Check all paths return properly in blocks. Closes #874.

src/comp/middle/tstate/ck.rs

@@ -145,7 +145,7 @@ fn check_states_against_conditions(fcx: &fn_ctxt, f: &_fn,
 
     /* Check that the return value is initialized */
     let post = aux::block_poststate(fcx.ccx, f.body);
-    if f.proto == ast::proto_fn &&
+    if f.proto != ast::proto_iter &&
            !promises(fcx, post, fcx.enclosing.i_return) &&
            !type_is_nil(fcx.ccx.tcx, ret_ty_of_fn(fcx.ccx.tcx, id)) &&
            f.decl.cf == return {

src/test/compile-fail/block-require-return.rs

@@ -0,0 +1,5 @@
+// error-pattern: not all control paths return
+fn force(f: &block() -> int) -> int { f() }
+fn main() {
+    log_err force({| | });
+}