[Bug c++/64679] Spurious redefinition error when parsing not-quite-most-vexing-parse declarations

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64679

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:ee9128189623ce43343ce326caa8d5eec2eccfec

commit r13-146-gee9128189623ce43343ce326caa8d5eec2eccfec
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue May 3 19:01:19 2022 -0400

    c++: wrong error with MVP and pushdecl [PR64679]

    This patch fixes the second half of 64679.  Here we issue a wrong
    "redefinition of 'int x'" for the following:

      struct Bar {
        Bar(int, int, int);
      };

      int x = 1;
      Bar bar(int(x), int(x), int{x}); // #1

    cp_parser_parameter_declaration_list does pushdecl every time it sees
    a named parameter, so the second "int(x)" causes the error.  That's
    premature, since this turns out to be a constructor call after the
    third argument!

    If the first parameter is parenthesized, we can't push until we've
    established we're looking at a function declaration.  Therefore this
    could be fixed by some kind of lookahead.  I thought about introducing a
    lightweight variant of cp_parser_parameter_declaration_list that would
    not have any side effects and would return as soon as it figures out
    whether it's looking at a declaration or expression.  Since that would
    require fairly nontrivial changes, I wanted something simpler.

    Something like delaying the pushdecl until we've reached the ')'
    following the parameter-declaration-clause.  But we must push the
    parameters before processing a default argument, as in:

      Bar bar(int(a), int(b), int c = sizeof(a));  // valid

    Moreover, this code should still be accepted

      Bar f(int(i), decltype(i) j = 42);

    so this patch stashes parameters into a vector when parsing tentatively
    only when pushdecl-ing a parameter would result in a clash and an error
    about redefinition/redeclaration.  The stashed parameters are pushed at
    the end of a parameter-declaration-clause if it's followed by a ')', so
    that we still diagnose redefining a parameter.

            PR c++/64679

    gcc/cp/ChangeLog:

            * parser.cc (cp_parser_parameter_declaration_clause): Maintain
            a vector of parameters that haven't been pushed yet.  Push them at
the
            end of a valid parameter-declaration-clause.
            (cp_parser_parameter_declaration_list): Take a new auto_vec
parameter.
            Do not pushdecl while parsing tentatively when pushdecl-ing a
parameter
            would result in a hard error.
            (cp_parser_cache_defarg): Adjust the call to
            cp_parser_parameter_declaration_list.

    gcc/testsuite/ChangeLog:

            * g++.dg/parse/ambig11.C: New test.
            * g++.dg/parse/ambig12.C: New test.
            * g++.dg/parse/ambig13.C: New test.
            * g++.dg/parse/ambig14.C: New test.