From 4335f4462cefefdfd2068e4cdf8685a1de5bd3a9 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Mon, 28 Jan 2019 08:58:18 -0800
Subject: [PATCH] Tuples are known to be covaraint

---
 src/compiler/checker.ts | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 805fb77efad..7a037440b6c 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -13287,11 +13287,8 @@ namespace ts {
         }
 
         function getVariances(type: GenericType): Variance[] {
-            if (!strictFunctionTypes) {
-                return emptyArray;
-            }
-            if (type === globalArrayType || type === globalReadonlyArrayType) {
-                // Arrays are known to be covariant, no need to spend time computing this (emptyArray implies covariance for all parameters)
+            // Arrays and tuples are known to be covariant, no need to spend time computing this (emptyArray implies covariance for all parameters)
+            if (!strictFunctionTypes || type === globalArrayType || type === globalReadonlyArrayType || type.objectFlags & ObjectFlags.Tuple) {
                 return emptyArray;
             }
             return getVariancesWorker(type.typeParameters, type, getMarkerTypeReference);