From 375437ef3cb149d49b560b0b30aa5c305b9c6eaa Mon Sep 17 00:00:00 2001
From: Daniel Rosenwasser <Daniel.Rosenwasser@microsoft.com>
Date: Sat, 12 Nov 2016 12:59:43 -0800
Subject: [PATCH] Avoid checking for optionality in comparability checks.

---
 src/compiler/checker.ts | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 789f2bd8104..6be5c1c453f 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -7331,7 +7331,8 @@ namespace ts {
                                 return Ternary.False;
                             }
                             result &= related;
-                            if (sourceProp.flags & SymbolFlags.Optional && !(targetProp.flags & SymbolFlags.Optional)) {
+                            // When checking for comparability, be more lenient with optional properties.
+                            if (relation !== comparableRelation && sourceProp.flags & SymbolFlags.Optional && !(targetProp.flags & SymbolFlags.Optional)) {
                                 // TypeScript 1.0 spec (April 2014): 3.8.3
                                 // S is a subtype of a type T, and T is a supertype of S if ...
                                 // S' and T are object types and, for each member M in T..