typeChecker.typeToString() should print most derived alias

[jayphelps]

TypeScript Version: typescript@next

Code

Playground Link

type First<T, R> = (source: T) => R;
type Second<T> = First<T, T>;

declare function example(): Second<string>;

Expected behavior:

Using the compiler API, typeChecker.typeToString(signature.getReturnType()) of example returns the most derived alias, Second<string>

Actual behavior:

typeChecker.typeToString(signature.getReturnType()) returns First<string, string>

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IntelliSense in the playground and vscode both show the same thing as typeChecker.typeToString(returnType), so I assume they share the same underlying logic.

I tried examining the type object itself to see if I could reach into private APIs to get it, but I couldn't find it. I also tried various formatting flags, but none of them seemed to give me the desired behavior.

I searched for existing issues on this, #17433 seemed similar but digging into it, I think it's different. It's also possible this behavior was intentional of course, if so, lmk. I'm using the compiler API to generate custom documentation for rxjs and there are sometimes a number of aliases used which simplify much more complex structures and hide internals.

I could try to take a crack at a patch if someone knows where I might find the applicable logic. Seems like it would need to be on the type metadata generation side, since I can't find it in the type returned from signature.getReturnType()?

Cc/ @benlesh

[jayphelps]

This might actually be intended behavior after all, albeit an unfortunate one:

https://www.typescriptlang.org/docs/handbook/advanced-types.html

One difference is that interfaces create a new name that is used everywhere. Type aliases don’t create a new name — for instance, error messages won’t use the alias name. In the code below, hovering over interfaced in an editor will show that it returns an Interface, but will show that aliased returns object literal type.

It's not clear why this would be desired, but as always I could definitely be wrong! 🤡

It also turns out that this section is no longer accurate: microsoft/TypeScript-Handbook#682 It's possibel it's the intended behavior but the naming of the first alias slipped in accidentally from other changes.