From de8597ca8c31eec0e7c5e08bff149db8832fa978 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 20 May 2015 16:30:50 -0700
Subject: [PATCH] Add cache to instantiateType function to break cycles

---
 src/compiler/checker.ts | 14 ++++++++++++++
 src/compiler/types.ts   |  6 ++++++
 2 files changed, 20 insertions(+)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 9bd0d6475fe..e49deba6032 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -3866,6 +3866,19 @@ module ts {
         }
 
         function instantiateAnonymousType(type: ObjectType, mapper: TypeMapper): ObjectType {
+            // If this type has already been instantiated using this mapper, returned the cached result. This guards against
+            // infinite instantiations of cyclic types, e.g. "var x: { a: T, b: typeof x };"
+            if (mapper.mappings) {
+                for (let mapping of mapper.mappings) {
+                    if (mapping.type === type) {
+                        return mapping.result;
+                    }
+                }
+            }
+            else {
+                mapper.mappings = [];
+            }
+            // Instantiate the given type using the given mapper and cache the result
             let result = <ResolvedType>createObjectType(TypeFlags.Anonymous, type.symbol);
             result.properties = instantiateList(getPropertiesOfObjectType(type), mapper, instantiateSymbol);
             result.members = createSymbolTable(result.properties);
@@ -3875,6 +3888,7 @@ module ts {
             let numberIndexType = getIndexTypeOfType(type, IndexKind.Number);
             if (stringIndexType) result.stringIndexType = instantiateType(stringIndexType, mapper);
             if (numberIndexType) result.numberIndexType = instantiateType(numberIndexType, mapper);
+            mapper.mappings.push({ type, result });
             return result;
         }
 
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 86e680ca8b0..0f810c9fe61 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -1586,9 +1586,15 @@ module ts {
         Number,
     }
 
+    export interface TypeMapping {
+        type: Type;
+        result: Type;
+    }
+
     /* @internal */
     export interface TypeMapper {
         (t: TypeParameter): Type;
+        mappings?: TypeMapping[];  // Type mapping cache
     }
 
     /* @internal */