[C++20] [Modules] We don't merge deduction guide correctly

[ChuanqiXu9]

Reproducer:

// foo.h
template <typename T>
class Templ {
public:
    Templ(T a) {}
};

// A.cppm
module;
#include "foo.h"
export module M:A;
export using ::Templ;

// B.cppm
module;
#include "foo.h"
export module M:B;

// M.cppm
export module M;
export import :A;
export import :B;

// Use.cpp
import M;

void func() {
    Templ t(5);
}

Compile it by:

clang++ -std=c++20 --precompile A.cppm -o M-A.pcm
clang++ -std=c++20 --precompile B.cppm -o M-B.pcm
clang++ -std=c++20 --precompile M.cppm -fprebuilt-module-path=.  -o M.pcm
clang++ -std=c++20 Use.cpp -fprebuilt-module-path=. -fsyntax-only

then we'll meet:

Use.cpp:4:11: error: ambiguous deduction for template arguments of 'Templ'
    Templ t(5);
          ^
/home/chuanqi.xcq/workspace.xuchuanqi/llvm-project-for-work/build/modules_deduction_guide/foo.h:4:5: note: candidate function [with T = int]
    Templ(T a) {}
    ^
/home/chuanqi.xcq/workspace.xuchuanqi/llvm-project-for-work/build/modules_deduction_guide/foo.h:4:5: note: candidate function [with T = int]

The error message is clearly wrong.

A workaround could be to add a user-defined deduction guide:

// foo.h
template <typename T>
class Templ {
public:
    Templ(T a) {}
};

template<typename T>
Templ(T t) -> Templ<T>;

[llvmbot]

@llvm/issue-subscribers-clang-modules

[aaronmondal]

This manifests in libcxx/include/__iterator/back_insert_iterator.h. Applying the proposed workaround appears to temporarily fix the issue.

diff --git a/libcxx/include/__iterator/back_insert_iterator.h b/libcxx/include/__iterator/back_insert_iterator.h
index da58b860e3..9aec7aa7cf 100644
--- a/libcxx/include/__iterator/back_insert_iterator.h
+++ b/libcxx/include/__iterator/back_insert_iterator.h
@@ -67,6 +67,9 @@ back_inserter(_Container& __x)
     return back_insert_iterator<_Container>(__x);
 }

+template <class _Container> _LIBCPP_HIDE_FROM_ABI
+back_insert_iterator(_Container& __x) -> back_insert_iterator<_Container>;
+
 _LIBCPP_END_NAMESPACE_STD

 #endif // _LIBCPP___ITERATOR_BACK_INSERT_ITERATOR_H

[ChuanqiXu9]

Yeah, it looks like the compiler can't do a good job for implicit deduction guide now. But due to the deduction guide is mainly grammar sugar for me. So the priority may be relatively low now..