From c48c670ff0ce4f0d2ffb1d43aca2ec1bed1fa2ef Mon Sep 17 00:00:00 2001
From: Harald Anlauf <anlauf@gmx.de>
Date: Fri, 17 Mar 2023 22:24:49 +0100
Subject: [PATCH] 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.
---
 gcc/fortran/resolve.cc                | 10 ++++++++++
 gcc/testsuite/gfortran.dg/pr85877.f90 | 17 +++++++++++++++++
 2 files changed, 27 insertions(+)
 create mode 100644 gcc/testsuite/gfortran.dg/pr85877.f90

diff --git a/gcc/fortran/resolve.cc b/gcc/fortran/resolve.cc
index 46585879ddc..7ec65f16240 100644
--- a/gcc/fortran/resolve.cc
+++ b/gcc/fortran/resolve.cc
@@ -13661,6 +13661,16 @@ 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 00000000000..675faac0027
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/pr85877.f90
@@ -0,0 +1,17 @@
+! { dg-do compile }
+! PR fortran/85877
+! A procedure with the bind(c) attribute shall have an explicit interface
+! Contributed by G. Steinmetz
+
+subroutine p
+  bind(c) f     ! { dg-error "must be explicit" }
+  x = f()
+end
+
+subroutine s
+  interface
+     function g() bind(c)
+     end function g
+  end interface
+  x = g()
+end
--
2.35.3