From d37d50afb0a8f19e79445eec020bbe00d363cea8 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 1 Jul 2016 09:18:43 -0700
Subject: [PATCH] Ensure const enum members with same value have same type
 identity

---
 src/compiler/checker.ts | 17 ++++++++++++++---
 src/compiler/types.ts   |  1 +
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 4493682ed51..7d7ed2afb52 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -3667,12 +3667,23 @@ namespace ts {
             return links.declaredType;
         }
 
+        function createEnumType(symbol: Symbol): Type {
+            const type = createType(TypeFlags.Enum);
+            type.symbol = symbol;
+            return type;
+        }
+
+        function getEnumMemberType(symbol: Symbol): Type {
+            const links = getSymbolLinks(getParentOfSymbol(symbol));
+            const map = links.enumMemberTypes || (links.enumMemberTypes = {});
+            const value = "" + getEnumMemberValue(<EnumMember>symbol.valueDeclaration);
+            return map[value] || (map[value] = createEnumType(symbol));
+        }
+
         function getDeclaredTypeOfEnum(symbol: Symbol): Type {
             const links = getSymbolLinks(symbol);
             if (!links.declaredType) {
-                const type = createType(TypeFlags.Enum);
-                type.symbol = symbol;
-                links.declaredType = type;
+                links.declaredType = symbol.flags & SymbolFlags.EnumMember ? getEnumMemberType(symbol) : createEnumType(symbol);
             }
             return links.declaredType;
         }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 486f1e82a97..3dedd5da1e3 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2136,6 +2136,7 @@ namespace ts {
         isDeclarationWithCollidingName?: boolean;    // True if symbol is block scoped redeclaration
         bindingElement?: BindingElement;    // Binding element associated with property symbol
         exportsSomeValue?: boolean;         // True if module exports some value (not just types)
+        enumMemberTypes?: Map<Type>;        // Enum member types indexed by enum value
     }
 
     /* @internal */