From 6cf640ed3bb3d5c3f23993c2375c8fa8b71c763f Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Mon, 5 Feb 2018 13:56:22 -0800
Subject: [PATCH] Add regression test

---
 tests/cases/compiler/conditionalTypesASI.ts | 8 ++++++++
 1 file changed, 8 insertions(+)
 create mode 100644 tests/cases/compiler/conditionalTypesASI.ts

diff --git a/tests/cases/compiler/conditionalTypesASI.ts b/tests/cases/compiler/conditionalTypesASI.ts
new file mode 100644
index 00000000000..44d6d816851
--- /dev/null
+++ b/tests/cases/compiler/conditionalTypesASI.ts
@@ -0,0 +1,8 @@
+// @declaration: true
+
+// Repro from #21637
+
+interface JSONSchema4 {
+  a?: number
+  extends?: string | string[]
+}