[Bug c++/103403] [11/12 Regression] auto return type with a trailing return type of decl(auto) uses rvalue reference type instead of reference type

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103403

--- 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:abd7712f91c99690f8b0046ea168b2782afbac69

commit r12-5778-gabd7712f91c99690f8b0046ea168b2782afbac69
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Nov 30 21:07:25 2021 -0500

    c++: Fix for decltype(auto) and parenthesized expr [PR103403]

    In r11-4758, I tried to fix this problem:

      int &&i = 0;
      decltype(auto) j = i; // should behave like int &&j = i; error

    wherein do_auto_deduction was getting confused with a REFERENCE_REF_P
    and it didn't realize its operand was a name, not an expression, and
    deduced the wrong type.

    Unfortunately that fix broke this:

      int&& r = 1;
      decltype(auto) rr = (r);

    where 'rr' should be 'int &' since '(r)' is an expression, not a name.  But
    because I stripped the INDIRECT_REF with the r11-4758 change, we deduced
    'rr's type as if decltype had gotten a name, resulting in 'int &&'.

    I suspect I thought that the REF_PARENTHESIZED_P check when setting
    'bool id' in do_auto_deduction would handle the (r) case, but that's not
    the case; while the documentation for REF_PARENTHESIZED_P specifically says
    it can be set in INDIRECT_REF, we don't actually do so.

    This patch sets REF_PARENTHESIZED_P even on REFERENCE_REF_P, so that
    do_auto_deduction can use it.

    It also removes code in maybe_undo_parenthesized_ref that I think is
    dead -- and we don't hit it while running dg.exp.  To adduce more data,
    it also looks dead here:
    https://splichal.eu/lcov/gcc/cp/semantics.c.gcov.html
    (It's dead since r9-1417.)

    Also add a fixed test for c++/81176.

            PR c++/103403

    gcc/cp/ChangeLog:

            * cp-gimplify.c (cp_fold): Don't recurse if
maybe_undo_parenthesized_ref
            doesn't change its argument.
            * pt.c (do_auto_deduction): Don't strip REFERENCE_REF_P trees if
they
            are REF_PARENTHESIZED_P.  Use stripped_init when checking for
            id-expression.
            * semantics.c (force_paren_expr): Set REF_PARENTHESIZED_P on
            REFERENCE_REF_P trees too.
            (maybe_undo_parenthesized_ref): Remove dead code.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/decltype-auto2.C: New test.
            * g++.dg/cpp1y/decltype-auto3.C: New test.
            * g++.dg/cpp1y/decltype-auto4.C: New test.
            * g++.dg/cpp1z/decomp-decltype1.C: New test.