From 129a4f190808ee3bda05fa76e32d1d92d8f667aa Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Sat, 5 Mar 2016 16:16:19 -0800
Subject: [PATCH] Check return type includes 'undefined' in function with
 implicit return

---
 src/compiler/checker.ts              | 3 +++
 src/compiler/diagnosticMessages.json | 4 ++++
 2 files changed, 7 insertions(+)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 6d09cbffce8..f15af886c36 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11095,6 +11095,9 @@ namespace ts {
                 // NOTE: having returnType !== undefined is a precondition for entering this branch so func.type will always be present
                 error(func.type, Diagnostics.A_function_whose_declared_type_is_neither_void_nor_any_must_return_a_value);
             }
+            else if (returnType && strictNullChecks && !isTypeAssignableTo(undefinedType, returnType)) {
+                error(func.type, Diagnostics.Function_lacks_ending_return_statement_and_return_type_does_not_include_undefined);
+            }
             else if (compilerOptions.noImplicitReturns) {
                 if (!returnType) {
                     // If return type annotation is omitted check if function has any explicit return statements.
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 0b70ab98ce7..bd8701d9cea 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -1091,6 +1091,10 @@
         "category": "Error",
         "code": 2365
     },
+    "Function lacks ending return statement and return type does not include 'undefined'.": {
+        "category": "Error",
+        "code": 2366
+    },
     "Type parameter name cannot be '{0}'": {
         "category": "Error",
         "code": 2368