A type unified by conditional types loses some types

[falsandtru]

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms:

Code

export type ElChildren =
  | ElChildren.Void
  | ElChildren.Text;
export namespace ElChildren {
  export type Void = undefined;
  export type Text = string;
}

type Relax<C extends ElChildren> = C extends ElChildren.Text ? ElChildren.Text : C;

export class Elem<
  C extends ElChildren,
  > {
  constructor(
    private children_: Relax<C>,
  ) {
  }
}

new Elem(undefined as ElChildren.Void);
new Elem('' as ElChildren.Text);
new Elem('' as ElChildren.Void | ElChildren.Text); // error
new Elem('' as ElChildren); // error

Expected behavior:

pass

Actual behavior:

Argument of type 'ElChildren' is not assignable to parameter of type 'undefined'.
  Type 'string' is not assignable to type 'undefined'.

Playground Link: http://www.typescriptlang.org/play/index.html#src=export%20type%20ElChildren%20%3D%0D%0A%20%20%7C%20ElChildren.Void%0D%0A%20%20%7C%20ElChildren.Text%3B%0D%0Aexport%20namespace%20ElChildren%20%7B%0D%0A%20%20export%20type%20Void%20%3D%20undefined%3B%0D%0A%20%20export%20type%20Text%20%3D%20string%3B%0D%0A%7D%0D%0A%0D%0Atype%20Relax%3CC%20extends%20ElChildren%3E%20%3D%20C%20extends%20ElChildren.Text%20%3F%20ElChildren.Text%20%3A%20C%3B%0D%0A%0D%0Aexport%20class%20Elem%3C%0D%0A%20%20C%20extends%20ElChildren%2C%0D%0A%20%20%3E%20%7B%0D%0A%20%20constructor(%0D%0A%20%20%20%20private%20children_%3A%20Relax%3CC%3E%2C%0D%0A%20%20)%20%7B%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0Anew%20Elem(undefined%20as%20ElChildren.Void)%3B%0D%0Anew%20Elem(''%20as%20ElChildren.Text)%3B%0D%0Anew%20Elem(''%20as%20ElChildren.Void%20%7C%20ElChildren.Text)%3B%0D%0Anew%20Elem(''%20as%20ElChildren)%3B

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