Macro Error in FunctionK.lift

[channingwalton]

Hi,
I was wrapping doobie's liftIO in FunctionK:

val lifter: FunctionK[IO, ConnectionIO] = FunctionK.lift(Async[ConnectionIO].liftIO)

and got

Error:(24, 80) Unexpected tree {
  <synthetic> val eta$0$1: cats.effect.Async[doobie.free.connection.ConnectionIO] = cats.effect.Async.apply[doobie.free.connection.ConnectionIO](free.this.connection.AsyncConnectionIO);
  ((ioa: cats.effect.IO[Nothing]) => eta$0$1.liftIO[Nothing](ioa))
} when lifting to FunctionK
  val lifter: FunctionK[IO, ConnectionIO] = FunctionK.lift(Async[ConnectionIO].liftIO)

This fixed it:

val acio = Async[ConnectionIO]
val lifter: FunctionK[IO, ConnectionIO] = FunctionK.lift(acio.liftIO)

@milessabin suggested the macro might have been confused about whether it was supposed to be looking at the Async.apply or the liftIO.

[kailuowang]

There have been some attempts to replace that macro with non macro solutions, but they fell short on being fully backward compatible.
But I think cats remains committed to remove all macro usages in the code base to ease transition to Scala 3.
In short, this issue is probably low priority.

[joroKr21]

I think it's confused by the eta-expansion.

Which happens to avoid calling Async[ConnectionIO] on every function invocation instead of just once (imagine if Async[ConnectionIO] had side effects).

[channingwalton]

Awesome, thank you.