assert in foreach causes too few argument lists for macro invocation

Question

I'm using scala test to check if Array contains Arrays of given size:

result.map(_.length == 2).foreach(assert)

This causes compilation error:

Error:(34, 39) too few argument lists for macro invocation
    result.map(_.length == 2).foreach(assert)

although intellij does not indicate compilation error. How to test it?

Answer 1

This is just a bug in the compiler. You can reproduce it with a much simpler macro you define yourself:

scala> import scala.language.experimental.macros
import scala.language.experimental.macros

scala> import scala.reflect.macros.blackbox.Context
import scala.reflect.macros.blackbox.Context

scala> object IncrementMacro { def inc(c: Context)(i: c.Expr[Int]) = i }
defined object IncrementMacro

scala> object Increment { def inc(i: Int): Int = macro IncrementMacro.inc }
defined object Increment

scala> List(1, 2, 3).map(Increment.inc)
<console>:15: error: too few argument lists for macro invocation
       List(1, 2, 3).map(Increment.inc)
                                   ^

scala> List(1, 2, 3).map(Increment.inc _)
<console>:15: error: macros cannot be eta-expanded
   List(1, 2, 3).map(Increment.inc _)
                               ^

scala> List(1, 2, 3).map(Increment.inc(_))
res1: List[Int] = List(1, 2, 3)

This is on 2.12.8, but I feel like I remember first noticing this back in the 2.10 days. There might be an issue for it, or there might not be, but the moral of the story is that Scala's macros interact with other language features—like eta expansion in this case—in weird ways, and in my view you're best off just memorizing the workarounds, like assert(_) here.