Add regression test

ahejlsberg · ahejlsberg · commit 6cf640ed3bb3 · 2018-02-05T13:56:22.000-08:00
diff --git a/tests/cases/compiler/conditionalTypesASI.ts b/tests/cases/compiler/conditionalTypesASI.ts
@@ -0,0 +1,8 @@
+// @declaration: true
+
+// Repro from #21637
+
+interface JSONSchema4 {
+  a?: number
+  extends?: string | string[]
+}
