[Bug c++/105550] New: Missing copy elision with conditional operator in NSDMI

[Bug c++/105550] New: Missing copy elision with conditional operator in NSDMI

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105550

            Bug ID: 105550
           Summary: Missing copy elision with conditional operator in
                    NSDMI
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

Consider:

struct A {
  const A* p = this;
};

struct E {
  A a = true ? A{} : A{};
};

constexpr E e{};

$ ./cc1plus -quiet q.C
q.C:9:15: error: ‘E{A{((const A*)(&<anonymous>))}}’ is not a constant
expression
    9 | constexpr E e{};
      |               ^

The ?: shouldn't preclude copy elision.  'e' must be static for this to work.

Bug 96004 with a similar title doesn't seem to be a duplicate of this one.

[Bug c++/105550] Missing copy elision with conditional operator in NSDMI

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105550

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
   Last reconfirmed|                            |2022-05-10
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
             Blocks|                            |100252
     Ever confirmed|0                           |1


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100252
[Bug 100252] [9/10/11/12/13 Regression] Internal compiler error during template
instantiation

[Bug c++/105550] Missing copy elision with conditional operator

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105550

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Missing copy elision with   |Missing copy elision with
                   |conditional operator in     |conditional operator
                   |NSDMI                       |

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
This is not tied to NSDMIs; this one fails the same:

struct A {
  const A* p = this;
};

constexpr A a = true ? A{} : A{};

[Bug c++/105550] Missing copy elision with conditional operator

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105550

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
// PR c++/105550

struct A {
  const A *p = this;
};

struct B {
  const B *p = this;
  constexpr operator A() { return {}; }
};

struct E {
  A a1 = true ? A{} : A{};
  A a2 = true ? A{} : B{};
  A a3 = false ? A{} : B{};
  A a4 = false ? B{} : B{};
  A a5 = A{};
  A a6 = B{};
  A a7 = false ? B{} : (true ? A{} : A{});
  A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
};

constexpr E e{};

constexpr A
foo (A a)
{
  return a;
}

constexpr A
bar (A)
{
  return {};
}

constexpr A baz() { return {}; }

constexpr A a1 = true ? A{} : A{};
constexpr A a2 = true ? A{} : B{};
constexpr A a3 = false ? A{} : B{};
constexpr A a4 = false ? B{} : B{};
constexpr A a5 = A{};
constexpr A a6 = B{};
constexpr A a7 = false ? B{} : (true ? A{} : A{});
constexpr A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
constexpr A a9 = (A{});
constexpr A a10 = (true, A{});
constexpr A a11 = bar (A{});
//static_assert(a10.p == &a10, ""); // bug?
constexpr A a12 = baz ();
//static_assert(a11.p == &a11, ""); // bug?

//constexpr A a13 = foo (A{}); // error!

[Bug c++/105550] Missing copy elision with conditional operator

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105550

--- Comment #3 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:ecd11acacd6be57af930fa617d7c31ecb40e7f74

commit r13-1397-gecd11acacd6be57af930fa617d7c31ecb40e7f74
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed May 25 18:11:31 2022 -0400

    c++: fix broken copy elision with nested TARGET_EXPRs [PR105550]

    In this problem, we are failing to properly perform copy elision with
    a conditional operator, so this:

      constexpr A a = true ? A{} : A{};

    fails with:

      error: 'A{((const A*)(&<anonymous>))}' is not a constant expression

    The whole initializer is

      TARGET_EXPR <D.2395, 1 ? TARGET_EXPR <D.2393, {.p=(const struct A *)
&<PLACEHOLDER_EXPR struct A>}> : TARGET_EXPR <D.2394, {.p=(const struct A *)
&<PLACEHOLDER_EXPR struct A>}>>

    where the outermost TARGET_EXPR is elided, but not the nested ones.
    Then we end up replacing the PLACEHOLDER_EXPRs with the temporaries the
    TARGET_EXPRs represent, which is precisely what should *not* happen with
    copy elision.

    I've tried the approach of tweaking ctx->object, but I ran into gazillion
    problems with that.  I thought that I would let
cxx_eval_constant_expression
    /TARGET_EXPR create a new object only when ctx->object was null, then
    adjust setting of ctx->object in places like cxx_bind_parameters_in_call
    and cxx_eval_component_reference but that failed completely.  Sometimes
    ctx->object has to be reset, sometimes it cannot be reset, 'this' needed
    special handling, etc.  I gave up.

    Instead, this patch strips TARGET_EXPRs from the operands of ?: like
    we do in various other places in constexpr.c.

            PR c++/105550

    gcc/cp/ChangeLog:

            * constexpr.cc (cxx_eval_conditional_expression): Strip
TARGET_EXPRs.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/nsdmi-aggr16.C: Remove FIXME.
            * g++.dg/cpp1y/nsdmi-aggr17.C: Remove FIXME.
            * g++.dg/cpp0x/constexpr-elision1.C: New test.
            * g++.dg/cpp1y/constexpr-elision1.C: New test.

[Bug c++/105550] Missing copy elision with conditional operator

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105550

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

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

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed for GCC 13.

[Bug c++/105550] Missing copy elision with conditional operator

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105550

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |13.0