Avoid checking for optionality in comparability checks.

DanielRosenwasser · DanielRosenwasser · commit 375437ef3cb1 · 2016-11-12T12:59:43.000-08:00
diff --git 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..
