Revise intersection optimizations to include string, number and symbol

src/compiler/checker.ts

@@ -8533,6 +8533,15 @@ namespace ts {
             return binarySearch(types, type, getTypeId, compareValues) >= 0;
         }
 
+        function insertType(types: Type[], type: Type): boolean {
+            const index = binarySearch(types, type, getTypeId, compareValues);
+            if (index < 0) {
+                types.splice(~index, 0, type);
+                return true;
+            }
+            return false;
+        }
+
         // Return true if the given intersection type contains
         // more than one unit type or,
         // an object type and a nullable type (null or undefined), or
@@ -8700,7 +8709,7 @@ namespace ts {
                     includes & TypeFlags.Undefined ? includes & TypeFlags.NonWideningType ? undefinedType : undefinedWideningType :
                         neverType;
             }
-            return getUnionTypeFromSortedList(typeSet, includes & TypeFlags.NotUnit ? 0 : TypeFlags.UnionOfUnitTypes, aliasSymbol, aliasTypeArguments);
+            return getUnionTypeFromSortedList(typeSet, includes & TypeFlags.NotPrimitiveUnion ? 0 : TypeFlags.UnionOfPrimitiveTypes, aliasSymbol, aliasTypeArguments);
         }
 
         function getUnionTypePredicate(signatures: ReadonlyArray<Signature>): TypePredicate | undefined {
@@ -8823,26 +8832,62 @@ namespace ts {
             }
         }
 
-        // When intersecting unions of unit types we can simply intersect based on type identity.
-        // Here we remove all unions of unit types from the given list and replace them with a
-        // a single union containing an intersection of the unit types.
-        function intersectUnionsOfUnitTypes(types: Type[]) {
-            const unionIndex = findIndex(types, t => (t.flags & TypeFlags.UnionOfUnitTypes) !== 0);
-            const unionType = <UnionType>types[unionIndex];
-            let intersection = unionType.types;
-            let i = types.length - 1;
-            while (i > unionIndex) {
+        // Check that the given type has a match in every union. A given type is matched by
+        // an identical type, and a literal type is additionally matched by its corresponding
+        // primitive type.
+        function eachUnionContains(unionTypes: UnionType[], type: Type) {
+            for (const u of unionTypes) {
+                if (!containsType(u.types, type)) {
+                    const primitive = type.flags & TypeFlags.StringLiteral ? stringType :
+                        type.flags & TypeFlags.NumberLiteral ? numberType :
+                        type.flags & TypeFlags.UniqueESSymbol ? esSymbolType :
+                        undefined;
+                    if (!primitive || !containsType(u.types, primitive)) {
+                        return false;
+                    }
+                }
+            }
+            return true;
+        }
+
+        // Remove all unions of primitive types from the given list and replace them with a
+        // single union containing an intersection of those primitive types.
+        function intersectUnionsOfPrimitiveTypes(types: Type[]) {
+            let unionTypes: UnionType[] | undefined;
+            const index = findIndex(types, t => (t.flags & TypeFlags.UnionOfPrimitiveTypes) !== 0);
+            let i = index + 1;
+            // Remove all but the first union of primitive types and collect them in
+            // the unionTypes array.
+            while (i < types.length) {
                 const t = types[i];
-                if (t.flags & TypeFlags.UnionOfUnitTypes) {
-                    intersection = filter(intersection, u => containsType((<UnionType>t).types, u));
+                if (t.flags & TypeFlags.UnionOfPrimitiveTypes) {
+                    (unionTypes || (unionTypes = [<UnionType>types[index]])).push(<UnionType>t);
                     orderedRemoveItemAt(types, i);
                 }
-                i--;
+                else {
+                    i++;
+                }
             }
-            if (intersection === unionType.types) {
+            // Return false if there was only one union of primitive types
+            if (!unionTypes) {
                 return false;
             }
-            types[unionIndex] = getUnionTypeFromSortedList(intersection, unionType.flags & TypeFlags.UnionOfUnitTypes);
+            // We have more than one union of primitive types, now intersect them. For each
+            // type in each union we check if the type is matched in every union and if so
+            // we include it in the result.
+            const checked: Type[] = [];
+            const result: Type[] = [];
+            for (const u of unionTypes) {
+                for (const t of u.types) {
+                    if (insertType(checked, t)) {
+                        if (eachUnionContains(unionTypes, t)) {
+                            insertType(result, t);
+                        }
+                    }
+                }
+            }
+            // Finally replace the first union with the result
+            types[index] = getUnionTypeFromSortedList(result, TypeFlags.UnionOfPrimitiveTypes);
             return true;
         }
 
@@ -8883,7 +8928,7 @@ namespace ts {
                 return typeSet[0];
             }
             if (includes & TypeFlags.Union) {
-                if (includes & TypeFlags.UnionOfUnitTypes && intersectUnionsOfUnitTypes(typeSet)) {
+                if (includes & TypeFlags.UnionOfPrimitiveTypes && intersectUnionsOfPrimitiveTypes(typeSet)) {
                     // When the intersection creates a reduced set (which might mean that *all* union types have
                     // disappeared), we restart the operation to get a new set of combined flags. Once we have
                     // reduced we'll never reduce again, so this occurs at most once.
@@ -13980,7 +14025,7 @@ namespace ts {
             if (type.flags & TypeFlags.Union) {
                 const types = (<UnionType>type).types;
                 const filtered = filter(types, f);
-                return filtered === types ? type : getUnionTypeFromSortedList(filtered, type.flags & TypeFlags.UnionOfUnitTypes);
+                return filtered === types ? type : getUnionTypeFromSortedList(filtered, type.flags & TypeFlags.UnionOfPrimitiveTypes);
             }
             return f(type) ? type : neverType;
         }

src/compiler/types.ts

@@ -3675,7 +3675,7 @@ namespace ts {
         /* @internal */
         FreshLiteral            = 1 << 25,  // Fresh literal or unique type
         /* @internal */
-        UnionOfUnitTypes        = 1 << 26,  // Type is union of unit types
+        UnionOfPrimitiveTypes   = 1 << 26,  // Type is union of primitive types
         /* @internal */
         ContainsWideningType    = 1 << 27,  // Type is or contains undefined or null widening type
         /* @internal */
@@ -3720,7 +3720,7 @@ namespace ts {
         Narrowable = Any | Unknown | StructuredOrInstantiable | StringLike | NumberLike | BooleanLike | ESSymbol | UniqueESSymbol | NonPrimitive,
         NotUnionOrUnit = Any | Unknown | ESSymbol | Object | NonPrimitive,
         /* @internal */
-        NotUnit = Any | String | Number | Boolean | Enum | ESSymbol | Void | Never | StructuredOrInstantiable,
+        NotPrimitiveUnion = Any | Unknown | Enum | Void | Never | StructuredOrInstantiable,
         /* @internal */
         RequiresWidening = ContainsWideningType | ContainsObjectLiteral,
         /* @internal */