From 5c6c1a3efe640d565e1d0bf0e0399b02b3866cc6 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Thu, 27 Jun 2019 16:48:00 -1000
Subject: [PATCH] Add regression test

---
 tests/cases/compiler/infiniteConstraints.ts | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/tests/cases/compiler/infiniteConstraints.ts b/tests/cases/compiler/infiniteConstraints.ts
index e914b738efc..a1dcc754260 100644
--- a/tests/cases/compiler/infiniteConstraints.ts
+++ b/tests/cases/compiler/infiniteConstraints.ts
@@ -36,3 +36,15 @@ const shouldBeError = ensureNoDuplicates({main: value("dup"), alternate: value("
 
 type Cond<T> = T extends number ? number : never;
 declare function function1<T extends {[K in keyof T]: Cond<T[K]>}>(): T[keyof T]["foo"];
+
+// Repro from #31823
+
+export type Prepend<Elm, T extends unknown[]> =
+  T extends unknown ?
+  ((arg: Elm, ...rest: T) => void) extends ((...args: infer T2) => void) ? T2 :
+  never :
+  never;
+export type ExactExtract<T, U> = T extends U ? U extends T ? T : never : never;
+
+type Conv<T, U = T> =
+  { 0: [T]; 1: Prepend<T, Conv<ExactExtract<U, T>>>;}[U extends T ? 0 : 1];