Ensure this works for RegExp subclasses

[ljharb]

I don't see spec text yet, so it may not be a problem yet - but let's make sure that

class Foo extends RegExp {} ; Foo.make`${foo}`

returns an instanceof Foo, not just RegExp.

[mikesamuel]

So we would use the this value as the constructor?

Would it be preferable to invoke the constructor assuming it takes (source, flags) or to try to do something Object.create based and then call the RegExp constructor to initialize if possible?

[bergus]

Just use Reflect.construct. I think it's safe to assume it takes (source, flags). Using RegExpInitialize directly should be avoided.

[ljharb]

It's the same logic as Array.from/Array.of imo - it uses the this value for the species-or-constructor.

Why would I want to create a RegExp and then use Reflect.construct in order to get a subclass of RegExp, when I only want the subclass already? Wouldn't that call RegExpInitialize twice when I only need it once?

[bergus]

Yeah, just like Array.of/Array.from/Promise.resolve - not using @@species though, that's for instance methods.

So new this(pattern, flags) (or Reflect.construct(this, [pattern, flags])); maybe with isConstructor(this) ? this : RegExp.
My hint at Reflect was unnecessary, I mistakenly thought that we could use it for variadic arguments - which are not needed here.

[ljharb]

yeah basically new this in the implementation of make should work fine, presuming that it includes a spec step that throws if it doesn't have a [[RegExpMatcher]] internal slot.

[bergus]

Nah, there is no [[RegExpMatcher]] slot on regex constructor functions, it's an instance slot.
I don't think it's possible to detect whether a function is an ES6 subclass that extends RegExp and calls super to it, but I don't think it's necessary either. There's nothing wrong with being lax about what the constructor does - it's not our business.
The closest you could get afaik is RegExp == this || RegExp.isPrototypeOf(this), but even that can be fooled.

[ljharb]

ah, good call

[mikesamuel]

Pull request: #18