Fix default arguments xhp

Summary: mangle xhp classes when used as default arguments

Reviewed By: vladima

Differential Revision: D7172431

fbshipit-source-id: a56bb79f175e76b232dc835507823fb93495f90e

hphp/hack/src/hhbc/hhbc_hhas.ml

@@ -1064,6 +1064,8 @@ and string_of_param_default_value ~env expr =
    e1 ^ s ^ e2
  in
  let fmt_class_name ~is_class_constant cn =
    let cn = if SU.Xhp.is_xhp (Utils.strip_ns cn)
    then SU.Xhp.mangle cn else cn in
    let cn = (Php_escaping.escape (SU.strip_global_ns cn)) in
    if is_class_constant then "\\\\" ^ cn else cn in
  let get_special_class_name ~env ~is_class_constant id =