[Bug c++/110164] New: Improve diagnostic for incomplete standard library types due to missing include

["rs2740 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110164

            Bug ID: 110164
           Summary: Improve diagnostic for incomplete standard library
                    types due to missing include
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

If I forget to include a header before using a sufficiently well-known standard
library type, GCC helpfully reminds me of the header:

$ echo 'std::array<int, 10> x;' | g++ -x c++ -

<stdin>:1:6: error: ‘array’ in namespace ‘std’ does not name a template type
<stdin>:1:1: note: ‘std::array’ is defined in header ‘<array>’; did you forget
to ‘#include <array>’?

But if I happen to have a different standard library header included that
happens to bring in a forward declaration of the type, the error message is
less helpful:

$ echo -e '#include <map>\nstd::array<int, 10> x;' | g++ -x c++ -

<stdin>:2:21: error: aggregate ‘std::array<int, 10> x’ has incomplete type and
cannot be defined

It would be nice if the latter case also has a hint about the potential missing
include.