Propose: Add "where" support for enum in generic type defintions

[Korporal]

Currently there's no way to restrict a generic type argument to be an enum. One "workaround" is to restrict the type to a struct perhaps adding IComparable etc to the "where" clause.

So this is a proposal to add this feature to C#.

This was inspired by this related issue:

dotnet/roslyn#18126

This has come up numerous time in certain kinds of designs, it isn't a frequent issue but does arise and does seem like a gap in the language.

Thx

[HaloFour]

Dupe of #104.

[temporaryfile]

#104 just disappeared, so...

Generic constraints make extremely good sense with enum inheritance. Example from the future:

public enum WindowMessage
{
    WM_NULL = 0,
}

public enum ListViewMessage : WindowMessage
{
    LVM_FIRST = 0x1000,
    LVM_EDITLABELW = (LVM_FIRST + 118),
}

public static IntPtr SendMessage<TMessage>(
    HWND hwnd,
    TMessage Msg = WindowMessage.WM_NULL,
    UIntPtr wParam = default(UIntPtr),
    IntPtr lParam = default(IntPtr))
    where TMessage : WindowMessage
{
    // [....]
}

Wait till I show you the example with NT kernel object access flags.