Why does this code typecheck in Scala 2.11 and what can I do about it?

Question

The following macro is a simplified version of Shapeless's illTyped that tries to typecheck some code that you provide as a string. It returns None if it succeeds, and the exception as an Option[String] if it fails.

import scala.language.experimental.macros
import scala.reflect.macros.TypecheckException
import scala.reflect.macros.whitebox.Context

def typecheck_impl(c: Context)(code: c.Expr[String]): c.Expr[Option[String]] = {
  import c.universe._

  val Expr(Literal(Constant(codeStr: String))) = code

  try {
    c.typecheck(c.parse(codeStr))
    c.Expr(q"None: Option[String]")
  } catch {
    case e: TypecheckException =>
      c.Expr(q"Some(${ e.toString }): Option[String]")
  }
}

def typecheck(code: String): Option[String] = macro typecheck_impl

Now suppose I've got a case class Foo. Because it's a case class, Foo will have an extractor automatically generated for it, but let's also define our own extractor Bar that does the same thing:

object Test {
  case class Foo(i: Int, c: Char)

  object Bar {
    def unapply(foo: Foo): Option[(Int, Char)] = Some((foo.i, foo.c))
  }
}

Now we can write the following:

scala> import Test._
import Test._

scala> val Foo(x, y) = Foo(1, 'a')
x: Int = 1
y: Char = a

scala> val Bar(x, y) = Foo(1, 'a')
x: Int = 1
y: Char = a

scala> val Foo(x, y, z) = Foo(1, 'a')
<console>:15: error: wrong number of arguments for pattern Test.Foo(i: Int,c: Char)
       val Foo(x, y, z) = Foo(1, 'a')
              ^

scala> val Bar(x, y, z) = Foo(1, 'a')
<console>:15: error: too many patterns for object Bar offering (Int, Char): expected 2, found 3
       val Bar(x, y, z) = Foo(1, 'a')
           ^

scala> typecheck("val Foo(x, y) = Foo(1, 'a')")
res0: Option[String] = None

scala> typecheck("val Bar(x, y) = Foo(1, 'a')")
res1: Option[String] = None

scala> typecheck("val Foo(x, y, z) = Foo(1, 'a')")
res2: Option[String] = Some(scala.reflect.macros.TypecheckException: wrong number of arguments for pattern Test.Foo(i: Int,c: Char))

None of this is surprising—the stuff you'd think would compile compiles, the stuff you wouldn't doesn't, and our macro agrees. But then you try this:

scala> typecheck("val Bar(x, y, z) = Foo(1, 'a')")
<macro>:1: error: too many patterns for object Bar offering (Int, Char): expected 2, found 3
val Bar(x, y, z) = Foo(1, 'a')
    ^

And the macro just chokes. Changing the catch block to handle any old throwable gives the same result. The equivalent code worked as expected in 2.10.

How can I capture this error so that my macro works as expected in 2.11?

Answer 1

This is a bug that is now fixed in https://github.com/scala/scala/pull/3876. Let's hope the fix will get included in 2.11.2.