Problem with Applicative CEs and 'Zero'

[gusty]

Repro steps

This script works fine, but when we uncomment the BindReturn member it fails because it seems it doesn't use the Zero method.

type Builder () =
    member        __.ReturnFrom (expr) = expr                                  : seq<'T>
    member inline __.Return (x: 'T) = Seq.singleton x                          : seq<'T>
    member inline __.Yield  (x: 'T) = Seq.singleton x                          : seq<'T>
    member inline __.Bind (p: seq<'T>, rest: 'T->seq<'U>) = Seq.collect rest p : seq<'U>

    member inline __.Delay (expr: _->seq<'T>) = Seq.delay expr : seq<'T>
    member        __.Run f = f : seq<'T>

    member inline __.Zero () = Seq.empty : seq<'T>

    // member inline __.BindReturn   (x : seq<'T>, f: 'T -> 'U) : seq<'U> = Seq.map f x

let seqbuilder= new Builder ()

let _pythags = seqbuilder {
  let! z = seq [1..50]
  let! x = seq [1..z]
  let! y = seq [x..z]
  if (x*x + y*y = z*z) then return (x, y, z) }

Expected behavior

Compile fine with the BindReturn member

Actual behavior

Doesn't compile with the BindReturn member

Known workarounds

Add an else branch. but still looks like a regression.

Related information

F# 5.0

[dsyme]

Yes, this code (with the BindReturn uncommented) compiles with --langversion:4.7 but not --langversion:5.0

[dsyme]

@cartermp I guess this isn't totally a regression (or at least not such a severe one), since we don't really expect BindReturn methods to be present unless people are using 5.0. So it's more a bug in the applicatives feature.

I'll fix it now neverthelss but change the priority

[gusty]

@dsyme It's true that it's not 100% a regression, but it hit us. In F#+ we started using it, and it worked fine.

But suddenly, our CIs upgraded to F# 5 and some tests started failing, then we realized that by shipping this optimization we included a breaking change. So we did an emergency release to take it out for the moment.