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From: "cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/101906] Constant evaluation failure in concepts
Date: Mon, 12 Sep 2022 21:06:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101906

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:c3ba0eaaa223f7b8208d279e3f39ff134912f9e9

commit r13-2633-gc3ba0eaaa223f7b8208d279e3f39ff134912f9e9
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Jun 7 14:19:53 2022 -0400

    c++: template-id arguments are evaluated [PR101906]

    Here we're neglecting to clear cp_unevaluated_operand when substituting
    into the arguments of the alias template-id 'skip<(T(), 0), T>' with T=
=3DA,
    which means cp_unevaluated_operand remains set during mark_used for
    A::A() and so we don't synthesize it.  Later constant evaluation for
    the substituted template argument '(A(), 0)' (from coerce_template_parm=
s)
    fails with "'constexpr A::A()' used before its definition" since it was
    never synthesized.

    This doesn't happen with a class template because tsubst_aggr_type
    clears cp_unevaluated_operand during substitution thereof.  But since
    template arguments are generally manifestly constant-evaluated, which in
    turn are evaluated even in an unevaluated operand, we should be clearing
    cp_unevaluated_operand more broadly whenever substituting into any set
    of template arguments.  To that end this patch makes us clear it during
    tsubst_template_args.

            PR c++/101906

    gcc/cp/ChangeLog:

            * pt.cc (tsubst_template_args): Set cp_evaluated here.
            (tsubst_aggr_type): Not here.

    gcc/testsuite/ChangeLog:

            * g++.dg/template/evaluated1.C: New test.
            * g++.dg/template/evaluated1a.C: New test.
            * g++.dg/template/evaluated1b.C: New test.
            * g++.dg/template/evaluated1c.C: New test.=