From a2928b8fd31ae3f0f3370a521daff506bf378087 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Thu, 30 Aug 2018 13:13:21 -0700
Subject: [PATCH] Optimize relationship checks for arrays and tuples

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

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 2bd5d40bffa..44c7eba605d 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11829,6 +11829,9 @@ namespace ts {
                             errorInfo = saveErrorInfo;
                         }
                     }
+                    else if (isTupleType(source) && (isArrayType(target) || isReadonlyArrayType(target)) || isArrayType(source) && isReadonlyArrayType(target)) {
+                        return isRelatedTo(getIndexTypeOfType(source, IndexKind.Number) || anyType, getIndexTypeOfType(target, IndexKind.Number) || anyType, reportErrors);
+                    }
                     // Even if relationship doesn't hold for unions, intersections, or generic type references,
                     // it may hold in a structural comparison.
                     // In a check of the form X = A & B, we will have previously checked if A relates to X or B relates