Properly narrow union types containing string and number

Author: ahejlsberg

src/compiler/checker.ts

@@ -8461,6 +8461,28 @@ namespace ts {
             return f(type) ? type : neverType;
         }
 
+        function mapType(type: Type, f: (t: Type) => Type): Type {
+            return type.flags & TypeFlags.Union ? getUnionType(map((<UnionType>type).types, f)) : f(type);
+        }
+
+        function extractTypesOfKind(type: Type, kind: TypeFlags) {
+            return filterType(type, t => (t.flags & kind) !== 0);
+        }
+
+        // Return a new type in which occurrences of the string and number primitive types in
+        // typeWithPrimitives have been replaced with occurrences of string literals and numeric
+        // literals in typeWithLiterals, respectively.
+        function replacePrimitivesWithLiterals(typeWithPrimitives: Type, typeWithLiterals: Type) {
+            if (isTypeSubsetOf(stringType, typeWithPrimitives) && maybeTypeOfKind(typeWithLiterals, TypeFlags.StringLiteral) ||
+                isTypeSubsetOf(numberType, typeWithPrimitives) && maybeTypeOfKind(typeWithLiterals, TypeFlags.NumberLiteral)) {
+                return mapType(typeWithPrimitives, t =>
+                    t.flags & TypeFlags.String ? extractTypesOfKind(typeWithLiterals, TypeFlags.String | TypeFlags.StringLiteral) :
+                    t.flags & TypeFlags.Number ? extractTypesOfKind(typeWithLiterals, TypeFlags.Number | TypeFlags.NumberLiteral) :
+                    t);
+            }
+            return typeWithPrimitives;
+        }
+
         function isIncomplete(flowType: FlowType) {
             return flowType.flags === 0;
         }
@@ -8791,16 +8813,12 @@ namespace ts {
                             assumeTrue ? TypeFacts.EQUndefined : TypeFacts.NEUndefined;
                     return getTypeWithFacts(type, facts);
                 }
-                if (type.flags & TypeFlags.String && isTypeOfKind(valueType, TypeFlags.StringLiteral) ||
-                    type.flags & TypeFlags.Number && isTypeOfKind(valueType, TypeFlags.NumberLiteral)) {
-                    return assumeTrue? valueType : type;
-                }
                 if (type.flags & TypeFlags.NotUnionOrUnit) {
                     return type;
                 }
                 if (assumeTrue) {
                     const narrowedType = filterType(type, t => areTypesComparable(t, valueType));
-                    return narrowedType.flags & TypeFlags.Never ? type : narrowedType;
+                    return narrowedType.flags & TypeFlags.Never ? type : replacePrimitivesWithLiterals(narrowedType, valueType);
                 }
                 if (isUnitType(valueType)) {
                     const regularType = getRegularTypeOfLiteralType(valueType);
@@ -8849,9 +8867,7 @@ namespace ts {
                 const discriminantType = getUnionType(clauseTypes);
                 const caseType =
                     discriminantType.flags & TypeFlags.Never ? neverType :
-                    type.flags & TypeFlags.String && isTypeOfKind(discriminantType, TypeFlags.StringLiteral) ? discriminantType :
-                    type.flags & TypeFlags.Number && isTypeOfKind(discriminantType, TypeFlags.NumberLiteral) ? discriminantType :
-                    filterType(type, t => isTypeComparableTo(discriminantType, t));
+                    replacePrimitivesWithLiterals(filterType(type, t => isTypeComparableTo(discriminantType, t)), discriminantType);
                 if (!hasDefaultClause) {
                     return caseType;
                 }

src/compiler/types.ts

@@ -2644,7 +2644,7 @@ namespace ts {
         // 'Narrowable' types are types where narrowing actually narrows.
         // This *should* be every type other than null, undefined, void, and never
         Narrowable = Any | StructuredType | TypeParameter | StringLike | NumberLike | BooleanLike | ESSymbol,
-        NotUnionOrUnit = Any | String | Number | ESSymbol | ObjectType,
+        NotUnionOrUnit = Any | ESSymbol | ObjectType,
         /* @internal */
         RequiresWidening = ContainsWideningType | ContainsObjectLiteral,
         /* @internal */