[nullsafe] --no-nullsafe-optimistic-third-party-in-default-mode is optimistic about Inheritance check

Summary: This is needed for migration purposes.

Reviewed By: artempyanykh

Differential Revision: D26543726

fbshipit-source-id: 2dd8928bb

infer/src/nullsafe/InheritanceRule.ml

@@ -95,6 +95,13 @@ module ReportableViolation = struct
 end
 
 let check type_role ~base ~overridden =
+  if Nullability.equal Nullability.ThirdPartyNonnull base then
+     (* In context of inheritance check, third party declarations in base are treated optimistically.
+        Meaning return values are assumed [@Nullable] and params are assumed [@NonNull].
+        This is done for compatibility reasons so existing [@Nullsafe] classes are preserved Nullsafe.
+         *)
+  Ok ()
+  else
     let subtype, supertype =
       match type_role with
       | Ret ->