Relax visibility rules for type-aliases when 'declaration' compiler option is set.

One common thing I use type-aliases for is to give shorter, more expressive names to commonly used types throughout a file.

This is especially useful for callback signatures that see common use throughout a module, as well as string-literal types used in a union type, like type HttpMethods = 'get' | 'post' | 'put' | 'delete'.

Let's look at the following simplified example:

type CharCallback = (c: string, i: number) => string;

export function mapOnChar(str: string, fn: CharCallback): string {
  const newStr = [];
  for (let i = 0, lim = str.length; i < lim; i++)
    newStr.push(fn(str.charAt(i), i));
  return newStr.join('');
}

When using the declaration compiler option, this example fails to compile with the following error:
error TS4078: Parameter 'fn' of exported function has or is using private name 'CharCallback'.

This initially makes sense; the type-alias is not being exported so it is private. However, if you look at the type-alias, the only thing "private" about it is the name it was given, and an alias' name is not really important to or needed for a definition file.

There is no reason that the un-exported type-alias in the example cannot be automatically de-aliased back into (c: string, i: number) => string and that used in its place when the definition file is emitted.

However, if the alias in the example were to be exported, then it should not be de-aliased in the definition file.

I should point out that this suggestion is considering type-aliases only. If the CharCallback type was re-written as interface CharCallback { (c: string, i: number): string }, then that would be a good case to raise an error. An interface is generally considered more "concrete" than a simple type-alias, and so its name should be preserved and used in the definition file.

Inizia con l’esempio TypeScript segnalato che usa un alias CharCallback non esportato, una funzione esportata e l’opzione del compilatore per le dichiarazioni. Segui l’emissione delle dichiarazioni e il controllo di visibilità TS4078; il lavoro è completato quando gli alias non esportati possono essere inseriti inline nelle dichiarazioni emesse, mentre gli alias esportati rimangono denominati e il caso interface continua a essere rifiutato.