Question

According to the documentation, an enumeration type constraint of the form
: enum<underlying-type> is “not intended for common use”. What would be the uncommon use it is meant for?

Answer 1

The first thing that jumps to mind is wrapping the members of System.Enum with strongly typed wrappers:

// gets all underlying values for which an enum is defined
let definedVals<'a, 'b when 'a : enum<'b>> = 
  System.Enum.GetValues(typeof<'a>) 
  |> Seq.cast<'a> 
  |> Seq.map (LanguagePrimitives.EnumToValue)

let ints = definedVals<System.ConsoleColor,_>