From ea55de3e988cbc291f88adfd1eae48619a7cfcc6 Mon Sep 17 00:00:00 2001
From: Nathan Shively-Sanders <293473+sandersn@users.noreply.github.com>
Date: Mon, 6 Nov 2017 15:44:16 -0800
Subject: [PATCH] Eagerly fall back to TupleBase

---
 src/compiler/checker.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 7aedfa43a5d..264b5dad105 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4993,7 +4993,7 @@ namespace ts {
         function getBaseTypes(type: InterfaceType): BaseType[] {
             if (!type.resolvedBaseTypes) {
                 if (type.objectFlags & ObjectFlags.Tuple) {
-                    type.resolvedBaseTypes = [createTypeFromGenericGlobalType(globalTupleBaseType || globalArrayType, [getUnionType(type.typeParameters)])];
+                    type.resolvedBaseTypes = [createTypeFromGenericGlobalType(globalTupleBaseType, [getUnionType(type.typeParameters)])];
                 }
                 else if (type.symbol.flags & (SymbolFlags.Class | SymbolFlags.Interface)) {
                     if (type.symbol.flags & SymbolFlags.Class) {
@@ -24526,7 +24526,7 @@ namespace ts {
 
             // TODO: ReadonlyArray and TupleBase should always be available, but haven't been required previously
             globalReadonlyArrayType = <GenericType>getGlobalTypeOrUndefined("ReadonlyArray" as __String, /*arity*/ 1);
-            globalTupleBaseType = <GenericType>getGlobalTypeOrUndefined("TupleBase" as __String, /*arity*/ 1);
+            globalTupleBaseType = <GenericType>getGlobalTypeOrUndefined("TupleBase" as __String, /*arity*/ 1) || globalArrayType;
             anyReadonlyArrayType = globalReadonlyArrayType ? createTypeFromGenericGlobalType(globalReadonlyArrayType, [anyType]) : anyArrayType;
             globalThisType = <GenericType>getGlobalTypeOrUndefined("ThisType" as __String, /*arity*/ 1);
         }