From f774ecf4ec1e86b83f251720467d8f4732bf244f Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Thu, 10 Mar 2016 14:30:42 -0800
Subject: [PATCH] Remove 'undefined' from type of binding element with
 non-undefined default value

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

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 3511305e168..10e4d50d930 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -2678,6 +2678,11 @@ namespace ts {
                     type = createArrayType(elementType);
                 }
             }
+            // In strict null checking mode, if a default value of a non-undefined type is specified, remove
+            // undefined from the final type.
+            if (strictNullChecks && declaration.initializer && !(getNullableKind(checkExpressionCached(declaration.initializer)) & TypeFlags.Undefined)) {
+                type = removeNullableKind(type, TypeFlags.Undefined);
+            }
             return type;
         }