[Bug fortran/108663] Accepts invalid bug with pdtXXX

[pault at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108663

--- Comment #5 from Paul Thomas <pault at gcc dot gnu.org> ---
(In reply to Steve Kargl from comment #1)
> > $ gfortran-13-20221218 -c z1.f90   # missing error
> > $
> > $ gfortran-13-20230115 -c z1.f90
> > z1.f90:12:7:
> > 
> >    12 |    use m, only: t, pdtt, s
> >       |       1
> > internal compiler error: in check_complete_insertion, at hash-table.h:578
> 
> For the record, the entity 'pdtt' is not specified
> in the module.  It is constructed by gfortran in 
> decl.cc(gfc_get_pdt_instance) when parsing a parameterized
> derived type.  The symbol pdtt means PDT type t, and it
> appears to inserted into the namespace.  A likely better
> name would have been _PDT_t, because the leading underscore
> means the name cannot conflict with a user's code and
> gfortran downcases all parsed code so PDT shows that the
> symbol is artificial.

The trouble with a leading non-alpha character is that U/C vs. L/C is used to
distinguish the derived type vs the constructor.

I have been trying to think of a suitable separator between the "Pdt" prefix
and the rest of the name.

Can you remember why '.' was deemed to be unsuitable for class names?

Paul