[Bug c++/105841] [12/13 Regression] Change in behavior of CTAD for alias templates

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105841

--- 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 underlying
    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.