[Bug c++/114709] New: Incorrect handling of inactive union member access via pointer to member in constant evaluated context

["420 at zerberste dot es" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114709

            Bug ID: 114709
           Summary: Incorrect handling of inactive union member access via
                    pointer to member in constant evaluated context
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: 420 at zerberste dot es
  Target Milestone: ---

According to https://standards.pydong.org/c++/class.union#general-note-1 an
inactive union member may be used to inspect the initial sequence common with
the active union member, given both are struct types. Furthermore
https://standards.pydong.org/c++/class.mem#general-26 tells us the behavior of
reading a non-static member m of such a common initial sequence is _as if_ the
corresponding member were nominated. Since it is not expression-equivalent, we
are still accessing the wrong member as far as the abstract machine is
concerned. Hence https://standards.pydong.org/c++/expr.const#5.10 still
applies.

GCC implements this correctly for the simple case. To reuse the example from
[class.mem]/26 consider this code fragment:
```cpp
struct T1 { int a, b; };
struct T2 { int c; double d; };
union U { T1 t1; T2 t2; };

consteval int f() {
  U u = { { 1, 2 } }; // active member is t1
  return u.t2.c;      // access through t2
}

static constexpr auto foo = f();
```
https://godbolt.org/z/x14n47T45

GCC correctly errors with the diagnostic 
> accessing 'U::t2' member instead of initialized 'U::t1' member in constant expression


However, consider the following code fragment:
```cpp
struct T1 { int a, b; };
struct T2 { int c; double d; };
union U { T1 t1; T2 t2; };

consteval int f() {
  U u = { { 1, 2 } };  // active member is t1
  return u.t2.*&T2::c; // access through t2
}

static constexpr auto foo = f();
```
https://godbolt.org/z/dK95jEPjq

GCC 12 up until trunk accept this code. GCC 11 and Clang don't.