Union arguments are unexpectedly expanded

[arnog]

Search terms

unions, arguments

type Union = "a" | "b";
class A {
 foo(a: Union) { /*...*/ }
}

Expected Behavior

The reflections generated by Typedoc preserve the reference to theUnion type:

type Union = "a" | "b";
class A {
 foo(a: Union { /*...*/ }
}

Actual Behavior

The reflections generated by Typedoc "expand" the union, and lose reference to the Union type:

type Union = "a" | "b";
class A {
 foo(a: "a" | "b") { /*...*/ }
}

Environment

• Typedoc version: 0.20.16. This did not happen in 0.19.

In case that matters, I'm seeing this when using in the JSON output.

[Gerrit0]

This was caused by the switch in 0.20 to use the type checker more to get types - particularly to resolve method types with generic classes. I suspect TypeScript 4.2 will fix this - https://devblogs.microsoft.com/typescript/announcing-typescript-4-2-beta/#smarter-type-alias-preservation

[arnog]

Right, that makes sense...

[Gerrit0]

TypeScript 4.2 contained some changes which makes this specific case better. If you update your TS version, you should see that union types are better about preserving their names if they are aliased somewhere.