more precise narrow.getValType

Summary:
- narrow.getValType is used for elaborating `maps:get/2` and `maps:get/3`
- returning NoneType if we can safely deduce it. See the tests

Reviewed By: michalmuskala

Differential Revision: D66961955

fbshipit-source-id: 0a0488b8e486fb32a8d719512a0f010b158ca562

eqwalizer/src/main/scala/com/whatsapp/eqwalizer/tc/Narrow.scala

@@ -183,7 +183,14 @@ class Narrow(pipelineContext: PipelineContext) {
     subtype.join(t.vType, t.props.values.map(_.tp))
 
   def getValType(key: Key, t: MapType): Type =
-    t.props.get(key).map(_.tp).getOrElse(t.vType)
+    t.props.get(key).map(_.tp).getOrElse {
+      // key represents a literal type
+      // so we can use subtyping for testing non-empty intersection
+      if (subtype.subType(Key.asType(key), t.kType))
+        t.vType
+      else
+        NoneType
+    }
 
   def withRequiredProp(k: Key, t: MapType): Option[MapType] =
     t.props.get(k) match {