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.

Reproduce the diagnostic in the linked TypeScript Playground using the TypeCaseA, TypeCaseB, TypeCaseAB, and Service.func1/func2/func3 examples. Compare the generic return-type handling in func1 with the accepted func2 and func3 cases. Done means func1 produces no error while preserving the expected TypeCaseAB return type.