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This solves a problem when using libstdc++ with Clang, due to Clang more eagerly instantiating constexpr function templates during argument deduction. G++ has some shortcuts to avoid this problem, but Clang doesn't, and it's not clear that it's strictly speaking a bug in Clang or if it's following the standard. By making these constructors explicit we stop them being considered by overload resolution for copying these functors, which stops us ending up back in the std::function SFINAE checks. I'm also using _GLIBCXX_MOVE to turn some internal copies into moves, because otherwise using something like std::function with <algorithm> results in a number of potentially expensive copies. PR libstdc++/78991 * include/bits/predefined_ops.h (_Iter_comp_iter, _Iter_comp_val) (_Val_comp_iter, _Iter_equals_val, _Iter_pred, _Iter_comp_to_val) (_Iter_comp_to_iter, _Iter_negate): Make constructors explicit and move function objects. (__iter_comp_iter, __iter_comp_val, __val_comp_iter, __pred_iter) (__iter_comp_val, __iter_comp_iter, __negate): Move function objects. * testsuite/25_algorithms/sort/78991.cc: New test. Tested powerpc64le-linux, committed to trunk. I'll backport the 'explicit' constructors (but not the _GLIBCXX_MOVE changes) to the branches too.
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