From c80f532e01a9a23e957231d3121a435465ea8d59 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Mon, 8 Oct 2018 09:59:47 -0700
Subject: [PATCH] Add missing check in getTypeAtSwitchClause

---
 src/compiler/checker.ts | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 852bf39c16d..c37066858a0 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -14972,9 +14972,12 @@ namespace ts {
             }
 
             function getTypeAtSwitchClause(flow: FlowSwitchClause): FlowType {
+                const expr = flow.switchStatement.expression;
+                if (containsMatchingReferenceDiscriminant(reference, expr)) {
+                    return declaredType;
+                }
                 const flowType = getTypeAtFlowNode(flow.antecedent);
                 let type = getTypeFromFlowType(flowType);
-                const expr = flow.switchStatement.expression;
                 if (isMatchingReference(reference, expr)) {
                     type = narrowTypeBySwitchOnDiscriminant(type, flow.switchStatement, flow.clauseStart, flow.clauseEnd);
                 }