[Bug fortran/64958] Warn if INTENT(IN) is changed by passing to no-intent argument

["tkoenig at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64958

--- Comment #3 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
If it is forbidden to modify an actual argument, we could make
a tempoary copy of that argument, pass that to a procedure where
it maches an intent(unknown) dummy argument, then do the comparison.

So, we would replace

subroutine bar(n)
  integer, intent(in) :: n
  call dusty(n)
end subroutine bar

with

subroutine bar(n)
  integer, intent(in) :: n
  integer :: shadow_n

  shadow_n = n
  call dusty(shadow_n)
  if (n /= shadow_n) call _gfortran_runtime_error("bletchful")
end subroutine bar

where dusty has no explicit interface or the first dummy argument
of dusty has unknown intent.

It would probably make most sense for scalar intrinsic types inculding
character variables.  Arrays of intrinsic types might also be a possibility,
but with decreasing return on investment, so to speak.