[Bug c++/92789] New: Non-obvious ?: behaviour with structurally equivalent types

[rsandifo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92789

            Bug ID: 92789
           Summary: Non-obvious ?: behaviour with structurally equivalent
                    types
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rsandifo at gcc dot gnu.org
  Target Milestone: ---
            Target: aarch64*-*-*

As noted in https://gcc.gnu.org/ml/gcc-patches/2019-12/msg00128.html
build_conditional_expr_1 uses same_type_p to check whether the "then"
and "else" types are the same.  If they are, it uses the "then"
type for the result.

However, same_type_p uses structural equality when
TYPE_STRUCTURAL_EQUALITY_P is true.  This means that we can treat
two distinct but equivalent types as "the same" and use the "then"
type for the result.  For example:

  #include <arm_neon.h>
  typedef uint8_t gnu_uint8_t __attribute__ ((vector_size (16)));
  void f(gnu_uint8_t x) {}
  void g(uint8x16_t y) {}

correctly mangles f as _Z1fDv16_h and g as _Z1g12__Uint8x16_t.  But:

  void h1(decltype(*(int *)nullptr ? *(gnu_uint8_t *)nullptr : *(uint8x16_t
*)nullptr)) {}
  void h2(decltype(*(int *)nullptr ? *(uint8x16_t *)nullptr : *(gnu_uint8_t
*)nullptr)) {}

mangles h1 as _Z2h1RDv16_h and h2 as _Z2h2R12__Uint8x16_t.

See https://gcc.gnu.org/ml/gcc-patches/2019-12/msg00173.html for
Jason's comments on how this should work.