[Bug libstdc++/94033] [10 Regression] is_trivially_copy_constructible<> fails with compiler error on complicated object with private default constructor
cvs-commit at gcc dot gnu.org
gcc-bugzilla@gcc.gnu.org
Fri Sep 11 10:53:02 GMT 2020
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94033
--- Comment #21 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-8 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:
https://gcc.gnu.org/g:1521433e2b6bff583b227fe391fe9dcf02f6000e
commit r8-10445-g1521433e2b6bff583b227fe391fe9dcf02f6000e
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Wed Mar 18 23:19:12 2020 +0000
libstdc++: Fix is_trivially_constructible (PR 94033)
This attempts to make is_nothrow_constructible more robust (and
efficient to compile) by not depending on is_constructible. Instead the
__is_constructible intrinsic is used directly. The helper class
__is_nt_constructible_impl which checks whether the construction is
non-throwing now takes a bool template parameter that is substituted by
the result of the instrinsic. This fixes the reported bug by not using
the already-instantiated (and incorrect) value of std::is_constructible.
I don't think it really fixes the problem in general, because
std::is_nothrow_constructible itself could already have been
instantiated in a context where it gives the wrong result. A proper fix
needs to be done in the compiler.
Backported to the gcc-8 and gcc-9 branches to fix PR 96999.
PR libstdc++/94033
* include/std/type_traits (__is_nt_default_constructible_atom):
Remove.
(__is_nt_default_constructible_impl): Remove.
(__is_nothrow_default_constructible_impl): Remove.
(__is_nt_constructible_impl): Add bool template parameter. Adjust
partial specializations.
(__is_nothrow_constructible_impl): Replace class template with
alias
template.
(is_nothrow_default_constructible): Derive from alias template
__is_nothrow_constructible_impl instead of
__is_nothrow_default_constructible_impl.
* testsuite/20_util/is_nothrow_constructible/94003.cc: New test.
(cherry picked from commit b3341826531e80e02f194460b4fbe1b0541c0463)
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