[Bug c++/81145] New: bogus "dereferencing type-punned pointer will break strict-aliasing rules" diagnostic with incomplete class type

[richard-gccbugzilla at metafoo dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81145

            Bug ID: 81145
           Summary: bogus "dereferencing type-punned pointer will break
                    strict-aliasing rules" diagnostic with incomplete
                    class type
           Product: gcc
           Version: 7.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: richard-gccbugzilla at metafoo dot co.uk
  Target Milestone: ---

Testcase:

struct B { int n; };
struct A
#ifndef BUG
 { B b; }
#endif
;

extern A a;
int n = reinterpret_cast<B*>(&a)->n;

This produces a bogus "dereferencing type-punned pointer will break
strict-aliasing rules" diagnostic with -DBUG, but correctly suppresses the
warning when the definition of A is visible.

Note that under C++ P0137R1 / DR1776, the A object is pointer-interconvertible
with its b member, so the reinterpret_cast produces a usable pointer to a.b,
regardless of whether the definition of A is visible.

Perhaps GCC's TBAA should be less aggressive when comparing an incomplete type
against (at least) a standard-layout type?