Invalid change of value conversion warning message

[William Clodius]

The file

module test_int
  use, intrinsic:: iso_fortran_env

  integer(int64), parameter ::               &
      e18 = 1000000000000000000_int64,       &
      e19 = ( -huge(1_int64) +               &
              (e18-223372036854775807_int64) &
              ) - 2

end module test_int

when compiled using

fortran -std=f2008 -fmax-errors=10 -Wall -Wimplicit-interface -O3 -fbacktrace -fwhole-file -I mod_dir -c -o test_int.o test_int.f90 

on Mac OS X version 10.11.4

with version

gfortran --version
GNU Fortran (GCC) 6.1.0
Copyright (C) 2016 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.

produces the message

test_int.f90:5:11:

       e18 = 1000000000000000000_int64,       &
           1
Warning: Change of value in conversion from ‘INTEGER(8)’ to ‘REAL(4)’ at (1) [-Wconversion]

Putting in an 
implicit none
statement doesn’t change the message.