[patch, fortran] Reject character lengths unequal to one in BIND(C)

[Thomas Koenig]

Hello world,

At the moment, gfortran silently accepts

  type, bind(c) :: a
     character(len=2,kind=c_char) :: b
  end type a

translating this into I don't know what. With the -fc-prototypes
option, we now get

typedef struct a {
    char b;
} a;

which is clearly bogus. Any user code which has the broken code above
will then likely crash and burn when using a gfortran-generated
prototype.

The attached patch solves that particular problem by issuing errors
in this case.  OK for trunk?

Regards

        Thomas

2017-01-28  Thomas Koenig  <tkoe...@gcc.gnu.org>

        PR fortran/84073
        * resolve.c (resolve_component): Ensure BIND(C) character
        components have length one.
        (resolve_symbol): Likewise for variables.

2017-01-28  Thomas Koenig  <tkoe...@gcc.gnu.org>

        PR fortran/84073
        * gfortran.dg/bind_c_usage_31.f90: New test.

Index: resolve.c
===================================================================
--- resolve.c	(Revision 257131)
+++ resolve.c	(Arbeitskopie)
@@ -13557,6 +13557,17 @@ resolve_component (gfc_component *c, gfc_symbol *s
       return false;
     }
 
+  /* F2003, 15.2.1 - length has to be one.  */
+  if (sym->attr.is_bind_c && c->ts.type == BT_CHARACTER
+      && (c->ts.u.cl == NULL || c->ts.u.cl->length == NULL
+	  || !gfc_is_constant_expr (c->ts.u.cl->length)
+	  || mpz_cmp_si (c->ts.u.cl->length->value.integer, 1) != 0))
+    {
+      gfc_error ("Component %qs of BIND(C) type at %L must have length one",
+		 c->name, &c->loc);
+      return false;
+    }
+
   if (c->attr.proc_pointer && c->ts.interface)
     {
       gfc_symbol *ifc = c->ts.interface;
@@ -14804,6 +14815,15 @@ resolve_symbol (gfc_symbol *sym)
 		     "module level scope", sym->name, &(sym->declared_at));
 	  t = false;
 	}
+      else if (sym->ts.type == BT_CHARACTER
+	       && (sym->ts.u.cl == NULL || sym->ts.u.cl->length == NULL
+		   || !gfc_is_constant_expr (sym->ts.u.cl->length)
+		   || mpz_cmp_si (sym->ts.u.cl->length->value.integer, 1) != 0))
+	{
+	  gfc_error ("BIND(C) Variable %qs at %L must have length one",
+		     sym->name, &sym->declared_at);
+	  t = false;
+	}
       else if (sym->common_head != NULL && sym->attr.implicit_type == 0)
         {
           t = verify_com_block_vars_c_interop (sym->common_head);

! { dg-do compile }
! PR fortran/84073 - this was accepted before.
module mod
  use iso_c_binding
  type, bind(c) :: a
     character(len=2,kind=c_char) :: b ! { dg-error "must have length one" }
  end type a
  character(len=2), bind(C) :: c ! { dg-error "must have length one" }
end module mod