[PATCH] Fix Bug 109512 - accepts implicit dummy procedure even with "implicit none (external)"

[Jerry D]

The following fix provided by Steve.

Fairly self explanatory.
Regression tested x86_64-linux.

OK for mainline? Copied release manager as we are in Stage 4.

Regards,

Jerry

Author: Steven G. Kargl <ka...@gcc.gnu.org>
Date:   Fri Jan 16 14:05:16 2026 -0800

    Fortran: Fix accepts invalid implicit none (external)

    This patch yields an error for the test case which was
    previously being accepted even though implicit none (external)
    was being specified.

            PR fortran/109512

    gcc/fortran/ChangeLog:

            * resolve.cc (resolve_call): Check if an external attribute is
            required on a call to an external function.

    gcc/testsuite/ChangeLog:

            * gfortran.dg/pr109512.f90: New test.

diff --git a/gcc/fortran/resolve.cc b/gcc/fortran/resolve.cc
index be72132c79d..dfc2803a7c8 100644
--- a/gcc/fortran/resolve.cc
+++ b/gcc/fortran/resolve.cc
@@ -4167,6 +4167,16 @@ resolve_call (gfc_code *c)
   /* Resume assumed_size checking.  */
   need_full_assumed_size--;

+  /* If 'implicit none (external)' and the symbol is a dummy argument,
+     check for an 'external' attribute.  */
+  if (csym->ns->has_implicit_none_export
+      && csym->attr.external == 0 && csym->attr.dummy == 1)
+    {
+      gfc_error ("Dummy procedure %qs at %L requires an EXTERNAL attribute",
+                csym->name, &c->loc);
+      return false;
+    }
+
   /* If external, check for usage.  */
   if (csym && is_external_proc (csym))
     resolve_global_procedure (csym, &c->loc, 1);
diff --git a/gcc/testsuite/gfortran.dg/pr109512.f90 
b/gcc/testsuite/gfortran.dg/pr109512.f90
new file mode 100644
index 00000000000..f2b4cd53003
-- /dev/null
+++ b/gcc/testsuite/gfortran.dg/pr109512.f90
@@ -0,0 +1,5 @@
+! { dg-do compile }
+subroutine foo(bar)
+  implicit none (external)
+  call bar(1)  ! { dg-error "requires an EXTERNAL attribute" }
+end subroutine foo