From 8f460a8446e46b48815cd96a187b101e71f61cfb Mon Sep 17 00:00:00 2001
From: Wesley Wigham <wwigham@gmail.com>
Date: Mon, 3 May 2021 12:46:44 -0700
Subject: [PATCH] Add single-element fastpath to getSupertypeOrUnion

---
 src/compiler/checker.ts | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 4d95f128b1a0d..7225159484145 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -19954,6 +19954,9 @@ namespace ts {
         // of those literal types. Otherwise, return the leftmost type for which no type to the
         // right is a supertype.
         function getSupertypeOrUnion(types: Type[]): Type {
+            if (types.length === 1) {
+                return types[0];
+            }
             return literalTypesWithSameBaseType(types) ?
                 getUnionType(types) :
                 reduceLeft(types, (s, t) => isTypeSubtypeOf(s, t) ? t : s)!;