From 720964f729b816b0cc4f644c325ec45413695735 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Sat, 5 Oct 2019 19:14:48 -0700
Subject: [PATCH] Fix bounds check

---
 src/compiler/checker.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index e791258c48f..0e46fd76497 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -19106,7 +19106,7 @@ namespace ts {
                         const flowType = getTypeAtFlowNode(flow.antecedent);
                         const type = getTypeFromFlowType(flowType);
                         const narrowedType = predicate.type ? narrowTypeByTypePredicate(type, predicate, flow.node, /*assumeTrue*/ true) :
-                            predicate.kind === TypePredicateKind.AssertsIdentifier && predicate.parameterIndex < flow.node.arguments.length ? narrowTypeByAssertion(type, flow.node.arguments[predicate.parameterIndex]) :
+                            predicate.kind === TypePredicateKind.AssertsIdentifier && predicate.parameterIndex >= 0 && predicate.parameterIndex < flow.node.arguments.length ? narrowTypeByAssertion(type, flow.node.arguments[predicate.parameterIndex]) :
                             type;
                         return narrowedType === type ? flowType : createFlowType(narrowedType, isIncomplete(flowType));
                     }