Yeah the biggest problem is that there are millions of pre-existing codebases all over the web that mozilla needs to make sure don't break. Since it is a speculative engine, the possibility of breaking even a small percentage of those codebases becomes very likely, so it's really not an easy thing to do.
I'll be curious if it's good enough to enable by default, or if the developer needs to flag the engine somehow to enable the feature.
Since this type inference is speculative and dynamic, if the speculative type optimizations fail you can fall back to less optimized code, or interpreted JS, that handles the badly-typed code. Nothing will break.
I'll be curious if it's good enough to enable by default, or if the developer needs to flag the engine somehow to enable the feature.