diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 16b2091733a..d2ae5b4cea8 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -580,6 +580,21 @@ namespace ts {
             Both = Source | Target,
         }
 
+        const enum TypeIncludes {
+            Any = 1 << 0,
+            Undefined = 1 << 1,
+            Null = 1 << 2,
+            Never = 1 << 3,
+            NonWideningType = 1 << 4,
+            String = 1 << 5,
+            Number = 1 << 6,
+            ESSymbol = 1 << 7,
+            LiteralOrUniqueESSymbol = 1 << 8,
+            ObjectType = 1 << 9,
+            EmptyObject = 1 << 10,
+            Union = 1 << 11,
+        }
+
         const enum MembersOrExportsResolutionKind {
             resolvedExports = "resolvedExports",
             resolvedMembers = "resolvedMembers"
@@ -5516,6 +5531,8 @@ namespace ts {
             sig.minArgumentCount = minArgumentCount;
             sig.hasRestParameter = hasRestParameter;
             sig.hasLiteralTypes = hasLiteralTypes;
+            sig.target = undefined;
+            sig.mapper = undefined;
             return sig;
         }
 
@@ -7419,21 +7436,6 @@ namespace ts {
             return links.resolvedType;
         }
 
-        interface TypeSet extends Array<Type> {
-            containsAny?: boolean;
-            containsUndefined?: boolean;
-            containsNull?: boolean;
-            containsNever?: boolean;
-            containsNonWideningType?: boolean;
-            containsString?: boolean;
-            containsNumber?: boolean;
-            containsESSymbol?: boolean;
-            containsLiteralOrUniqueESSymbol?: boolean;
-            containsObjectType?: boolean;
-            containsEmptyObject?: boolean;
-            unionIndex?: number;
-        }
-
         function getTypeId(type: Type) {
             return type.id;
         }
@@ -7458,28 +7460,28 @@ namespace ts {
             return false;
         }
 
-        function addTypeToUnion(typeSet: TypeSet, type: Type) {
+        function addTypeToUnion(typeSet: Type[], includes: TypeIncludes, type: Type) {
             const flags = type.flags;
             if (flags & TypeFlags.Union) {
-                addTypesToUnion(typeSet, (<UnionType>type).types);
+                includes = addTypesToUnion(typeSet, includes, (<UnionType>type).types);
             }
             else if (flags & TypeFlags.Any) {
-                typeSet.containsAny = true;
+                includes |= TypeIncludes.Any;
             }
             else if (!strictNullChecks && flags & TypeFlags.Nullable) {
-                if (flags & TypeFlags.Undefined) typeSet.containsUndefined = true;
-                if (flags & TypeFlags.Null) typeSet.containsNull = true;
-                if (!(flags & TypeFlags.ContainsWideningType)) typeSet.containsNonWideningType = true;
+                if (flags & TypeFlags.Undefined) includes |= TypeIncludes.Undefined;
+                if (flags & TypeFlags.Null) includes |= TypeIncludes.Null;
+                if (!(flags & TypeFlags.ContainsWideningType)) includes |= TypeIncludes.NonWideningType;
             }
             else if (!(flags & TypeFlags.Never || flags & TypeFlags.Intersection && isEmptyIntersectionType(<IntersectionType>type))) {
                 // We ignore 'never' types in unions. Likewise, we ignore intersections of unit types as they are
                 // another form of 'never' (in that they have an empty value domain). We could in theory turn
                 // intersections of unit types into 'never' upon construction, but deferring the reduction makes it
                 // easier to reason about their origin.
-                if (flags & TypeFlags.String) typeSet.containsString = true;
-                if (flags & TypeFlags.Number) typeSet.containsNumber = true;
-                if (flags & TypeFlags.ESSymbol) typeSet.containsESSymbol = true;
-                if (flags & TypeFlags.StringOrNumberLiteralOrUnique) typeSet.containsLiteralOrUniqueESSymbol = true;
+                if (flags & TypeFlags.String) includes |= TypeIncludes.String;
+                if (flags & TypeFlags.Number) includes |= TypeIncludes.Number;
+                if (flags & TypeFlags.ESSymbol) includes |= TypeIncludes.ESSymbol;
+                if (flags & TypeFlags.StringOrNumberLiteralOrUnique) includes |= TypeIncludes.LiteralOrUniqueESSymbol;
                 const len = typeSet.length;
                 const index = len && type.id > typeSet[len - 1].id ? ~len : binarySearch(typeSet, type, getTypeId, compareValues);
                 if (index < 0) {
@@ -7489,14 +7491,16 @@ namespace ts {
                     }
                 }
             }
+            return includes;
         }
 
         // Add the given types to the given type set. Order is preserved, duplicates are removed,
         // and nested types of the given kind are flattened into the set.
-        function addTypesToUnion(typeSet: TypeSet, types: Type[]) {
+        function addTypesToUnion(typeSet: Type[], includes: TypeIncludes, types: Type[]): TypeIncludes {
             for (const type of types) {
-                addTypeToUnion(typeSet, type);
+                includes = addTypeToUnion(typeSet, includes, type);
             }
+            return includes;
         }
 
         function containsIdenticalType(types: Type[], type: Type) {
@@ -7520,7 +7524,7 @@ namespace ts {
             return false;
         }
 
-        function isSetOfLiteralsFromSameEnum(types: TypeSet): boolean {
+        function isSetOfLiteralsFromSameEnum(types: Type[]): boolean {
             const first = types[0];
             if (first.flags & TypeFlags.EnumLiteral) {
                 const firstEnum = getParentOfSymbol(first.symbol);
@@ -7536,7 +7540,7 @@ namespace ts {
             return false;
         }
 
-        function removeSubtypes(types: TypeSet) {
+        function removeSubtypes(types: Type[]) {
             if (types.length === 0 || isSetOfLiteralsFromSameEnum(types)) {
                 return;
             }
@@ -7549,15 +7553,15 @@ namespace ts {
             }
         }
 
-        function removeRedundantLiteralTypes(types: TypeSet) {
+        function removeRedundantLiteralTypes(types: Type[], includes: TypeIncludes) {
             let i = types.length;
             while (i > 0) {
                 i--;
                 const t = types[i];
                 const remove =
-                    t.flags & TypeFlags.StringLiteral && types.containsString ||
-                    t.flags & TypeFlags.NumberLiteral && types.containsNumber ||
-                    t.flags & TypeFlags.UniqueESSymbol && types.containsESSymbol ||
+                    t.flags & TypeFlags.StringLiteral && includes & TypeIncludes.String ||
+                    t.flags & TypeFlags.NumberLiteral && includes & TypeIncludes.Number ||
+                    t.flags & TypeFlags.UniqueESSymbol && includes & TypeIncludes.ESSymbol ||
                     t.flags & TypeFlags.StringOrNumberLiteral && t.flags & TypeFlags.FreshLiteral && containsType(types, (<LiteralType>t).regularType);
                 if (remove) {
                     orderedRemoveItemAt(types, i);
@@ -7579,15 +7583,15 @@ namespace ts {
             if (types.length === 1) {
                 return types[0];
             }
-            const typeSet = [] as TypeSet;
-            addTypesToUnion(typeSet, types);
-            if (typeSet.containsAny) {
+            const typeSet: Type[] = [];
+            const includes = addTypesToUnion(typeSet, 0, types);
+            if (includes & TypeIncludes.Any) {
                 return anyType;
             }
             switch (unionReduction) {
                 case UnionReduction.Literal:
-                    if (typeSet.containsLiteralOrUniqueESSymbol) {
-                        removeRedundantLiteralTypes(typeSet);
+                    if (includes & TypeIncludes.LiteralOrUniqueESSymbol) {
+                        removeRedundantLiteralTypes(typeSet, includes);
                     }
                     break;
                 case UnionReduction.Subtype:
@@ -7595,8 +7599,8 @@ namespace ts {
                     break;
             }
             if (typeSet.length === 0) {
-                return typeSet.containsNull ? typeSet.containsNonWideningType ? nullType : nullWideningType :
-                    typeSet.containsUndefined ? typeSet.containsNonWideningType ? undefinedType : undefinedWideningType :
+                return includes & TypeIncludes.Null ? includes & TypeIncludes.NonWideningType ? nullType : nullWideningType :
+                    includes & TypeIncludes.Undefined ? includes & TypeIncludes.NonWideningType ? undefinedType : undefinedWideningType :
                         neverType;
             }
             return getUnionTypeFromSortedList(typeSet, aliasSymbol, aliasTypeArguments);
@@ -7674,39 +7678,42 @@ namespace ts {
             return links.resolvedType;
         }
 
-        function addTypeToIntersection(typeSet: TypeSet, type: Type) {
-            if (type.flags & TypeFlags.Intersection) {
-                addTypesToIntersection(typeSet, (<IntersectionType>type).types);
+        function addTypeToIntersection(typeSet: Type[], includes: TypeIncludes, type: Type) {
+            const flags = type.flags;
+            if (flags & TypeFlags.Intersection) {
+                includes = addTypesToIntersection(typeSet, includes, (<IntersectionType>type).types);
             }
-            else if (type.flags & TypeFlags.Any) {
-                typeSet.containsAny = true;
+            else if (flags & TypeFlags.Any) {
+                includes |= TypeIncludes.Any;
             }
-            else if (type.flags & TypeFlags.Never) {
-                typeSet.containsNever = true;
+            else if (flags & TypeFlags.Never) {
+                includes |= TypeIncludes.Never;
             }
             else if (getObjectFlags(type) & ObjectFlags.Anonymous && isEmptyObjectType(type)) {
-                typeSet.containsEmptyObject = true;
+                includes |= TypeIncludes.EmptyObject;
             }
-            else if ((strictNullChecks || !(type.flags & TypeFlags.Nullable)) && !contains(typeSet, type)) {
-                if (type.flags & TypeFlags.Object) {
-                    typeSet.containsObjectType = true;
+            else if ((strictNullChecks || !(flags & TypeFlags.Nullable)) && !contains(typeSet, type)) {
+                if (flags & TypeFlags.Object) {
+                    includes |= TypeIncludes.ObjectType;
                 }
-                if (type.flags & TypeFlags.Union && typeSet.unionIndex === undefined) {
-                    typeSet.unionIndex = typeSet.length;
+                if (flags & TypeFlags.Union) {
+                    includes |= TypeIncludes.Union;
                 }
-                if (!(type.flags & TypeFlags.Object && (<ObjectType>type).objectFlags & ObjectFlags.Anonymous &&
+                if (!(flags & TypeFlags.Object && (<ObjectType>type).objectFlags & ObjectFlags.Anonymous &&
                     type.symbol && type.symbol.flags & (SymbolFlags.Function | SymbolFlags.Method) && containsIdenticalType(typeSet, type))) {
                     typeSet.push(type);
                 }
             }
+            return includes;
         }
 
         // Add the given types to the given type set. Order is preserved, freshness is removed from literal
         // types, duplicates are removed, and nested types of the given kind are flattened into the set.
-        function addTypesToIntersection(typeSet: TypeSet, types: Type[]) {
+        function addTypesToIntersection(typeSet: Type[], includes: TypeIncludes, types: Type[]) {
             for (const type of types) {
-                addTypeToIntersection(typeSet, getRegularTypeOfLiteralType(type));
+                includes = addTypeToIntersection(typeSet, includes, getRegularTypeOfLiteralType(type));
             }
+            return includes;
         }
 
         // We normalize combinations of intersection and union types based on the distributive property of the '&'
@@ -7723,27 +7730,27 @@ namespace ts {
             if (types.length === 0) {
                 return emptyObjectType;
             }
-            const typeSet = [] as TypeSet;
-            addTypesToIntersection(typeSet, types);
-            if (typeSet.containsNever) {
+            const typeSet: Type[] = [];
+            const includes = addTypesToIntersection(typeSet, 0, types);
+            if (includes & TypeIncludes.Never) {
                 return neverType;
             }
-            if (typeSet.containsAny) {
+            if (includes & TypeIncludes.Any) {
                 return anyType;
             }
-            if (typeSet.containsEmptyObject && !typeSet.containsObjectType) {
+            if (includes & TypeIncludes.EmptyObject && !(includes & TypeIncludes.ObjectType)) {
                 typeSet.push(emptyObjectType);
             }
             if (typeSet.length === 1) {
                 return typeSet[0];
             }
-            const unionIndex = typeSet.unionIndex;
-            if (unionIndex !== undefined) {
+            if (includes & TypeIncludes.Union) {
                 // We are attempting to construct a type of the form X & (A | B) & Y. Transform this into a type of
                 // the form X & A & Y | X & B & Y and recursively reduce until no union type constituents remain.
+                const unionIndex = findIndex(typeSet, t => (t.flags & TypeFlags.Union) !== 0);
                 const unionType = <UnionType>typeSet[unionIndex];
                 return getUnionType(map(unionType.types, t => getIntersectionType(replaceElement(typeSet, unionIndex, t))),
-                UnionReduction.Literal, aliasSymbol, aliasTypeArguments);
+                    UnionReduction.Literal, aliasSymbol, aliasTypeArguments);
             }
             const id = getTypeListId(typeSet);
             let type = intersectionTypes.get(id);