Defer resolution of the true and false branches of conditional types

Author: ahejlsberg

src/compiler/checker.ts

@@ -394,7 +394,6 @@ namespace ts {
         const intersectionTypes = createMap<IntersectionType>();
         const literalTypes = createMap<LiteralType>();
         const indexedAccessTypes = createMap<IndexedAccessType>();
-        const conditionalTypes = createMap<Type>();
         const substitutionTypes = createMap<SubstitutionType>();
         const evolvingArrayTypes: EvolvingArrayType[] = [];
         const undefinedProperties = createMap<Symbol>() as UnderscoreEscapedMap<Symbol>;
@@ -3648,8 +3647,8 @@ namespace ts {
                     context.inferTypeParameters = (<ConditionalType>type).root.inferTypeParameters;
                     const extendsTypeNode = typeToTypeNodeHelper((<ConditionalType>type).extendsType, context);
                     context.inferTypeParameters = saveInferTypeParameters;
-                    const trueTypeNode = typeToTypeNodeHelper((<ConditionalType>type).trueType, context);
-                    const falseTypeNode = typeToTypeNodeHelper((<ConditionalType>type).falseType, context);
+                    const trueTypeNode = typeToTypeNodeHelper(getTrueTypeFromConditionalType(<ConditionalType>type), context);
+                    const falseTypeNode = typeToTypeNodeHelper(getFalseTypeFromConditionalType(<ConditionalType>type), context);
                     context.approximateLength += 15;
                     return createConditionalTypeNode(checkTypeNode, extendsTypeNode, trueTypeNode, falseTypeNode);
                 }
@@ -7674,7 +7673,7 @@ namespace ts {
                 // just `any`. This result is _usually_ unwanted - so instead here we elide an `any` branch from the constraint type,
                 // in effect treating `any` like `never` rather than `unknown` in this location.
                 const trueConstraint = getInferredTrueTypeFromConditionalType(type);
-                const falseConstraint = type.falseType;
+                const falseConstraint = getFalseTypeFromConditionalType(type);
                 type.resolvedDefaultConstraint = isTypeAny(trueConstraint) ? falseConstraint : isTypeAny(falseConstraint) ? trueConstraint : getUnionType([trueConstraint, falseConstraint]);
             }
             return type.resolvedDefaultConstraint;
@@ -10377,37 +10376,23 @@ namespace ts {
             if (checkType === wildcardType || extendsType === wildcardType) {
                 return wildcardType;
             }
-            const trueType = instantiateType(root.trueType, mapper);
-            const falseType = instantiateType(root.falseType, mapper);
-            const instantiationId = `${root.isDistributive ? "d" : ""}${getTypeId(checkType)}>${getTypeId(extendsType)}?${getTypeId(trueType)}:${getTypeId(falseType)}`;
-            const result = conditionalTypes.get(instantiationId);
-            if (result) {
-                return result;
-            }
-            const newResult = getConditionalTypeWorker(root, mapper, checkType, extendsType, trueType, falseType);
-            conditionalTypes.set(instantiationId, newResult);
-            return newResult;
-        }
-
-        function getConditionalTypeWorker(root: ConditionalRoot, mapper: TypeMapper | undefined, checkType: Type, extendsType: Type, trueType: Type, falseType: Type) {
             // Simplifications for types of the form `T extends U ? T : never` and `T extends U ? never : T`.
-            if (falseType.flags & TypeFlags.Never && getActualTypeVariable(trueType) === getActualTypeVariable(checkType)) {
+            if (root.falseType.flags & TypeFlags.Never && getActualTypeVariable(root.trueType) === getActualTypeVariable(root.checkType)) {
                 if (checkType.flags & TypeFlags.Any || isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(extendsType))) { // Always true
-                    return trueType;
+                    return instantiateType(root.trueType, mapper);
                 }
                 else if (isIntersectionEmpty(checkType, extendsType)) { // Always false
                     return neverType;
                 }
             }
-            else if (trueType.flags & TypeFlags.Never && getActualTypeVariable(falseType) === getActualTypeVariable(checkType)) {
+            else if (root.trueType.flags & TypeFlags.Never && getActualTypeVariable(root.falseType) === getActualTypeVariable(root.checkType)) {
                 if (!(checkType.flags & TypeFlags.Any) && isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(extendsType))) { // Always true
                     return neverType;
                 }
                 else if (checkType.flags & TypeFlags.Any || isIntersectionEmpty(checkType, extendsType)) { // Always false
-                    return falseType;
+                    return instantiateType(root.falseType, mapper);
                 }
             }
-
             const checkTypeInstantiable = maybeTypeOfKind(checkType, TypeFlags.Instantiable | TypeFlags.GenericMappedType);
             let combinedMapper: TypeMapper | undefined;
             if (root.inferTypeParameters) {
@@ -10425,49 +10410,55 @@ namespace ts {
             // We attempt to resolve the conditional type only when the check and extends types are non-generic
             if (!checkTypeInstantiable && !maybeTypeOfKind(inferredExtendsType, TypeFlags.Instantiable | TypeFlags.GenericMappedType)) {
                 if (inferredExtendsType.flags & TypeFlags.AnyOrUnknown) {
-                    return combinedMapper ? instantiateType(root.trueType, combinedMapper) : trueType;
+                    return instantiateType(root.trueType, combinedMapper || mapper);
                 }
                 // Return union of trueType and falseType for 'any' since it matches anything
                 if (checkType.flags & TypeFlags.Any) {
-                    return getUnionType([combinedMapper ? instantiateType(root.trueType, combinedMapper) : trueType, falseType]);
+                    return getUnionType([instantiateType(root.trueType, combinedMapper || mapper), instantiateType(root.falseType, mapper)]);
                 }
                 // Return falseType for a definitely false extends check. We check an instantiations of the two
                 // types with type parameters mapped to the wildcard type, the most permissive instantiations
                 // possible (the wildcard type is assignable to and from all types). If those are not related,
                 // then no instantiations will be and we can just return the false branch type.
                 if (!isTypeAssignableTo(getPermissiveInstantiation(checkType), getPermissiveInstantiation(inferredExtendsType))) {
-                    return falseType;
+                    return instantiateType(root.falseType, mapper);
                 }
                 // Return trueType for a definitely true extends check. We check instantiations of the two
                 // types with type parameters mapped to their restrictive form, i.e. a form of the type parameter
                 // that has no constraint. This ensures that, for example, the type
                 //   type Foo<T extends { x: any }> = T extends { x: string } ? string : number
                 // doesn't immediately resolve to 'string' instead of being deferred.
                 if (isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(inferredExtendsType))) {
-                    return combinedMapper ? instantiateType(root.trueType, combinedMapper) : trueType;
+                    return instantiateType(root.trueType, combinedMapper || mapper);
                 }
             }
             // Return a deferred type for a check that is neither definitely true nor definitely false
-            return getDeferredConditionalType(root, mapper, combinedMapper, checkType, extendsType, trueType, falseType);
+            return getDeferredConditionalType(root, mapper, combinedMapper, checkType, extendsType);
         }
 
-        function getDeferredConditionalType(root: ConditionalRoot, mapper: TypeMapper | undefined, combinedMapper: TypeMapper | undefined, checkType: Type, extendsType: Type, trueType: Type, falseType: Type) {
+        function getDeferredConditionalType(root: ConditionalRoot, mapper: TypeMapper | undefined, combinedMapper: TypeMapper | undefined, checkType: Type, extendsType: Type) {
             const erasedCheckType = getActualTypeVariable(checkType);
             const result = <ConditionalType>createType(TypeFlags.Conditional);
             result.root = root;
             result.checkType = erasedCheckType;
             result.extendsType = extendsType;
             result.mapper = mapper;
             result.combinedMapper = combinedMapper;
-            result.trueType = trueType;
-            result.falseType = falseType;
             result.aliasSymbol = root.aliasSymbol;
             result.aliasTypeArguments = instantiateTypes(root.aliasTypeArguments, mapper!); // TODO: GH#18217
             return result;
         }
 
+        function getTrueTypeFromConditionalType(type: ConditionalType) {
+            return type.resolvedTrueType || (type.resolvedTrueType = instantiateType(type.root.trueType, type.mapper));
+        }
+
+        function getFalseTypeFromConditionalType(type: ConditionalType) {
+            return type.resolvedFalseType || (type.resolvedFalseType = instantiateType(type.root.falseType, type.mapper));
+        }
+
         function getInferredTrueTypeFromConditionalType(type: ConditionalType) {
-            return type.resolvedInferredTrueType || (type.resolvedInferredTrueType = instantiateType(type.root.trueType, type.combinedMapper || type.mapper));
+            return type.resolvedInferredTrueType || (type.resolvedInferredTrueType = type.combinedMapper ? instantiateType(type.root.trueType, type.combinedMapper) : getTrueTypeFromConditionalType(type));
         }
 
         function getInferTypeParameters(node: ConditionalTypeNode): TypeParameter[] | undefined {
@@ -12987,8 +12978,8 @@ namespace ts {
                         if ((<ConditionalType>source).root.isDistributive === (<ConditionalType>target).root.isDistributive) {
                             if (result = isRelatedTo((<ConditionalType>source).checkType, (<ConditionalType>target).checkType, /*reportErrors*/ false)) {
                                 if (result &= isRelatedTo((<ConditionalType>source).extendsType, (<ConditionalType>target).extendsType, /*reportErrors*/ false)) {
-                                    if (result &= isRelatedTo((<ConditionalType>source).trueType, (<ConditionalType>target).trueType, /*reportErrors*/ false)) {
-                                        if (result &= isRelatedTo((<ConditionalType>source).falseType, (<ConditionalType>target).falseType, /*reportErrors*/ false)) {
+                                    if (result &= isRelatedTo(getTrueTypeFromConditionalType(<ConditionalType>source), getTrueTypeFromConditionalType(<ConditionalType>target), /*reportErrors*/ false)) {
+                                        if (result &= isRelatedTo(getFalseTypeFromConditionalType(<ConditionalType>source), getFalseTypeFromConditionalType(<ConditionalType>target), /*reportErrors*/ false)) {
                                             return result;
                                         }
                                     }
@@ -13147,8 +13138,8 @@ namespace ts {
                         // and Y1 is related to Y2.
                         if (isTypeIdenticalTo((<ConditionalType>source).extendsType, (<ConditionalType>target).extendsType) &&
                             (isRelatedTo((<ConditionalType>source).checkType, (<ConditionalType>target).checkType) || isRelatedTo((<ConditionalType>target).checkType, (<ConditionalType>source).checkType))) {
-                            if (result = isRelatedTo((<ConditionalType>source).trueType, (<ConditionalType>target).trueType, reportErrors)) {
-                                result &= isRelatedTo((<ConditionalType>source).falseType, (<ConditionalType>target).falseType, reportErrors);
+                            if (result = isRelatedTo(getTrueTypeFromConditionalType(<ConditionalType>source), getTrueTypeFromConditionalType(<ConditionalType>target), reportErrors)) {
+                                result &= isRelatedTo(getFalseTypeFromConditionalType(<ConditionalType>source), getFalseTypeFromConditionalType(<ConditionalType>target), reportErrors);
                             }
                             if (result) {
                                 errorInfo = saveErrorInfo;
@@ -15181,12 +15172,12 @@ namespace ts {
                 else if (source.flags & TypeFlags.Conditional && target.flags & TypeFlags.Conditional) {
                     inferFromTypes((<ConditionalType>source).checkType, (<ConditionalType>target).checkType);
                     inferFromTypes((<ConditionalType>source).extendsType, (<ConditionalType>target).extendsType);
-                    inferFromTypes((<ConditionalType>source).trueType, (<ConditionalType>target).trueType);
-                    inferFromTypes((<ConditionalType>source).falseType, (<ConditionalType>target).falseType);
+                    inferFromTypes(getTrueTypeFromConditionalType(<ConditionalType>source), getTrueTypeFromConditionalType(<ConditionalType>target));
+                    inferFromTypes(getFalseTypeFromConditionalType(<ConditionalType>source), getFalseTypeFromConditionalType(<ConditionalType>target));
                 }
                 else if (target.flags & TypeFlags.Conditional && !contravariant) {
-                    inferFromTypes(source, (<ConditionalType>target).trueType);
-                    inferFromTypes(source, (<ConditionalType>target).falseType);
+                    inferFromTypes(source, getTrueTypeFromConditionalType(<ConditionalType>target));
+                    inferFromTypes(source, getFalseTypeFromConditionalType(<ConditionalType>target));
                 }
                 else if (target.flags & TypeFlags.UnionOrIntersection) {
                     // We infer from types that are not naked type variables first so that inferences we

src/compiler/types.ts

@@ -4338,8 +4338,8 @@ namespace ts {
         root: ConditionalRoot;
         checkType: Type;
         extendsType: Type;
-        trueType: Type;
-        falseType: Type;
+        resolvedTrueType: Type;
+        resolvedFalseType: Type;
         /* @internal */
         resolvedInferredTrueType?: Type; // The `trueType` instantiated with the `combinedMapper`, if present
         /* @internal */