[Bug c++/103403] New: Decltype of rvalue reference

[Bug c++/103403] New: Decltype of rvalue reference

[enricomaria.dean6elis at gmail dot com]

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

            Bug ID: 103403
           Summary: Decltype of rvalue reference
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: enricomaria.dean6elis at gmail dot com
                CC: anders.granlund.0 at gmail dot com,
                    daniel.kruegler at googlemail dot com, jason at gcc dot gnu.org,
                    mpolacek at gcc dot gnu.org, trippels at gcc dot gnu.org,
                    webrown.cpp at gmail dot com
  Target Milestone: ---

Unlike the comment at the end of the original report claims, the bug seems to
be still present (or at least a different shade of it).

StackOverflow question:
https://stackoverflow.com/questions/70092776/validity-of-presenting-an-xvalue-as-an-lvalue
Repro: https://godbolt.org/z/h943G933a


+++ This bug was initially created as a clone of Bug #78209 +++

This came up on the isocpp mailing list yesterday:

markus@x4 /tmp % cat test.ii
int main() {
  int &&i = 0;
  decltype(auto) j = i;
  return j;
}

markus@x4 /tmp % clang++ test.ii
test.ii:3:18: error: rvalue reference to type 'int' cannot bind to lvalue of
type 'int'
  decltype(auto) j = i;
                 ^   ~
1 error generated.

markus@x4 /tmp % icpc test.ii
test.ii(3): error: an rvalue reference cannot be bound to an lvalue
    decltype(auto) j = i;
                       ^
compilation aborted for test.ii (code 2)

markus@x4 /tmp % g++ -Wall -Wextra test.ii
markus@x4 /tmp %

[Bug c++/103403] Decltype of rvalue reference

[pinskia at gcc dot gnu.org]

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is a totally different issue and unrelated issue from PR 78209

[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

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |11.3
     Ever confirmed|0                           |1
      Known to fail|                            |11.1.0
            Summary|Decltype of rvalue          |[11/12 Regression] auto
                   |reference                   |return type with a trailing
                   |                            |return type of decl(auto)
                   |                            |uses rvalue reference type
                   |                            |instead of reference type
             Status|UNCONFIRMED                 |NEW
      Known to work|                            |10.1.0, 10.3.0, 7.1.0
   Last reconfirmed|                            |2021-11-24

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Here is a C++14 testcase which shows this is also a regression:
template<typename T>
auto constexpr RtoL1(T&& r) -> decltype(auto) {
    return (r);
};
int main() {
    int t;
    int x{3};
    decltype (RtoL1(x+0)) y = t;
}

Confirmed.

[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

[rguenth at gcc dot gnu.org]

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

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[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

[mpolacek at gcc dot gnu.org]

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

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Started with my r11-4758, sigh.

[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

[enricomaria.dean6elis at gmail dot com]

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

--- Comment #4 from Enrico Maria De Angelis <enricomaria.dean6elis at gmail dot com> ---
So there is a relation between this problem and PR 78209. At least a
cause-effect relation.

[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]

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.

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

[mpolacek at gcc dot gnu.org]

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

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[11/12 Regression] auto     |[11 Regression] auto return
                   |return type with a trailing |type with a trailing return
                   |return type of decl(auto)   |type of decl(auto) uses
                   |uses rvalue reference type  |rvalue reference type
                   |instead of reference type   |instead of reference type

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed on trunk so far.

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

[rguenth at gcc dot gnu.org]

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

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.3                        |11.4

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 11.3 is being released, retargeting bugs to GCC 11.4.

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

[mpolacek at gcc dot gnu.org]

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

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.