[Bug c++/89062] class template argument deduction failure with parentheses

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89062

--- Comment #9 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:6186708312780bb2139da01946abdde39667e985

commit r12-2859-g6186708312780bb2139da01946abdde39667e985
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Aug 11 15:58:30 2021 -0400

    c++: most vexing parse and braced CTAD [PR89062]

    Here grokdeclarator is emitting the error

      error: class template placeholder âFooâ not permitted in this context

    during the tentative (and ultimately futile) parse of 'x' as a function
    declaration.  This happens because when parsing 'Foo{1}',
    cp_parser_parameter_declaration yields a parameter declaration with no
    declarator and whose type is a CTAD placeholder, and stops short of
    consuming the '{'.  The caller cp_parser_parameter_declaration_list then
    calls grokdeclarator on this declarator, hence the error, and soon
    thereafter we abort this tentative parse since the next token '{'
    doesn't make sense in the context of a parameter list.

    Note that we don't have this issue with parenthesized CTAD

      Foo<int> x(Foo(1));

    because in this case cp_parser_direct_declarator (called indirectly from
    c_p_p_d) consumes the '(' and returns cp_error_declarator instead of a
    NULL declarator (and also simulates a parse error), and grokdeclarator
    exits early for this declarator without emitting any error.

    Since grokdeclarator doesn't take a 'complain' parameter, to fix this we
    need to avoid calling grokdeclarator in this situation.  To that end
    this patch makes c_p_p_d simulate an error when a construct is a CTAD
    expression and definitely not a parameter declaration, so that c_p_p_d_l
    can avoid calling grokdeclarator by checking for this simulated error.
    Alternatively we could keep all this logic inside c_p_p_d_l and not
    touch c_p_p_d at all, but this approach seems slightly less adhoc.

            PR c++/89062

    gcc/cp/ChangeLog:

            * parser.c (cp_parser_parameter_declaration_list): Don't call
            grokdeclarator if cp_parser_error_occurred.
            (cp_parser_parameter_declaration): Simulate an error if we see
            the beginning of a CTAD form, i.e. if we see an opening brace
            after the decl-specifier-seq and the type is a CTAD placeholder.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1z/class-deduction97.C: New test.