rustc::regionck: fix ICE when getting signature of ty_err

Fixes #5062

src/librustc/middle/typeck/check/regionck.rs

@@ -460,6 +460,10 @@ fn constrain_call(rcx: @mut Rcx,
     debug!("constrain_call(call_expr=%s, implicitly_ref_args=%?)",
            call_expr.repr(tcx), implicitly_ref_args);
     let callee_ty = rcx.resolve_node_type(callee_id);
+    if ty::type_is_error(callee_ty) {
+        // Bail, as function type is unknown
+        return;
+    }
     let fn_sig = ty::ty_fn_sig(callee_ty);
 
     // `callee_region` is the scope representing the time in which the