Fortran: improve checking of procedures passed as actual argument [PR50377]

Procedures passed as actual argument require either an explicit interface
or must be declared EXTERNAL.  Add a check and generate an error (default)
or a warning when -std=legacy is specified.

	PR fortran/50377

gcc/fortran/ChangeLog:

	* resolve.cc (resolve_actual_arglist): Check procedure actual
	arguments.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr41011.f: Fix invalid testcase.
	* gfortran.dg/actual_procedure_2.f: New test.

gcc/fortran/resolve.cc

@@ -2295,6 +2295,30 @@ resolve_actual_arglist (gfc_actual_arglist *arg, procedure_type ptype,
 	  goto cleanup;
 	}
 
+      if (e->expr_type == EXPR_VARIABLE
+	  && e->ts.type == BT_PROCEDURE
+	  && no_formal_args
+	  && sym->attr.flavor == FL_PROCEDURE
+	  && sym->attr.if_source == IFSRC_UNKNOWN
+	  && !sym->attr.external
+	  && !sym->attr.intrinsic
+	  && !sym->attr.artificial
+	  && !sym->ts.interface)
+	{
+	  /* Emit a warning for -std=legacy and an error otherwise. */
+	  if (gfc_option.warn_std == 0)
+	    gfc_warning (0, "Procedure %qs at %L used as actual argument but "
+			 "does neither have an explicit interface nor the "
+			 "EXTERNAL attribute", sym->name, &e->where);
+	  else
+	    {
+	      gfc_error ("Procedure %qs at %L used as actual argument but "
+			 "does neither have an explicit interface nor the "
+			 "EXTERNAL attribute", sym->name, &e->where);
+	      goto cleanup;
+	    }
+	}
+
       first_actual_arg = false;
     }
 

gcc/testsuite/gfortran.dg/actual_procedure_2.f

@@ -0,0 +1,22 @@
+! { dg-do compile }
+! PR fortran/50377
+!
+! Reject procedures passed as actual argument if there is no explicit
+! interface and they are not declared EXTERNAL
+!
+! Contributed by Vittorio Zecca
+
+!     external sub      ! Required for valid code
+!     external fun      ! Required for valid code
+      call sub(sub)     ! { dg-error "used as actual argument" }
+      z = fun(fun)      ! { dg-error "used as actual argument" }
+      end
+
+      subroutine sub(y)
+      external y
+      end
+
+      real function fun(z)
+      external z
+      f = 1.
+      end

gcc/testsuite/gfortran.dg/pr41011.f

@@ -1,5 +1,7 @@
 ! { dg-do compile }
 ! { dg-options "-O3 -std=legacy" }
+      SUBROUTINE PR41011 (DCDX)
+      DIMENSION DCDX(*)
       CALL UVSET(NX,NY,NZ,HVAR,ZET,NP,DZ,DKM,UM,VM,UG,VG,TM,DCDX, ! { dg-warning "Rank mismatch|Invalid procedure argument" }
      *ITY,ISH,NSMT,F)
          CALL DCTDX(NX,NY,NX1,NFILT,C(MLAG),DCDX(MLAG),HELP,HELPA,