[Bug fortran/100724] New: -fwhole-program breaks module use

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100724

            Bug ID: 100724
           Summary: -fwhole-program breaks module use
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fx at gnu dot org
  Target Milestone: ---

I found that trying gfortran -fwhole-program failed to link a case I tried,
with undefined references to routines with interface blocks.  It's OK with
gfortran 10.
Here's a trivial example (on Debian 10, but I don't suppose that matters):

$ gfortran-11 --version
GNU Fortran (GCC) 11.1.0
$ cat test.f90
module tw
  interface
     real function twice (x)
     end function twice
  end interface
end module tw

real function twice (x)
  twice = 2*x
end function twice

use tw
read *, x
print *, twice (x)
end
$ gfortran-11 -O -fwhole-program test.f90 
/usr/bin/ld: /tmp/ccBKHiLp.o: in function `MAIN__':
test.f90:(.text+0x7d): undefined reference to `twice_'
collect2: error: ld returned 1 exit status