Generics return type of function reported not assignable.

Bug Report

🔎 Search Terms

return value, typescript, generics

🕗 Version & Regression Information

This changed between versions 3.5.1 and 3.6.3

⏯ Playground Link

https://www.typescriptlang.org/play?ts=3.6.3#code/C4TwDgpgBAKuEGECGBnCBBAPOgfFAvFAN5QAmSwSAXFOlAL4DcAUKJLPMmgEKbcFQUwAE4BLAHYBzPIRLlKAfhrdGUAMYB7UhBpCxU1U1bwOkLhl7oANFH6E9E6QLhnUGbHgA+pxG97ccFmY1ABtUFCgAZQhhADdRNWgiZihUqEkIYH8BBykcAApNbV0RRxt5JCVbAEoaF18ePjxktNb1DXEhKGEIFABXEOA6zj8mgTkKJBsi6CM2tJ7gPuFxbt6B4BZW+mYUtIzgLFx8ipp0Wp9zI+a9+cXl1YnKBi20nduoADM+8TUARnyF3qV14JFEpBo4j6AFsAEYxBg3eape4rKDAAAWohQADoDtx8gByP4AJgAzH9CdVXql3q1vr8SYDhq40OhQVBwZCYfDhIjiB9WppOsA1sBYkgQgJMdi8ZkCcTyZTqYKFpkHmKJSEaQxdvSfmoyczLm52ZgwRCoFC4Qj6DZck4WsjUasZbj8UTSRTCTZCQAWACsADYqTqdvQgA

💻 Code

type TypeCaseA<A> = { data: A };
type TypeCaseB<B = string> = { data?: B; code: string; };
type TypeCaseAB<A, B = string> = TypeCaseA<A> | TypeCaseB<B>;

class Service {
    getB<B = string>(code: string, data?: B): TypeCaseB<B> {
        const result: TypeCaseB<B> = { data, code };
        return result;
    }

    getA<A>(data: A): TypeCaseA<A> {
        return { data };
    }

    func1(): TypeCaseAB<{ id: number }> {
        /* The following line is blamed with:
          Type 'TypeCaseB<string | { id: number; }>' is not assignable to type 'TypeCaseAB<{ id: number; }, string>'.
            Type 'TypeCaseB<string | { id: number; }>' is not assignable to type 'TypeCaseB<string>'.
              Type 'string | { id: number; }' is not assignable to type 'string'.
                Type '{ id: number; }' is not assignable to type 'string'.
         */
        return this.getB('1231');
    }

    func2(): TypeCaseAB<{ id: number }> {
        const retval = this.getB('1231');
        return retval;
    }

    func3(): TypeCaseAB<{ id: number }, string> {
        return this.getB('1231', '456');
    }
}

🙁 Actual behavior

func1 is tested with the following error:

          Type 'TypeCaseB<string | { id: number; }>' is not assignable to type 'TypeCaseAB<{ id: number; }, string>'.
            Type 'TypeCaseB<string | { id: number; }>' is not assignable to type 'TypeCaseB<string>'.
              Type 'string | { id: number; }' is not assignable to type 'string'.
                Type '{ id: number; }' is not assignable to type 'string'.

There should be no difference from func2 which is tested correct.

🙂 Expected behavior

No error reported.

Reproduire le diagnostic dans le TypeScript Playground lié en utilisant les exemples TypeCaseA, TypeCaseB, TypeCaseAB et Service.func1/func2/func3. Comparer la gestion du type de retour générique dans func1 avec les cas acceptés func2 et func3. Le travail est terminé lorsque func1 ne produit aucune erreur tout en conservant le type de retour TypeCaseAB attendu.