Fortran: reject empty derived type with bind(C) attribute [PR101577]

PR fortran/101577

gcc/fortran/ChangeLog:

	* symbol.cc (verify_bind_c_derived_type): Generate error message
	for derived type with no components in standard conformance mode,
	indicating that this is a GNU extension.

gcc/testsuite/ChangeLog:

	* gfortran.dg/empty_derived_type.f90: Adjust dg-options.
	* gfortran.dg/empty_derived_type_2.f90: New test.

gcc/fortran/symbol.cc

@@ -4624,12 +4624,29 @@ verify_bind_c_derived_type (gfc_symbol *derived_sym)
      entity may be defined by means of C and the Fortran entity is said
      to be interoperable with the C entity.  There does not have to be such
      an interoperating C entity."
+
+     However, later discussion on the J3 mailing list
+     (https://mailman.j3-fortran.org/pipermail/j3/2021-July/013190.html)
+     found this to be a defect, and Fortran 2018 added in section 18.3.4
+     the following constraint:
+     "C1805: A derived type with the BIND attribute shall have at least one
+     component."
+
+     We thus allow empty derived types only as GNU extension while giving a
+     warning by default, or reject empty types in standard conformance mode.
   */
   if (curr_comp == NULL)
     {
-      gfc_warning (0, "Derived type %qs with BIND(C) attribute at %L is empty, "
-		   "and may be inaccessible by the C companion processor",
-		   derived_sym->name, &(derived_sym->declared_at));
+      if (!gfc_notify_std (GFC_STD_GNU, "Derived type %qs with BIND(C) "
+			   "attribute at %L has no components",
+			   derived_sym->name, &(derived_sym->declared_at)))
+	return false;
+      else if (!pedantic)
+	/* Generally emit warning, but not twice if -pedantic is given.  */
+	gfc_warning (0, "Derived type %qs with BIND(C) attribute at %L "
+		     "is empty, and may be inaccessible by the C "
+		     "companion processor",
+		     derived_sym->name, &(derived_sym->declared_at));
       derived_sym->ts.is_c_interop = 1;
       derived_sym->attr.is_bind_c = 1;
       return true;

gcc/testsuite/gfortran.dg/empty_derived_type.f90

@@ -1,4 +1,5 @@
 ! { dg-do compile }
+! { dg-options "" }
 module stuff
    implicit none
    type, bind(C) :: junk ! { dg-warning "may be inaccessible by the C companion" }

gcc/testsuite/gfortran.dg/empty_derived_type_2.f90

@@ -0,0 +1,11 @@
+! { dg-do compile }
+! { dg-additional-options "-std=f2018" }
+!
+! PR fortran/101577
+!
+! Contributed by Tobias Burnus
+
+type, bind(C) :: t ! { dg-error "has no components" }
+   ! Empty!
+end type t
+end