TailCall attribute does not lead to errors when recursive call is in a list comprehension

[Larocceau]

TailCall attribute does not lead to errors when recursive call is in a list comprehension

[dawedawe]

@dawedawe since you contributed to this feature this might be of interest to you

Thanks for the finding, I'll work on it.

[Larocceau]

@dawedawe since you contributed to this feature this might be of interest to you

[vzarytovskii]

Example project with a case where the bug occurs , and a slightly different version where the bug does not occur

// Nearly identical code that is flagged by the compiler;
// only difference is using List.Append instead of a list comprehension

Difference is that list expression will use list collector, and will result in something like

let tailOrNull = input.TailOrNull
let headOrDefault = input.HeadOrDefault
let listCollector = ListCollector<t>()
listCollector.AddMany(reverse(tailOrNull))
listCollector.Add(headOrDefault)
listCollector.Close()

But when the analysis happens, it probably thinks it's atomic or something.

[vzarytovskii]

Oh, yeah

[<TailCall>]
let rec recurse (list : list<int>) =
    [ yield! recurse list; () ]

Will result in something like (roughly):

DebugPoint((6,8--6,35), App(toList, [Op(Coerce, App(seq, [App(delay, [Lambda([unitVar], App(append, [.., ..]))])]))]))

Whereas

[<TailCall>]
let rec recurse (list : list<int>) =
    recurse list; ()

into

Sequential(DebugPoint((7,8--7,20), App(.., [..])), DebugPoint((7,22--7,24), ()))

We analyze the latter correctly as sequential expression and produce the warning, and we end up analysing the former as application and decide that it's tail recursive, which we shouldn't.