[Bug fortran/96986] New: [8 Regression] Explicit interface required: volatile argument for ENTRY subroutine

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96986

            Bug ID: 96986
           Summary: [8 Regression] Explicit interface required: volatile
                    argument for ENTRY subroutine
           Product: gcc
           Version: 8.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: foreese at gcc dot gnu.org
  Target Milestone: ---

Created attachment 49200
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49200&action=edit
Testcase which exhibits the regression

GCC 8.3.1 does not accept the following program, but all versions 4.x through
7.x and <= 8.3.0 do:

$ cat volatile.for
      subroutine volatile_test ()
        implicit none
        integer(4), volatile :: va

        entry fun_a()
        return

        entry fun_b(va)
          call fun_c()
        return
      end

      subroutine fun_c ()
        implicit none
        call fun_a()
        return
      end
$ gfortran -std=legacy -c volatile.for
volatile.for:15:18:

         call fun_a()
                  1
Error: Explicit interface required for ‘fun_a’ at (1): volatile argument


It seems that because fun_b() has a volatile argument, the subroutine
volatile_test() is marked as such. Subsequently the call to fun_a() from
fun_c() within the same procedure trips the error.

The regression appears to have been introduced in r269895 on gcc-9 which fixed
PR 78865. The fix was backported to gcc-8 by r270032 and released as part of
8.3.1.