[api] Add .getReducedType() method

packages/typescript/src/api/async/api.ts

@@ -1787,6 +1787,11 @@ export class Checker {
         return type.getApparentType();
     }
 
+    /** Get the reduced type of a type. Always returns a type. */
+    async getReducedType(type: Type): Promise<Type> {
+        return type.getReducedType();
+    }
+
     async getPropertiesOfType(type: Type): Promise<readonly Symbol[]> {
         return type.getProperties();
     }
@@ -2237,6 +2242,7 @@ class TypeObject implements Type {
     private default: number | false;
     private nonNullableType: number | false;
     private apparentType: number | false;
+    private reducedType: number | false;
     private properties: readonly Symbol[] | false;
     private apparentProperties: readonly Symbol[] | false;
     private callSignatures: readonly Signature[] | false;
@@ -2286,6 +2292,7 @@ class TypeObject implements Type {
         this.default = false;
         this.nonNullableType = false;
         this.apparentType = false;
+        this.reducedType = false;
         this.properties = false;
         this.apparentProperties = false;
         this.callSignatures = false;
@@ -2368,6 +2375,12 @@ class TypeObject implements Type {
         return result;
     }
 
+    async getReducedType(): Promise<Type> {
+        const result = await this.objectRegistry.fetchType(this, "getReducedType", this.reducedType);
+        this.reducedType = result.id;
+        return result;
+    }
+
     async getIndexInfos(): Promise<readonly IndexInfo[]> {
         if (this.indexInfos === false) {
             this.indexInfos = await this.objectRegistry.fetchIndexInfosOfType(this);

packages/typescript/src/api/async/types.ts

@@ -47,6 +47,9 @@ export interface Type {
     /** Get the apparent type of this type. */
     getApparentType(): Promise<Type>;
 
+    /** Get the reduced type of this type. */
+    getReducedType(): Promise<Type>;
+
     /** Get the call signatures of this type. */
     getCallSignatures(): Promise<readonly Signature[]>;
 

packages/typescript/src/api/proto.generated.ts

@@ -91,6 +91,7 @@ export interface APIMethodInfo {
     getPropertiesOfType: APIMethod<CheckerTypeParams, SymbolResponse[] | null>;
     getApparentPropertiesOfType: APIMethod<GetTypePropertyParams, SymbolResponse[]>;
     getApparentType: APIMethod<GetTypePropertyParams, TypeResponse>;
+    getReducedType: APIMethod<GetTypePropertyParams, TypeResponse>;
     getPropertyOfType: APIMethod<GetPropertyOfTypeParams, SymbolResponse | null>;
     getIndexInfosOfType: APIMethod<CheckerTypeParams, IndexInfoResponse[] | null>;
     getConstraintOfTypeParameter: APIMethod<GetTypePropertyParams, TypeResponse | null>;

packages/typescript/src/api/sync/api.ts

@@ -1795,6 +1795,11 @@ export class Checker {
         return type.getApparentType();
     }
 
+    /** Get the reduced type of a type. Always returns a type. */
+    getReducedType(type: Type): Type {
+        return type.getReducedType();
+    }
+
     getPropertiesOfType(type: Type): readonly Symbol[] {
         return type.getProperties();
     }
@@ -2245,6 +2250,7 @@ class TypeObject implements Type {
     private default: number | false;
     private nonNullableType: number | false;
     private apparentType: number | false;
+    private reducedType: number | false;
     private properties: readonly Symbol[] | false;
     private apparentProperties: readonly Symbol[] | false;
     private callSignatures: readonly Signature[] | false;
@@ -2294,6 +2300,7 @@ class TypeObject implements Type {
         this.default = false;
         this.nonNullableType = false;
         this.apparentType = false;
+        this.reducedType = false;
         this.properties = false;
         this.apparentProperties = false;
         this.callSignatures = false;
@@ -2376,6 +2383,12 @@ class TypeObject implements Type {
         return result;
     }
 
+    getReducedType(): Type {
+        const result = this.objectRegistry.fetchType(this, "getReducedType", this.reducedType);
+        this.reducedType = result.id;
+        return result;
+    }
+
     getIndexInfos(): readonly IndexInfo[] {
         if (this.indexInfos === false) {
             this.indexInfos = this.objectRegistry.fetchIndexInfosOfType(this);

packages/typescript/src/api/sync/types.ts

@@ -55,6 +55,9 @@ export interface Type {
     /** Get the apparent type of this type. */
     getApparentType(): Type;
 
+    /** Get the reduced type of this type. */
+    getReducedType(): Type;
+
     /** Get the call signatures of this type. */
     getCallSignatures(): readonly Signature[];
 

packages/typescript/test/async/api.test.ts

@@ -818,6 +818,47 @@ describe("Checker - getApparentType", () => {
     });
 });
 
+describe("Checker - getReducedType", () => {
+    test("returns the reduced type", async () => {
+        const api = spawnAPI({
+            "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
+            "/src/main.ts": `
+declare const TaggedError: <Tag extends string>(
+    tag: Tag,
+) => new <A extends Record<string, any> = {}>(
+    args: { readonly [P in keyof A]: A[P] },
+) => { readonly _tag: Tag } & Readonly<A>;
+
+class RateLimitError extends TaggedError("RateLimitError")<{
+    readonly retryAfter: number;
+}> {}
+
+class QuotaExceededError extends TaggedError("QuotaExceededError")<{
+    readonly limit: number;
+}> {}
+
+export type Result = RateLimitError | (RateLimitError & QuotaExceededError);`,
+        });
+        try {
+            const snapshot = await api.updateSnapshot({ openProject: "/tsconfig.json" });
+            const project = snapshot.getProject("/tsconfig.json")!;
+            const sourceFile = await project.program.getSourceFile("/src/main.ts");
+            assert.ok(sourceFile);
+            const typeAlias = sourceFile.statements.find(isTypeAliasDeclaration);
+            assert.ok(typeAlias);
+            const type = await project.checker.getTypeAtLocation(typeAlias);
+            assert.equal(type.isUnionType(), true);
+            assert.equal(type.isObjectType(), false);
+            const reducedType = await project.checker.getReducedType(type);
+            assert.equal(reducedType.isUnionType(), false);
+            assert.equal(reducedType.isObjectType(), true);
+        }
+        finally {
+            await api.close();
+        }
+    });
+});
+
 describe("Checker - getMemberInModuleExports", () => {
     test("returns a named export when present", async () => {
         const api = spawnAPI({

packages/typescript/test/sync/api.test.ts

@@ -826,6 +826,47 @@ describe("Checker - getApparentType", () => {
     });
 });
 
+describe("Checker - getReducedType", () => {
+    test("returns the reduced type", () => {
+        const api = spawnAPI({
+            "/tsconfig.json": JSON.stringify({ compilerOptions: { strict: true } }),
+            "/src/main.ts": `
+declare const TaggedError: <Tag extends string>(
+    tag: Tag,
+) => new <A extends Record<string, any> = {}>(
+    args: { readonly [P in keyof A]: A[P] },
+) => { readonly _tag: Tag } & Readonly<A>;
+
+class RateLimitError extends TaggedError("RateLimitError")<{
+    readonly retryAfter: number;
+}> {}
+
+class QuotaExceededError extends TaggedError("QuotaExceededError")<{
+    readonly limit: number;
+}> {}
+
+export type Result = RateLimitError | (RateLimitError & QuotaExceededError);`,
+        });
+        try {
+            const snapshot = api.updateSnapshot({ openProject: "/tsconfig.json" });
+            const project = snapshot.getProject("/tsconfig.json")!;
+            const sourceFile = project.program.getSourceFile("/src/main.ts");
+            assert.ok(sourceFile);
+            const typeAlias = sourceFile.statements.find(isTypeAliasDeclaration);
+            assert.ok(typeAlias);
+            const type = project.checker.getTypeAtLocation(typeAlias);
+            assert.equal(type.isUnionType(), true);
+            assert.equal(type.isObjectType(), false);
+            const reducedType = project.checker.getReducedType(type);
+            assert.equal(reducedType.isUnionType(), false);
+            assert.equal(reducedType.isObjectType(), true);
+        }
+        finally {
+            api.close();
+        }
+    });
+});
+
 describe("Checker - getMemberInModuleExports", () => {
     test("returns a named export when present", () => {
         const api = spawnAPI({

tsc/internal/api/proto.go

@@ -149,6 +149,7 @@ const (
 	MethodGetPropertiesOfType               Method = "getPropertiesOfType"
 	MethodGetApparentPropertiesOfType       Method = "getApparentPropertiesOfType"
 	MethodGetApparentType                   Method = "getApparentType"
+	MethodGetReducedType                    Method = "getReducedType"
 	MethodGetPropertyOfType                 Method = "getPropertyOfType"
 	MethodGetIndexInfosOfType               Method = "getIndexInfosOfType"
 	MethodGetConstraintOfTypeParameter      Method = "getConstraintOfTypeParameter"
@@ -487,6 +488,7 @@ var unmarshalers = map[Method]func([]byte) (any, error){
 	MethodGetPropertiesOfType:               unmarshallerFor[CheckerTypeParams],
 	MethodGetApparentPropertiesOfType:       unmarshallerFor[GetTypePropertyParams],
 	MethodGetApparentType:                   unmarshallerFor[GetTypePropertyParams],
+	MethodGetReducedType:                    unmarshallerFor[GetTypePropertyParams],
 	MethodGetPropertyOfType:                 unmarshallerFor[GetPropertyOfTypeParams],
 	MethodGetIndexInfosOfType:               unmarshallerFor[CheckerTypeParams],
 	MethodGetConstraintOfTypeParameter:      unmarshallerFor[GetTypePropertyParams],

tsc/internal/api/session.go

@@ -777,6 +777,8 @@ func (s *Session) HandleRequest(ctx context.Context, method string, params json.
 		return s.handleGetApparentPropertiesOfType(ctx, parsed.(*GetTypePropertyParams))
 	case string(MethodGetApparentType):
 		return s.handleGetApparentType(ctx, parsed.(*GetTypePropertyParams))
+	case string(MethodGetReducedType):
+		return s.handleGetReducedType(ctx, parsed.(*GetTypePropertyParams))
 	case string(MethodGetPropertyOfType):
 		return s.handleGetPropertyOfType(ctx, parsed.(*GetPropertyOfTypeParams))
 	case string(MethodGetIndexInfosOfType):
@@ -3062,6 +3064,22 @@ func (s *Session) handleGetApparentType(ctx context.Context, params *GetTypeProp
 	return setup.newTypeResponse(setup.checker.GetApparentType(t)), nil
 }
 
+// handleGetReducedType returns the reduced type of a type.
+func (s *Session) handleGetReducedType(ctx context.Context, params *GetTypePropertyParams) (*TypeResponse, error) {
+	setup, err := s.setupChecker(ctx, params.Snapshot, params.Project)
+	if err != nil {
+		return nil, err
+	}
+	defer setup.done()
+
+	t, err := setup.resolveTypeHandle(params.Type)
+	if err != nil {
+		return nil, err
+	}
+
+	return setup.newTypeResponse(setup.checker.GetReducedType(t)), nil
+}
+
 // handleGetIndexInfosOfType returns the index infos of a type.
 // @gen-proto-nullable
 func (s *Session) handleGetIndexInfosOfType(ctx context.Context, params *CheckerTypeParams) ([]*IndexInfoResponse, error) {

tsc/internal/checker/exports.go

@@ -294,6 +294,10 @@ func (c *Checker) GetApparentType(t *Type) *Type {
 	return c.getApparentType(t)
 }
 
+func (c *Checker) GetReducedType(t *Type) *Type {
+	return c.getReducedType(t)
+}
+
 // GetFullyQualifiedName returns the fully qualified name of a symbol, walking up
 // its parent chain (e.g. `"/path/to/module".Namespace.Name`).
 func (c *Checker) GetFullyQualifiedName(symbol *ast.Symbol) string {