From 815af7da175429166fff3ddfe54ccad74c6550b7 Mon Sep 17 00:00:00 2001
From: Andy <anhans@microsoft.com>
Date: Wed, 12 Jul 2017 07:45:02 -0700
Subject: [PATCH] getSwitchClauseTypes: exit early if getTypeOfSwitchClause is
 undefined (#16865)

---
 src/compiler/checker.ts | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 8bab67f7a38..e454b43d4a1 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11066,8 +11066,14 @@ namespace ts {
             if (!links.switchTypes) {
                 // If all case clauses specify expressions that have unit types, we return an array
                 // of those unit types. Otherwise we return an empty array.
-                const types = map(switchStatement.caseBlock.clauses, getTypeOfSwitchClause);
-                links.switchTypes = !contains(types, undefined) ? types : emptyArray;
+                links.switchTypes = [];
+                for (const clause of switchStatement.caseBlock.clauses) {
+                    const type = getTypeOfSwitchClause(clause);
+                    if (type === undefined) {
+                        return links.switchTypes = emptyArray;
+                    }
+                    links.switchTypes.push(type);
+                }
             }
             return links.switchTypes;
         }