Preserve type parameter types in narrowing

ahejlsberg · mhegazy · commit 709b4ebc83f8 · 2016-09-12T11:28:32.000-07:00
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -8665,7 +8665,7 @@ namespace ts {
                 // type. Otherwise, the types are completely unrelated, so narrow to an intersection of the
                 // two types.
                 const targetType = type.flags & TypeFlags.TypeParameter ? getApparentType(type) : type;
-                return isTypeSubtypeOf(candidate, targetType) ? candidate :
+                return isTypeSubtypeOf(candidate, type) ? candidate :
                     isTypeAssignableTo(type, candidate) ? type :
                     isTypeAssignableTo(candidate, targetType) ? candidate :
                     getIntersectionType([type, candidate]);
