Endilll is quite familiar with.

[ChuanqiXu9]

Thanks!

I took another look and now I feel this is more or less a mismatch.

The code is:

llvm-project/clang/lib/Sema/SemaDecl.cpp

Lines 3600 to 3609 in f69e267

	if (!Old) {
	if (UsingShadowDecl *Shadow = dyn_cast<UsingShadowDecl>(OldD)) {
	if (New->getFriendObjectKind()) {
	Diag(New->getLocation(), diag::err_using_decl_friend);
	Diag(Shadow->getTargetDecl()->getLocation(),
	diag::note_using_decl_target);
	Diag(Shadow->getIntroducer()->getLocation(), diag::note_using_decl)
	<< 0;
	return true;
	}

Here the story is, during the compilation of b.cpp, n::aa is looked up, but the templated function decl n::aa is not exported so we can't find it. We find the using decl n::aa. So we find the using decl n::aa before the actual function decl n::aa. Then we saw the friend decl, and we matched the above diagnostics.

This is the reason why it won't happen in normal codes, since in normal codes, in the reproducer, we will always see the actual function decl first and later the using decl.

Let me see how can we fix this.

[ChuanqiXu9]

Oh, when I try to work on this, I surprisingly find the reproducer is accepted now... I commit it directly to avoid regression at 2d81994

Feel free to ask for reopening this if I observe anything wrong.

[bangerth]

@ChuanqiXu9 Does whatever change cause this issue to be fixed also fix #138558?

[ChuanqiXu9]

@ChuanqiXu9 Does whatever change cause this issue to be fixed also fix #138558?

I didn't search the commit that makes this issue closed. But #138558 is different than the current one. I feel from what @AaronBallman said, the friend declaration won't be looked up. Since in the current issue, there is a declaration n::aa in the namespace (out of the friend declaration), but in your example, the friend declaration is defined inside the class. This is the difference.

We can see it clearly from the non modular example: https://godbolt.org/z/45jsr61WM

So... we can't fix it since it is not an "issue"

[bangerth]

Let me move the discussion to #138558.