fewerBraces lambdas cannot have `_` arguments

[sjrd]

Compiler version

Current head, i.e., 9585868
with -language:experimental.fewerBraces

Minimized code

object Test {
  def main(args: Array[String]): Unit =
    val xs = List((1, "hello"), (2, "bar"))
    val ys = xs.map (_, str) =>
      val len = str.length
      len * len
    println(ys)
}

Output

-- [E081] Type Error: tests/run/hello.scala:4:21 -------------------------------
4 |    val ys = xs.map (_, str) =>
  |                     ^
  |     Missing parameter type
  |
  |     I could not infer the type of the parameter _$1 of expanded function:
  |     _$1 => 
  |       xs.map(
  |         (_$1, str) => 
  |           {
  |             val len = str.length
  |             len * len
  |           }
  |       ).
1 error found

Expectation

I expect it to compile.

Workaround

Giving an (unused) name to the parameter makes it compile and run successfully:

object Test {
  def main(args: Array[String]): Unit =
    val xs = List((1, "hello"), (2, "bar"))
    val ys = xs.map (unused, str) =>
      val len = str.length
      len * len
    println(ys)
}

prints

List(25, 9)

[odersky]

Fixed in #15273