Given within for-comprehension not found

[qwbarch]

Compiler version

3.0.0

Minimized code

Declaring a result as given works as expected if I'm flat-mapping to another monadic value. For example:

for
   given Int <- List(0)
   x <- List(summon[Int])
yield x

Scastie: https://scastie.scala-lang.org/9nPXzZH0QEaVzz3zI4ER8g

However, if the next step in the for-comprehension is a variable, the given is not found. For example:

for
   given Int <- List(0)
   x = summon[Int]
yield x

Scastie: https://scastie.scala-lang.org/1cuJc8bcS4q59Y5ZoSQ4bw

Output

no implicit argument of type Int was found for parameter x of method summon in object Predef

Expectation

It should compile just like the first example.

[smarter]

@bishabosha is this perhaps related to #12631 ?

[bishabosha]

@bishabosha is this perhaps related to #12631 ?

I don't think so - it seems that the code path that recognises given pattern as irrefutable is only checked for generator enumerators not alias enumerators

https://github.com/lampepfl/dotty/blob/d9799a8e3374cd3bee8becb39c950022fb525f41/compiler/src/dotty/tools/dotc/ast/Desugar.scala#L1590

[dwijnand]

Looks like the translation happens during typer. (Couldn't tell just from comparing -Xprint outputs why things go how they go.)

[kubukoz]

A nasty workaround that's less unpleasant than no givens at all... adding a dummy value to flatMap on before you use the newly created given.

trait demo {
  trait A
  trait B
  def mkA: Option[A]
  def mkB(using A): B

  val dummy: Option[Any] = Some(42)

  def example = for {
    given A <- mkA
    _ <- dummy
    given B = mkB
  } yield ()
}

but yeah, it would be great if it just worked. FWIW it seems withFilter is involved, ignoring the <- dummy line with a type that doesn't have it issues:

value withFilter is not a member of demo.this.Op[demo.this.A]

code that produced this

trait demo {
  trait Op[X] {
    def flatMap[B](f: X => Op[B]): Op[B]
    def map[B](f: X => B): Op[B]
  }
  trait A
  trait B
  def mkA: Op[A]
  def mkB(using A): B

  val dummy: Op[Any]

  def example = for {
    given A <- mkA
    // _ <- dummy
    given B = mkB
  } yield ()
}

all tested on 3.0.2.

Update: still happens on 3.1.0-RC3

[bishabosha]

another example from #14836

trait C
object C:
  def provide: Option[C] = Some(new C {})

def consume(arg: Int)(using C): Unit = ()

val result1 = for {
  given C <- C.provide
  a = consume(42) // Error: no implicit of type C
} yield a


val result2 = for {
  c <- C.provide
  a = {
    given C = c
    consume(42) // this works fine
  }
} yield a