Basically, just this (true for `enum` and `not struct` constraints):  (found it while playing with @cartermp's [impossibility challenge](https://fsharp.slack.com/archives/C1R50TKEU/p1593620919399200)) Also, `unmanaged`, `equality`, `comparison` and `enum` constraints do not get colored correctly: 