[Bug gdb/28480] pretty printing getting confused

["jwakely.gcc at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28480

--- Comment #8 from Jonathan Wakely <jwakely.gcc at gmail dot com> ---
Standalone reproducer:

namespace std
{
#if __has_cpp_attribute(__no_unique_address__)
#warning using attr
  template<typename T>
    struct _Head_base
    {
      [[__no_unique_address__]] T _M_head_impl;
    };
#else
#warning not using attr
  template<typename T>
    struct _Head_base : T
    { };
#endif

  template<typename T>
    struct _Head_base<T*>
    {
      T* _M_head_impl;
    };

  template<unsigned long, typename ...> struct _Tuple_impl;

  template<typename T, typename U>
    struct _Tuple_impl<0, T, U> : _Tuple_impl<1, U>, _Head_base<T>
    { };

  template<typename U>
    struct _Tuple_impl<1, U> : _Head_base<U>
    { };

  template<typename T, typename U>
    struct tuple : _Tuple_impl<0, T, U>
    { };

  template<typename T> struct default_delete { };

  template<typename T, typename D = default_delete<T>>
    struct unique_ptr
    {
      unique_ptr(T* p) { _M_t._Head_base<T*>::_M_head_impl = p; }

      using __tuple_type = tuple<T*, D>;

      __tuple_type _M_t;
    };
}

struct datum
{
  const char* s;
  int i;
};

int
main()
{
  std::unique_ptr<datum> uptr (new datum);
  return 0;
}

compiled with GCC 9 or above this uses the [[no_unique_address]] attribute, an
GDB gets confused:

$ GCC 8 up.C -Wno-unused && ~/gcc/gdb/11/bin/gdb -q  -ex 'start' -ex n -ex  'p
uptr' a.out
up.C:11:2: warning: #warning not using attr [-Wcpp]
 #warning not using attr
  ^~~~~~~
Reading symbols from a.out...
Temporary breakpoint 1 at 0x40112a: file up.C, line 59.
Starting program: /tmp/a.out 

Temporary breakpoint 1, main () at up.C:59
59        std::unique_ptr<datum> uptr (new datum);
60        return 0;
$1 = std::unique_ptr<datum> = {get() = 0x416eb0}


$ GCC 9 up.C -Wno-unused && ~/gcc/gdb/11/bin/gdb -q  -ex 'start' -ex n -ex  'p
uptr' a.out
up.C:4:2: warning: #warning using attr [-Wcpp]
    4 | #warning using attr
      |  ^~~~~~~
Reading symbols from a.out...
Temporary breakpoint 1 at 0x40112a: file up.C, line 59.
Starting program: /tmp/a.out 

Temporary breakpoint 1, main () at up.C:59
59        std::unique_ptr<datum> uptr (new datum);
60        return 0;
$1 = std::unique_ptr<datum> = {get() = {<No data fields>}}

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