🚀 go-pugleaf

I was trying to pass a subroutine to another with the following test code.
-----------------------------------------------------------------------
	implicit double precision (a-h,o-z)
	external ext
	call sub(ext,x)
	write (6,10) x
10	format (/' x = ',1pd10.3)
	stop
	end
c
c
c
c
	subroutine sub(f,a)
	implicit double precision (a-h,o-z)
c
c  Compiler gives warning if 'f' is not declared external here.
c	external f
	call f(a)
	return
	end
c
c
c
c
	subroutine ext(b)
	implicit double precision (a-h,o-z)
	b = 1.234d0
	return
	end
-----------------------------------------------------------------------
Can someone tell me why the UNIX 4.2BSD f77 compiler gives the following
warning message ?

Warning on line 17 of test.f: Dummy procedure not declared EXTERNAL. Code may be wrong.

I thought a passed procedure only had to be declared external in the calling
procedure.
If this is not a bug, what is the compiler trying to warn me of ?
By the way, the program still seems to execute properly.

                                             Thank you,
             /\
        /\  /  \                             Chip Brewster
       /  \/    \/\                          University of Vermont
      /    \    /  \                         decvax!dartvax!uvm-cs!brewster
      +     m' m'    +                       brewster@uvm-cs
     / `m    m' `m  /
    +--+--+--+--+--+
    Vermont, Green Mountains and lots 'uv cows!
-- 
                                             Thank you,
             /\
        /\  /  \                             Chip Brewster
       /  \/    \/\                          University of Vermont
      /    \    /  \                         decvax!dartvax!uvm-cs!brewster
      +     m' m'    +                       brewster@uvm-cs
     / `m    m' `m  /                        (802) 656-3330  ext. 33
    +--+--+--+--+--+
    Vermont, Green Mountains and lots 'uv cows!