From 2e1dcd666c790347b02d0af46e4e928c7d45fdf7 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Sat, 10 Feb 2018 17:10:44 -0800
Subject: [PATCH] Add regression test

---
 .../types/conditional/conditionalTypes1.ts            | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/tests/cases/conformance/types/conditional/conditionalTypes1.ts b/tests/cases/conformance/types/conditional/conditionalTypes1.ts
index 3a6dab9a612..a0776ef2e0e 100644
--- a/tests/cases/conformance/types/conditional/conditionalTypes1.ts
+++ b/tests/cases/conformance/types/conditional/conditionalTypes1.ts
@@ -252,3 +252,14 @@ function f33<T, U>() {
     var z: T1;
     var z: T2;
 }
+
+// Repro from #21863
+
+function f40() {
+    type Eq<T, U> = T extends U ? U extends T ? true : false : false;
+    type If<S, T, U> = S extends false ? U : T;
+    type Omit<T extends object> = { [P in keyof T]: If<Eq<T[P], never>, never, P>; }[keyof T];
+    type Omit2<T extends object, U = never> = { [P in keyof T]: If<Eq<T[P], U>, never, P>; }[keyof T];
+    type A = Omit<{ a: void; b: never; }>;  // 'a'
+    type B = Omit2<{ a: void; b: never; }>;  // 'a'
+}