Fortran: fix "unstable" interfaces of external procedures [PR122206]

In the testcase repeated invocations of a function showed an apparently
unstable interface.  This was caused by trying to guess an (inappropriate)
interface of the external procedure after processing of the procedure
arguments in gfc_conv_procedure_call.  The mis-guessed interface showed up
in subsequent uses of the procedure symbol in gfc_conv_procedure_call.  The
solution is to check for an existing interface of an external procedure
before trying to wildly guess based on just the actual arguments.

	PR fortran/122206

gcc/fortran/ChangeLog:

	* trans-types.cc (gfc_get_function_type): Do not clobber an
	existing procedure interface.

gcc/testsuite/ChangeLog:

	* gfortran.dg/interface_abstract_6.f90: New test.

gcc/fortran/trans-types.cc

@@ -3441,6 +3441,7 @@ gfc_get_function_type (gfc_symbol * sym, gfc_actual_arglist *actual_args,
 	}
     }
   if (sym->backend_decl == error_mark_node && actual_args != NULL
+      && sym->ts.interface == NULL
       && sym->formal == NULL && (sym->attr.proc == PROC_EXTERNAL
 				 || sym->attr.proc == PROC_UNKNOWN))
     gfc_get_formal_from_actual_arglist (sym, actual_args);

gcc/testsuite/gfortran.dg/interface_abstract_6.f90

@@ -0,0 +1,53 @@
+! { dg-do compile }
+! { dg-options "-fdump-tree-original" }
+!
+! PR fortran/122206
+!
+! Verify that procedure interfaces are "stable"
+
+module test_example
+  use, intrinsic :: iso_c_binding, only: c_double, c_int
+  implicit none
+
+  abstract interface
+     function simple_interface(iarg1, arg2) bind(c) result(res)
+       import c_double, c_int
+       integer(c_int), value, intent(in) :: iarg1
+       real(c_double), value, intent(in) :: arg2
+       real(c_double) :: res
+     end function simple_interface
+  end interface
+
+  procedure(simple_interface), bind(c,name="simple_function") :: simple_function
+
+  interface
+     function other_interface(iarg1, arg2) result(res)
+       import c_double, c_int
+       integer(c_int), value, intent(in) :: iarg1
+       real(c_double), value, intent(in) :: arg2
+       real(c_double) :: res
+     end function other_interface
+  end interface
+
+  procedure(other_interface) :: other_function
+
+contains
+  subroutine test_example_interface
+    implicit none
+    integer(c_int) :: iarg1 = 2
+    real(c_double) :: arg2  = 10.
+    real(c_double) :: val1, val2
+
+    val1 = simple_function(iarg1, arg2)
+    val2 = simple_function(iarg1, arg2)
+    if (val1 /= val2) stop 1
+
+    val1 = other_function(iarg1, arg2)
+    val2 = other_function(iarg1, arg2)
+    if (val1 /= val2) stop 2
+
+  end subroutine test_example_interface
+end module test_example
+
+! { dg-final { scan-tree-dump-times "simple_function \\(iarg1, arg2\\);" 2 "original"} }
+! { dg-final { scan-tree-dump-times "other_function \\(iarg1, arg2\\);" 2 "original"} }