[Bug c++/108759] New: "mandatory copy elision" not implemented during constant evaluation redux

[Bug c++/108759] New: "mandatory copy elision" not implemented during constant evaluation redux

[herring at lanl dot gov]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108759

            Bug ID: 108759
           Summary: "mandatory copy elision" not implemented during
                    constant evaluation redux
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: herring at lanl dot gov
  Target Milestone: ---

Extending the test case in (the invalid) #94537 to have a user-provided
destructor does not cause the "mandatory copy elision" to take place even
though the implementation is no longer permitted to introduce a temporary:

  struct A;
  extern const A a;
  struct A {
    bool special=this==&a;
    constexpr ~A() {}
  };
  constexpr A get() {return {};}  // or "return A();"
  constexpr A a=get();
  static_assert(a.special);

[Bug c++/108759] "mandatory copy elision" not implemented during constant evaluation redux

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108759

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-02-13
             Status|UNCONFIRMED                 |NEW
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed, we haven't elided the TARGET_EXPR:

*<retval> = TARGET_EXPR <D.2533, {.special=(const struct A *)
&<PLACEHOLDER_EXPR struct A> == &a}>

[Bug c++/108759] "mandatory copy elision" not implemented during constant evaluation redux

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108759

Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |arthur.j.odwyer at gmail dot com

--- Comment #2 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
I believe what I'm going to say is simply a duplicate of Davis's report, but
just in case it's different, I'll add mine here. This is a slightly modified
version of the example from [class.copy.elision], merely modified so that g()
returns a C++17 prvalue instead of relying on NRVO/copy-elision.
Original example: https://eel.is/c++draft/class.copy.elision#2

Modified example:

// https://godbolt.org/z/n43jPs1Kj
struct A {
  void *p;
  constexpr A(): p(this) {}
};
constexpr A g() {
  return A();
}
constexpr A b = g();    // well-formed, b.p points to b
static_assert(b.p == &b);

MSVC and Clang accept. GCC incorrectly rejects:

<source>:11:19: error: 'A{((void*)(&<anonymous>))}' is not a constant
expression
   11 | constexpr A b = g();    // well-formed, b.p points to b
      |                   ^
<source>:12:19: error: non-constant condition for static assertion
   12 | static_assert(b.p == &b);
      |               ~~~~^~~~~