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#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

💻 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.

Reproduziere die Diagnose im verlinkten TypeScript Playground anhand der Beispiele TypeCaseA, TypeCaseB, TypeCaseAB und Service.func1/func2/func3. Vergleiche die Behandlung generischer Rückgabetypen in func1 mit den akzeptierten Fällen func2 und func3. Als abgeschlossen gilt die Aufgabe, wenn func1 keinen Fehler erzeugt und dabei den erwarteten Rückgabetyp TypeCaseAB beibehält.