[Bug c++/92652] function call to lambda expression that return true does not satisfy the constraint in requires-clause if using return type deduction

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92652

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Patrick Palka
<ppa...@gcc.gnu.org>:

https://gcc.gnu.org/g:03c344ad180e094140be514a5e7cbaf95b5dcd2e

commit r10-8214-g03c344ad180e094140be514a5e7cbaf95b5dcd2e
Author: Patrick Palka <ppa...@redhat.com>
Date:   Fri May 29 14:17:02 2020 -0400

    c++: lambdas inside constraints [PR92652]

    When parsing a constraint-expression, a requires-clause or a
    requires-expression, we temporarily increment processing_template_decl
    so that we always obtain template trees which we could later reduce via
    substitution even when not inside a template.

    But incrementing processing_template_decl when we're already inside a
    template has the unintended side effect of shifting up the template
    parameter levels of a lambda defined inside one of these constructs,
    which leads to confusion later during substitution into the lambda.

    This patch fixes this issue by incrementing processing_template_decl
    during parsing of these constructs only if it is 0.

    gcc/cp/ChangeLog:

            PR c++/92652
            PR c++/93698
            PR c++/94128
            * parser.c (cp_parser_requires_clause_expression): Temporarily
            increment processing_template_decl only if it is 0.
            (cp_parser_constraint_expression): Likewise.
            (cp_parser_requires_expression): Likewise.

    gcc/testsuite/ChangeLog:

            PR c++/92652
            PR c++/93698
            PR c++/94128
            * g++.dg/cpp2a/concepts-lambda8.C: New test.
            * g++.dg/cpp2a/concepts-lambda9.C: New test.
            * g++.dg/cpp2a/concepts-lambda10.C: New test.

    (cherry picked from commit 020d86db8896f088435830595640e6fc21bc64ad)