Add cache of anonymous object type instantiations to TypeMapper

Author: ahejlsberg

src/compiler/checker.ts

@@ -4487,6 +4487,15 @@ namespace ts {
         }
 
         function instantiateAnonymousType(type: ObjectType, mapper: TypeMapper): ObjectType {
+            if (mapper.instantiations) {
+                let cachedType = mapper.instantiations[type.id];
+                if (cachedType) {
+                    return cachedType;
+                }
+            }
+            else {
+                mapper.instantiations = [];
+            }
             // Mark the anonymous type as instantiated such that our infinite instantiation detection logic can recognize it
             let result = <ResolvedType>createObjectType(TypeFlags.Anonymous | TypeFlags.Instantiated, type.symbol);
             result.properties = instantiateList(getPropertiesOfObjectType(type), mapper, instantiateSymbol);
@@ -4497,6 +4506,7 @@ namespace ts {
             let numberIndexType = getIndexTypeOfType(type, IndexKind.Number);
             if (stringIndexType) result.stringIndexType = instantiateType(stringIndexType, mapper);
             if (numberIndexType) result.numberIndexType = instantiateType(numberIndexType, mapper);
+            mapper.instantiations[type.id] = result;
             return result;
         }
 

src/compiler/types.ts

@@ -1953,6 +1953,7 @@ namespace ts {
     /* @internal */
     export interface TypeMapper {
         (t: TypeParameter): Type;
+        instantiations?: Type[];    // Cache of instantiations created using this type mapper.
         context?: InferenceContext; // The inference context this mapper was created from.
                                     // Only inference mappers have this set (in createInferenceMapper).
                                     // The identity mapper and regular instantiation mappers do not need it.