Support coercing on a function type

[TheSpyder]

I recently discovered that coercion works for variants where everything is represented by the same primitive type:
https://rescript-lang.org/docs/manual/latest/variant#coercing-variants-to-primitives

This is great, but I have a function that returns one of these variants and I can't coerce that. It seems to work for no-argument functions, but not functions with arguments.
https://rescript-lang.org/try?version=v11.1.1&code=C4TwDgpgBAhgzgSQHbCgXigHygAXgCgEYBKKAeSWmzznwCZSAVAdwHstcCBmJgCwCcIEAFDCANhFStKyYAC5YiFOnKVxkqMFayV+aRB1yAfFACWKYqImpBwAK78ksgKpJTqDPlJoTFEdahbBycUAGVzAHMJXQB9b181dVQtWQAxQl0gxxc3VGMofDtc9BNzYGJLAJSUVLpMyWDZcKQo6Hz8AHIYErMLYiA

The specific example I'm trying to model is array sort, so I'd need it to work on 2-argument functions as well.
https://rescript-lang.org/try?version=v11.1.1&code=C4TwDgpgBAxg9gWzAQwE4EsDOcB2UC8AUFFAD5QACymAFALQCMAlFADISaYAqAFsjsTKVqNAAwsAogEcArsgA2g8lVrMoAcVQRkwCKl79ChOJDwApTADoAgqlTIQAJkLyIwKNlTv8UGmlQANFAAfgBmAFy+AOTIQTEs+AB8sIgoGNg4CYmCnsAAkjgACvLIMBAA6ujAPH52dImY8uhlAGKoiGJMQTShUOHJ6DjATExAA

[cristianoc]

One observation: the coercion in the example seems to work if the argument type is not polymorphic, e.g. float.
Coercion was not modified for function types, it still uses the original implementation, but the way it's been implemented for variants might interfere with the pre-existing mechanism in certain cases (e.g. polymorphic argument).

[TheSpyder]

Ah, that makes sense. Unfortunately the way I'm trying to use it with Array sort requires polymorphism.

For now I've just used an obj magic.

[cristianoc]

@TheSpyder any more tests for #6828 ?

[TheSpyder]

@cristianoc the second example is the one I pulled from my codebase, I haven't come across any others (although I haven't really been looking). I appreciate you looking at this!

[cristianoc]

@cristianoc the second example is the one I pulled from my codebase, I haven't come across any others (although I haven't really been looking). I appreciate you looking at this!

Great, that example is covered already.

[cristianoc]

Done in #6828