Hang on broken unapply macro

[scabug]

I had forgotten the outcome of discussion last year about name-based pattern matching, so trying to do patternless boolean extraction, I tried this broken form, which hangs the compiler, also on trunk:

package bug

import scala.language.experimental.macros
import scala.reflect.macros.{ WhiteboxContext => Context }
//import scala.reflect.macros.whitebox.Context

object Max {
  def impl(c: Context)(any: c.Expr[Any]): c.Expr[Any] = {
    import c.universe._
    def fail(msg: String) = c.abort(c.enclosingPosition, msg)
    val t = c.macroApplication match {
      case q"$_.unapply($unargs)" =>
        /* hangs
        */
        q"""
          new { 
            def isEmpty = false
            def get = this
            def unapply(x: String) = this
          }.unapply($unargs)
        """
        /*
        if get returns Unit or Boolean:
        wrong number of patterns for <$anon: AnyRef> offering Unit: expected 1, found 0
        */
        /* straightforward
        q"""
          new {
            def unapply(x: String) = true
          }.unapply($unargs)
        """
        */
      case _ => fail("bad appl")
    }
    c.Expr[Any](t)
  }
}

with sample usage

package bug

import scala.language.experimental.macros
import org.junit.Test

object X {
  def unapply(any: Any): Any = macro Max.impl
} 
    
class BugTest {
  @Test def bug(): Unit = {
    "any" match {
      case X() =>
      case _ => ???
    }   
  }
}

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-8158?orig=1
Reporter: @som-snytt
Affected Versions: 2.11.0-M7

[scabug]

@xeno-by said (edited on Jan 18, 2014 9:59:51 AM UTC):
I first thought that the hang is caused by infinite macro expansion, but apparently both doTypedUnapply and the macro impl are called only once. The problem is however localized in typer, because -Xprint:typer never prints anything. upd. In particular patmat.alignPatterns(fun1, args).unexpandedFormals seems to be at fault.

[scabug]

@xeno-by said:
Allright, here's what's going on. The macro emits

{
  final class $anon extends scala.AnyRef {
    def <init>() = {
      super.<init>();
      ()
    };
    def isEmpty = false;
    def get = this;
    def unapply(x: String) = this
  };
  new $anon()
}.unapply(<unapply-selector>)

Then this code under the alias of fun1 gets into https://github.com/xeno-by/scala/blob/master/src/compiler/scala/tools/nsc/typechecker/PatternTypers.scala#L306. Then patmat.alignPatterns(fun1, args) is called, which figures out that there's an arityError (https://github.com/xeno-by/scala/blob/master/src/compiler/scala/tools/nsc/transform/patmat/ScalacPatternExpanders.scala#L108), which gets reported by stringifying the type of fun1, which looks like TypeRef(NoPrefix, TypeName("$anon"), List()).

Apparently it's the stringification that causes an infinite loop, because the type is self-referential. I thought we detect these cases already, but apparently not quite. I'll figure this out and submit a pull request later today.

[scabug]

@som-snytt said:
Thanks, I'd started looking through that code to see what I was supposed to do. I'll finish reading it at my leisure.

I'd hoped to work in a pun on "The Secret World of Arrietty" (or ArityError) before the bug was fixed.

[scabug]

@xeno-by said:
scala/scala#3385