Merge pull request #10839 from Microsoft/fixNarrowingWithTypeParameter

Fix narrowing with constrained type parameter

src/compiler/checker.ts

@@ -8729,7 +8729,7 @@ 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]);