From d7f8f0072e5ff8d6731f4f656f3319b4bd0d8e4f Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Thu, 4 Jul 2019 16:26:54 -1000
Subject: [PATCH] Include conditional types in top-level type parameter check

---
 src/compiler/checker.ts | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 40b86ad679..9b540573c2 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -15208,7 +15208,11 @@ namespace ts {
         }
 
         function isTypeParameterAtTopLevel(type: Type, typeParameter: TypeParameter): boolean {
-            return type === typeParameter || !!(type.flags & TypeFlags.UnionOrIntersection) && some((<UnionOrIntersectionType>type).types, t => isTypeParameterAtTopLevel(t, typeParameter));
+            return !!(type === typeParameter ||
+                type.flags & TypeFlags.UnionOrIntersection && some((<UnionOrIntersectionType>type).types, t => isTypeParameterAtTopLevel(t, typeParameter)) ||
+                type.flags & TypeFlags.Conditional && (
+                    isTypeParameterAtTopLevel(getTrueTypeFromConditionalType(<ConditionalType>type), typeParameter) ||
+                    isTypeParameterAtTopLevel(getFalseTypeFromConditionalType(<ConditionalType>type), typeParameter)));
         }
 
         /** Create an object with properties named in the string literal type. Every property has type `any` */