[C++][concepts] error implicit instantiation of undefined template reported from checking constraint satisfaction

[thbwd]

#include <string>

template <template <typename> class SB>
concept Trait = requires(SB<int> a) { a.holder0; };

template <typename T> void set_property(T);

template <template <typename> class SB, typename ST>
void set_property(SB<ST> *) requires Trait<SB>;

void initialize(std::string *name) { set_property(name); }

Produces the following output:

% /opt/homebrew/Cellar/llvm/19.1.7_1/bin/clang test.cpp -std=c++20
In file included from test.cpp:1:
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/string:820:42: error: implicit instantiation of undefined template 'std::char_traits<int>'
  820 |   static_assert(is_same<_CharT, typename traits_type::char_type>::value,
      |                                          ^
test.cpp:4:40: note: in instantiation of template class 'std::basic_string<int>' requested here
    4 | concept Trait = requires(SB<int> a) { a.holder0; };
      |                                        ^
test.cpp:4:39: note: in instantiation of requirement here
    4 | concept Trait = requires(SB<int> a) { a.holder0; };
      |                                       ^~~~~~~~~
test.cpp:4:17: note: while substituting template arguments into constraint expression here
    4 | concept Trait = requires(SB<int> a) { a.holder0; };
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.cpp:9:38: note: while checking the satisfaction of concept 'Trait<std::basic_string>' requested here
    9 | void set_property(SB<ST> *) requires Trait<SB>;
      |                                      ^~~~~~~~~
test.cpp:9:38: note: while substituting template arguments into constraint expression here
    9 | void set_property(SB<ST> *) requires Trait<SB>;
      |                                      ^~~~~~~~~
test.cpp:11:38: note: while checking constraint satisfaction for template 'set_property<std::basic_string, char>' required here
   11 | void initialize(std::string *name) { set_property(name); }
      |                                      ^~~~~~~~~~~~
test.cpp:11:38: note: in instantiation of function template specialization 'set_property<std::basic_string, char>' requested here
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__fwd/string.h:23:29: note: template is declared here
   23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
      |                             ^
1 error generated.

This compiles fine with 18.1.8.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (thbwd)

``` #include <string>

template <template <typename> class SB>
concept Trait = requires(SB<int> a) { a.holder0; };

template <typename T> void set_property(T);

template <template <typename> class SB, typename ST>
void set_property(SB<ST> *) requires Trait<SB>;

void initialize(std::string *name) { set_property(name); }

Produces the following output:

% /opt/homebrew/Cellar/llvm/19.1.7_1/bin/clang test.cpp -std=c++20
In file included from test.cpp:1:
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/string:820:42: error: implicit instantiation of undefined template 'std::char_traits<int>'
820 | static_assert(is_same<_CharT, typename traits_type::char_type>::value,
| ^
test.cpp:4:40: note: in instantiation of template class 'std::basic_string<int>' requested here
4 | concept Trait = requires(SB<int> a) { a.holder0; };
| ^
test.cpp:4:39: note: in instantiation of requirement here
4 | concept Trait = requires(SB<int> a) { a.holder0; };
| ^~~~~~~~~
test.cpp:4:17: note: while substituting template arguments into constraint expression here
4 | concept Trait = requires(SB<int> a) { a.holder0; };
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.cpp:9:38: note: while checking the satisfaction of concept 'Trait<std::basic_string>' requested here
9 | void set_property(SB<ST> *) requires Trait<SB>;
| ^~~~~~~~~
test.cpp:9:38: note: while substituting template arguments into constraint expression here
9 | void set_property(SB<ST> *) requires Trait<SB>;
| ^~~~~~~~~
test.cpp:11:38: note: while checking constraint satisfaction for template 'set_property<std::basic_string, char>' required here
11 | void initialize(std::string *name) { set_property(name); }
| ^~~~~~~~~~~~
test.cpp:11:38: note: in instantiation of function template specialization 'set_property<std::basic_string, char>' requested here
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__fwd/string.h:23:29: note: template is declared here
23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
| ^
1 error generated.

This compiles fine with 18.1.8.
</details>

[zyn0217]

Our template template parameter expert @mizvekov

[mizvekov]

I can't reproduce this on compiler explorer.

Your example, as you posted it, works on every compiler, including past versions of clang.

They all pick the unconstrained version of set_property. You can see this if you delete that overload: https://compiler-explorer.com/z/3ahfjGn5d

Remove that delete, and they all work: https://compiler-explorer.com/z/zbzY8M43c

[thbwd]

@mizvekov Hm, that's very odd. I can reliably reproduce this locally with exactly the code and command I posted. I will see if I can reproduce this on another Mac.

[thbwd]

@mizvekov I can also reproduce this on a secondary Mac just by running brew install llvm and pasting the code into test.cpp and running clang on it. Could this be OS specific? Or a configuration issue?

[mizvekov]

Yep, I confirm this on a MacOS machine as well.

[cor3ntin]

https://compiler-explorer.com/z/8aY4P98Mz reproducing requires libc++ (which is the default on mac)

[mizvekov]

It's not a problem to do with template template parameters either: https://compiler-explorer.com/z/6EdqrT6av

[cor3ntin]

And this boils down to https://compiler-explorer.com/z/KP1vs1ceM - which i think we are correct to reject per
https://eel.is/c++draft/char.traits#specializations.general-1 @ldionne @philnik777

[mizvekov]

Yeah, and the error doesn't happen in the immediate context either, so I don't think this should be confined to the concept.

[philnik777]

Yes, instantiating basic_string<int> should be rejected. We didn't have a default implementation of char_traits since LLVM 16 though, so I'm not sure whether this is actually related. If incomplete types should be rejected here, then it's correct. That's not for me to answer though.