[Bug libstdc++/126800] New: Heterogeneous insertion into unordered_map doesn't work.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126800

            Bug ID: 126800
           Summary: Heterogeneous insertion into unordered_map doesn't
                    work.
           Product: gcc
           Version: 16.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xbenes3 at fi dot muni.cz
  Target Milestone: ---

Created attachment 65302
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=65302&action=edit
the preprocessed file (*.i*) that triggers the bug

#include <print>
#include <unordered_map>

struct other {};

struct key {
        key() = default;
        key(other) { std::println("key constructed from other"); }
};

struct hash {
        using is_transparent = void;
        auto operator()(auto&&) const noexcept { return 0zu; }
};

struct equal {
        using is_transparent = void;
        auto operator()(auto&&, auto&&) const noexcept { return true; }
};

int main() {
        std::unordered_map<key, int, hash, equal> m{ { key{}, 0 } };
        m.try_emplace(other{}, 0);  // key constructed from other
        m[other{}] = 1;             // key constructed from other
}

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/16/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc/configure
--enable-languages=ada,c,c++,d,fortran,go,lto,m2,objc,obj-c++,rust,cobol
--enable-bootstrap --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://gitlab.archlinux.org/archlinux/packaging/packages/gcc/-/issues
--with-build-config=bootstrap-lto --with-gcc-major-version-only
--with-linker-hash-style=gnu --with-system-zlib --enable-cet=auto
--enable-checking=release --enable-clocale=gnu --enable-default-pie
--enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object
--enable-libstdcxx-backtrace --enable-link-serialization=1
--enable-linker-build-id --enable-lto --enable-multilib --enable-plugin
--enable-shared --enable-threads=posix --disable-fixincludes --disable-libssp
--disable-libstdcxx-pch --disable-werror
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 16.2.1 20260810 (GCC) 

$ g++ -std=c++26 hlookup.cpp
$ ./a.out
key constructed from other
key constructed from other

The expected result was no output at all, because both try_emplace and
operator[] should be heterogeneous (both the hash and the equality function
objects are transparent).

The problem might be in /usr/include/c++/16/bits/hashtable.h on line 1110
(inside try_emplace(const_iterator, _KType&& __k, _Args&&... __args)), where
the code calls _M_locate(__k), but _M_locate only takes const key_type& (the
heterogeneous version is _M_locate_tr).