[Bug c++/100368] New: Missing guaranteed elision in constexpr evaluation

[Bug c++/100368] New: Missing guaranteed elision in constexpr evaluation

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100368

            Bug ID: 100368
           Summary: Missing guaranteed elision in constexpr evaluation
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

struct S {
  S() = default;
  S(const S&) = delete;
};

struct array {
  S s[2];
};

struct PS {
  constexpr array operator*() const { return {}; }
};

struct W {
  constexpr W(const PS& p)   // 1
    : t(*p) {}
  array t;
};

W w(PS{});

This issues a spurious complaint on trunk and then ICEs:

source>: In constructor 'constexpr W::W(const PS&)':
<source>:17:14: error: use of deleted function 'array::array(array&&)'
   17 |     : t(*p) {}
      |              ^
<source>:7:8: note: 'array::array(array&&)' is implicitly deleted because the
default definition would be ill-formed:
    7 | struct array {
      |        ^~~~~
<source>:7:8: error: use of deleted function 'S::S(S&&)'
<source>:4:3: note: declared here
    4 |   S(S&&) = delete;
      |   ^
<source>:17:14: internal compiler error: tree check: expected target_expr, have
error_mark in bot_manip, at cp/tree.c:3095
   17 |     : t(*p) {}
      |              ^
0x1d002c9 internal_error(char const*, ...)
        ???:0
0x67b4e3 tree_check_failed(tree_node const*, char const*, int, char const*,
...)
        ???:0
0x13967e3 walk_tree_1(tree_node**, tree_node* (*)(tree_node**, int*, void*),
void*, hash_set<tree_node*, false, default_hash_traits<tree_node*> >*,
tree_node* (*)(tree_node**, int*, tree_node* (*)(tree_node**, int*, void*),
void*, hash_set<tree_node*, false, default_hash_traits<tree_node*> >*))
        ???:0
0x9ad0ae break_out_target_exprs(tree_node*, bool)
        ???:0
0x72aa0a maybe_save_constexpr_fundef(tree_node*)
        ???:0
0x7b0711 finish_function(bool)
        ???:0
0x8e2c4d c_parse_file()
        ???:0
0xa63232 c_common_parse_file()
        ???:0
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.

Removing the constexpr on the line marked #1 makes it compile. Both clang and
MSVC accept the original code.

[Bug c++/100368] Missing guaranteed elision in constexpr evaluation

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100368

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
                 CC|                            |ppalka at gcc dot gnu.org
   Last reconfirmed|                            |2021-05-03

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed, looks like this never worked.

[Bug c++/100368] Missing guaranteed elision in constexpr evaluation

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100368

Patrick Palka <ppalka at gcc dot gnu.org> changed:

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

[Bug c++/100368] Missing guaranteed elision in constexpr evaluation

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100368

--- Comment #2 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:88834c7d05acf5ce4eaccda56fb04436595e2a52

commit r12-1068-g88834c7d05acf5ce4eaccda56fb04436595e2a52
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed May 26 08:37:30 2021 -0400

    c++: constexpr and copy elision within mem init [PR100368]

    In the testcase below, the member initializer b(f()) inside C's default
    constructor is encoded as a TARGET_EXPR wrapping the CALL_EXPR f() in
    C++17 mode.  During massaging of this constexpr constructor,
    build_target_expr_with_type called from bot_manip on this initializer
    tries to add an extra copy using B's implicitly deleted copy constructor
    rather than just preserving the copy elision.

    Since it's wrong to introduce an extra copy when initializing a
    temporary from a CALL_EXPR, this patch makes build_target_expr_with_type
    avoid calling force_rvalue in this case.  Additionally, bot_manip should
    be copying TARGET_EXPRs in a more oblivious manner, so this patch makes
    bot_manip use force_target_expr instead of build_target_expr_with_type.
    And since bot_manip is now no longer a caller, we can remove the void
    initializer handling in build_target_expr_with_type.

            PR c++/100368

    gcc/cp/ChangeLog:

            * tree.c (build_target_expr_with_type): Don't call force_rvalue
            on CALL_EXPR initializer.  Simplify now that bot_manip is no
            longer a caller.
            (bot_manip): Use force_target_expr instead of
            build_target_expr_with_type.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1z/elide6.C: New test.

[Bug c++/100368] Missing guaranteed elision in constexpr evaluation

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100368

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.0
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 12.