Error assigning result of std::bind_front() to std::function with libstdc++ 14 headers

[HighCommander4]

Steps to reproduce

Given the following code:

#include <functional>

struct Logger {
    void log(int answer) const;
};

int main() {
    Logger l;
    std::function<void(int)> fn = std::bind_front(&Logger::log, &l);
}

compile with recent clang against libstdc++ 14 headers (depending on the system, that may require adding --gcc-toolchain=/path/to/gcc-14-installation to the clang command):

$ clang-22 -c -std=c++23 test.cpp

Expected results

Compiles successfully.

This is what happens when compiling with gcc 14, and also when compiling with clang using other libstdc++ versions (at least I tested with 12 and 15 and they work fine).

Actual results

$ clang-22 -c -std=c++23 test.cpp
test.cpp:9:30: error: no viable conversion from '_Bind_front_t<void (Logger::*)(int) const, Logger *>' (aka '_Bind_front<void (Logger::*)(int) const, Logger *>') to
      'std::function<void (int)>'
    9 |     std::function<void(int)> fn = std::bind_front(&Logger::log, &l);
      |                              ^    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
gcc-14.2/lib/gcc/x86_64-pc-linux-gnu/14.2.0/../../../../include/c++/14.2.0/bits/std_function.h:375:7: note: candidate constructor not viable:
      no known conversion from '_Bind_front_t<void (Logger::*)(int) const, Logger *>' (aka '_Bind_front<void (Logger::*)(int) const, Logger *>') to 'nullptr_t' (aka 'std::nullptr_t') for
      1st argument
  375 |       function(nullptr_t) noexcept
      |       ^        ~~~~~~~~~
gcc-14.2/lib/gcc/x86_64-pc-linux-gnu/14.2.0/../../../../include/c++/14.2.0/bits/std_function.h:386:7: note: candidate constructor not viable:
      no known conversion from '_Bind_front_t<void (Logger::*)(int) const, Logger *>' (aka '_Bind_front<void (Logger::*)(int) const, Logger *>') to 'const function<void (int)> &' for 1st
      argument
  386 |       function(const function& __x)
      |       ^        ~~~~~~~~~~~~~~~~~~~
gcc-14.2/lib/gcc/x86_64-pc-linux-gnu/14.2.0/../../../../include/c++/14.2.0/bits/std_function.h:404:7: note: candidate constructor not viable:
      no known conversion from '_Bind_front_t<void (Logger::*)(int) const, Logger *>' (aka '_Bind_front<void (Logger::*)(int) const, Logger *>') to 'function<void (int)> &&' for 1st
      argument
  404 |       function(function&& __x) noexcept
      |       ^        ~~~~~~~~~~~~~~
gcc-14.2/lib/gcc/x86_64-pc-linux-gnu/14.2.0/../../../../include/c++/14.2.0/bits/std_function.h:435:2: note: candidate template ignored:
      requirement 'std::function<void (int)>::_Callable<std::_Bind_front<void (Logger::*)(int) const, Logger *>, std::_Bind_front<void (Logger::*)(int) const, Logger *>,
      std::__invoke_result<std::_Bind_front<void (Logger::*)(int) const, Logger *> &, int>>::value' was not satisfied [with _Functor = _Bind_front_t<void (Logger::*)(int) const, Logger *>]
  435 |         function(_Functor&& __f)
      |         ^
1 error generated.

(Originally reported at clangd/clangd#2455.)

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Nathan Ridge (HighCommander4)

**Steps to reproduce**

Given the following code:

#include <functional>

struct Logger {
    void log(int answer) const;
};

int main() {
    Logger l;
    std::function<void(int)> fn = std::bind_front(&Logger::log, &l);
}

compile with recent clang against libstdc++ 14 headers (depending on the system, that may require adding --gcc-toolchain=/path/to/gcc-14-installation to the clang command):

$ clang-22 -c -std=c++23 test.cpp

Expected results

Compiles successfully.

This is what happens when compiling with gcc 14, and also when compiling with clang using other libstdc++ versions (at least I tested with 12 and 15 and they work fine).

Actual results

$ clang-22 -c -std=c++23 test.cpp
test.cpp:9:30: error: no viable conversion from '_Bind_front_t<void (Logger::*)(int) const, Logger *>' (aka '_Bind_front<void (Logger::*)(int) const, Logger *>') to
      'std::function<void (int)>'
    9 |     std::function<void(int)> fn = std::bind_front(&Logger::log, &l);
      |                              ^    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
gcc-14.2/lib/gcc/x86_64-pc-linux-gnu/14.2.0/../../../../include/c++/14.2.0/bits/std_function.h:375:7: note: candidate constructor not viable:
      no known conversion from '_Bind_front_t<void (Logger::*)(int) const, Logger *>' (aka '_Bind_front<void (Logger::*)(int) const, Logger *>') to 'nullptr_t' (aka 'std::nullptr_t') for
      1st argument
  375 |       function(nullptr_t) noexcept
      |       ^        ~~~~~~~~~
gcc-14.2/lib/gcc/x86_64-pc-linux-gnu/14.2.0/../../../../include/c++/14.2.0/bits/std_function.h:386:7: note: candidate constructor not viable:
      no known conversion from '_Bind_front_t<void (Logger::*)(int) const, Logger *>' (aka '_Bind_front<void (Logger::*)(int) const, Logger *>') to 'const function<void (int)> &' for 1st
      argument
  386 |       function(const function& __x)
      |       ^        ~~~~~~~~~~~~~~~~~~~
gcc-14.2/lib/gcc/x86_64-pc-linux-gnu/14.2.0/../../../../include/c++/14.2.0/bits/std_function.h:404:7: note: candidate constructor not viable:
      no known conversion from '_Bind_front_t<void (Logger::*)(int) const, Logger *>' (aka '_Bind_front<void (Logger::*)(int) const, Logger *>') to 'function<void (int)> &&' for 1st
      argument
  404 |       function(function&& __x) noexcept
      |       ^        ~~~~~~~~~~~~~~
gcc-14.2/lib/gcc/x86_64-pc-linux-gnu/14.2.0/../../../../include/c++/14.2.0/bits/std_function.h:435:2: note: candidate template ignored:
      requirement 'std::function<void (int)>::_Callable<std::_Bind_front<void (Logger::*)(int) const, Logger *>, std::_Bind_front<void (Logger::*)(int) const, Logger *>,
      std::__invoke_result<std::_Bind_front<void (Logger::*)(int) const, Logger *> &, int>>::value' was not satisfied [with _Functor = _Bind_front_t<void (Logger::*)(int) const, Logger *>]
  435 |         function(_Functor&& __f)
      |         ^
1 error generated.

(Originally reported at clangd/clangd#2455.)

[frederick-vs-ja]

Looks like that this error doesn't occur in C++20 mode (Godbolt link).

[frederick-vs-ja]

Ah, I see. This is because of that libstdc++ used to implement forward_like with deduced return type, and the internal transformation trait __like_t relies on forward_like, but Clang doesn't support such technique.

When explicit object parameter is supported (__cpp_explicit_this_parameter is defined), libstdc++ implements the operator() with explicit this parameter and relies on __like_t. However, if forward_like is implemented with a deduced return type and __like_t relies on it, Clang can't give correct result type because it treats forward_like as an intrinsic, which is reported in #101614.

libstdc++ changed the implementation strategy later (per gcc-mirror/gcc@8256d5c and gcc-mirror/gcc@140aab2).

So, can we close this as duplicate and suggest users to use libstdc++ 14.3.0 or later?

[HighCommander4]

So, can we close this as duplicate and suggest users to use libstdc++ 14.3.0 or later?

+1

I found #131708, an older report of basically the same issue, which was duped over to #101614.

[xim]

So, can we close this as duplicate and suggest users to use libstdc++ 14.3.0 or later?

Fine for me. Worked around in clangd using

git diff --git a/.clangd b/.clangd
index b84871929dc5..ed2903fcf293 100644
--- a/.clangd
+++ b/.clangd
@@ -1,3 +1,5 @@
+CompileFlags:
+  Add: ['-fno-builtin-std-forward_like']

[HighCommander4]

Thanks all for digging up the previously filed issue and workaround.