From e3b4c9097006bca2d491c751bd5cadf026cf95f3 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Sat, 22 Apr 2017 17:08:52 -0700
Subject: [PATCH] Properly account for 'this' parameter in type predicate
 checks

---
 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 cb28cb0bd5c..8549f63df6f 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11565,7 +11565,7 @@ namespace ts {
                 }
 
                 if (isIdentifierTypePredicate(predicate)) {
-                    const predicateArgument = callExpression.arguments[predicate.parameterIndex];
+                    const predicateArgument = callExpression.arguments[predicate.parameterIndex - (signature.thisParameter ? 1 : 0)];
                     if (predicateArgument) {
                         if (isMatchingReference(reference, predicateArgument)) {
                             return getNarrowedType(type, predicate.type, assumeTrue, isTypeSubtypeOf);