Fortran: procedures with BIND(C) attribute require explicit interface [PR85877]

gcc/fortran/ChangeLog:

	PR fortran/85877
	* resolve.cc (resolve_fl_procedure): Check for an explicit interface
	of procedures with the BIND(C) attribute (F2018:15.4.2.2).

gcc/testsuite/ChangeLog:

	PR fortran/85877
	* gfortran.dg/pr85877.f90: New test.

2 files changed, 35 insertions, 0 deletions

diff --git a/gcc/fortran/resolve.cc b/gcc/fortran/resolve.cc
index ba603b4..f3f3051 100644
--- a/gcc/fortran/resolve.cc
+++ b/gcc/fortran/resolve.cc
@@ -13871,6 +13871,16 @@ check_formal:
 	    }
 	}
     }
+
+  /* F2018:15.4.2.2 requires an explicit interface for procedures with the
+     BIND(C) attribute.  */
+  if (sym->attr.is_bind_c && sym->attr.if_source == IFSRC_UNKNOWN)
+    {
+      gfc_error ("Interface of %qs at %L must be explicit",
+		 sym->name, &sym->declared_at);
+      return false;
+    }
+
   return true;
 }
 
diff --git a/gcc/testsuite/gfortran.dg/pr85877.f90 b/gcc/testsuite/gfortran.dg/pr85877.f90
new file mode 100644
index 0000000..d8f08cb
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/pr85877.f90
@@ -0,0 +1,25 @@
+! { dg-do compile }
+! PR fortran/85877
+! A procedure with the bind(c) attribute shall have an explicit interface
+! Contributed by G. Steinmetz
+
+function f() bind(c)
+  f = 42.
+end
+
+subroutine p
+  bind(c) f     ! { dg-error "must be explicit" }
+  x = f()
+end
+
+function g() bind(c)
+  g = 42.
+end
+
+subroutine s
+  interface
+     function g() bind(c)
+     end function g
+  end interface
+  x = g()
+end