[Bug fortran/99922] New: Bind(C) with assumed length character should work

[everythingfunctional at protonmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99922

            Bug ID: 99922
           Summary: Bind(C) with assumed length character should work
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: everythingfunctional at protonmail dot com
  Target Milestone: ---

As of (at least) Fortran 2018, bind(C) should be allowed for procedures with
assumed length character variables, as one can pass CFI_cdesc_t* from the C
side and it's supposed to work.

MWE below

! main.f90
program main
    implicit none

    interface
        subroutine say_hello_c() bind(C)
        end subroutine
    end interface

    call say_hello_c()
end program

! say_hello_fortran.f90
subroutine say_hello_fortran(name) bind(C)
    use iso_c_binding, only: c_char

    implicit none

    character(len=*, kind=c_char), intent(in) :: name

    print *, "Hello from Fortran, " // name // "!"
end subroutine

! say_hello_c.c
#include <string.h>
#include <ISO_Fortran_binding.h>

extern void say_hello_fortran(CFI_cdesc_t * name_descriptor);

void say_hello_c() {
    char * name = "World";
    CFI_cdesc_t name_descriptor;
    int error_code = CFI_establish(
            &name_descriptor,
            name,
            CFI_attribute_other,
            CFI_type_char,
            strlen(name),
            0,
            NULL);
    say_hello_fortran(&name_descriptor);
}

Compiling say_hello_fortran.f90 gives the error

say_hello_fortran.f90:1:33:

    1 | subroutine say_hello_fortran(name) bind(C)
      |                                 1
Error: Character argument ‘name’ at (1) must be length 1 because procedure
‘say_hello_fortran’ is BIND(C)

But the relevant text from the standard says (from section 18.3.6):

any dummy argument without the VALUE attribute corresponds to a formal
parameter of the prototype that is of a pointer type, and either
the dummy argument is a nonallocatable nonpointer variable of type CHARACTER
with assumed character length and the formal parameter is a pointer to
CFI_cdesc_t,

So this is supposed to be allowed, and I can confirm that I can compile and
execute the above example and obtain the expected result with Intel (ifort and
icc).

output of gfortran --version:
GNU Fortran (Ubuntu 10.2.0-13ubuntu1) 10.2.0
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.