exclude enum _value_ from uninitialized attribute error

Summary:
per Eric's comment in python/typing#1804 (comment)

_value_ is magic for enums, and we shouldn't give uninitialized attr errors for it

Reviewed By: migeed-z

Differential Revision: D59833300

fbshipit-source-id: 59391ee56aecbab8b648648d27927258b2b6d3fc

source/analysis/typeCheck.ml

@@ -7585,20 +7585,24 @@ let emit_errors_on_exit (module Context : Context) ~errors_sofar ~resolution ()
                   ->
                  let parent = Type.Primitive (ClassSummary.name definition |> Reference.show) in
                  let expected = annotation in
+                 let is_enum =
+                   GlobalResolution.less_or_equal
+                     global_resolution
+                     ~left:parent
+                     ~right:Type.enumeration
+                 in
                  if Type.is_top expected then
                    None
+                 else if is_enum && String.equal name "_value_" then
+                   (* _value_ is automatically initialized for enums *)
+                   None
                  else
                    let error_kind =
                      if is_protocol then
                        Error.Protocol (Reference.create original_class_name)
                      else if is_abstract then
                        Error.Abstract (Reference.create original_class_name)
-                     else if
-                       GlobalResolution.less_or_equal
-                         global_resolution
-                         ~left:parent
-                         ~right:Type.enumeration
-                     then
+                     else if is_enum then
                        Error.Enumeration
                      else
                        Error.Class