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From: "cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/105841] [12/13 Regression] Change in behavior of CTAD for
 alias templates
Date: Thu, 09 Mar 2023 15:27:47 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105841

--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:148cbb153dafd6b21d83c00787acd430aec68a3d

commit r13-6553-g148cbb153dafd6b21d83c00787acd430aec68a3d
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Feb 9 12:51:51 2023 -0800

    c++: add __is_deducible trait [PR105841]

    C++20 class template argument deduction for an alias template involves
    adding a constraint that the template arguments for the alias template =
can
    be deduced from the return type of the deduction guide for the underlyi=
ng
    class template.  In the standard, this is modeled as defining a class
    template with a partial specialization, but it's much more efficient to
    implement with a trait that directly tries to perform the deduction.

    The first argument to the trait is a template rather than a type, so
various
    places needed to be adjusted to accommodate that.

            PR c++/105841

    gcc/ChangeLog:

            * doc/extend.texi (Type Traits):: Document __is_deducible.

    gcc/cp/ChangeLog:

            * cp-trait.def (IS_DEDUCIBLE): New.
            * cxx-pretty-print.cc (pp_cxx_trait): Handle non-type.
            * parser.cc (cp_parser_trait): Likewise.
            * tree.cc (cp_tree_equal): Likewise.
            * pt.cc (tsubst_copy_and_build): Likewise.
            (type_targs_deducible_from): New.
            (alias_ctad_tweaks): Use it.
            * semantics.cc (trait_expr_value): Handle CPTK_IS_DEDUCIBLE.
            (finish_trait_expr): Likewise.
            * constraint.cc (diagnose_trait_expr): Likewise.
            * cp-tree.h (type_targs_deducible_from): Declare.

    gcc/testsuite/ChangeLog:

            * g++.dg/ext/is_deducible1.C: New test.=