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[PATCH] PR78991 make __gnu_cxx::__ops constructors explicit


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.

Attachment: patch.txt
Description: Text document


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