[Bug c++/124331] New: [reflection] data member comparison mishap

[mpolacek at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124331

            Bug ID: 124331
           Summary: [reflection] data member comparison mishap
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

Thanks to Dan K. for reporting & reducing this.

```
#include <meta>

template <typename T>
void f() {
    constexpr auto ctx = std::meta::access_context::current();
    static constexpr auto rng =
std::define_static_array(nonstatic_data_members_of(^^T, ctx));
    static_assert(rng[0] == ^^T::mem);
};

struct S { int mem; };
auto force_instantiation = (f<S>, 0);
```

fails with

t.C:7:26: error: static assertion failed
    7 |     static_assert(rng[0] == ^^T::mem);
  • the comparison reduces to ‘(^^S::mem == ^^S::mem)’

because compare_reflections gets:

(gdb) p lhs
$7 = <field_decl 0x7fffe63041e0 mem>
(gdb) p rhs
$8 = <component_ref 0x7fffe5f23b10>