Consider disallowing multiple `[LegacyFactoryFunction]` identifiers per interface

[ExE-Boss]

Currently, the following is allowed:

[LegacyFactoryFunction=A(),
 LegacyFactoryFunction=B(optional DOMString src),
 LegacyFactoryFunction=C(optional long width, optional long height)]
interface MultipleLegacyFactoryFunctionInterface {};

But it’s not used anywhere, and Chromium’s WebIDL processor doesn’t support it, and WebIDL2JS doesn’t intend to support it either (https://github.com/jsdom/webidl2js/pull/213#issuecomment-621277733).

---

This doesn’t intend to deprecate overloaded [LegacyFactoryFunction]s, which share the same identifier:

[LegacyFactoryFunction=Overloaded(),
 LegacyFactoryFunction=Overloaded(optional DOMString src),
 LegacyFactoryFunction=Overloaded(optional long width, optional long height)]
interface OverloadedLegacyFactoryFunctionInterface {};

See also

• https://github.com/whatwg/webidl/issues/1045

[annevk]

Do we need overloaded though? HTML doesn't use it.

[domenic]

Right, I think we should pare down LegacyFactoryFunction to the minimum, probably.

[TimothyGu]

Neither Blink nor Gecko allows multiple NamedConstructors either.

[domenic]

We should in fact deprecate overloaded [LegacyFactoryFunction]s since nothing uses them and nothing will do so.