Include conditional types in top-level type parameter check

Author: ahejlsberg

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` */