if type errors

[Erik Demaine]

I was trying to write a proper type for our compile function (once it becomes async):

export function compile<T extends CompileOptions>(src: string, options?: T): Promise<
  if T extends { ast: true }
    CivetAST
  else
    if T extends { sourceMap: true }
      code: string
      sourceMap: SourceMap
    else
      string
>

The current compilation has multiple bugs:

export function compile<T extends CompileOptions>(src: string, options?: T): Promise<
  (T extends { ast: true }?
    CivetAST
  :
    (T extends { sourceMap: true }?
      {code: string}:never)),
      {sourceMap: SourceMap},
    else,
      string
>

It's missing two close parentheses, and it says else, instead of the second :.

Also, it would be nice to be able to write else if here.

Related, it's not possible to have a newline after : in a ?: type:

type CompileOutput<T extends CompileOptions> =
  T extends { ast: true } ? CivetAST :
  T extends { sourceMap: true } ? {
    code: string,
    sourceMap: SourceMap,
  } : string