Quasiquote splicing for empty lists misbehaves during trait instantiation

scabug commented 10 years ago

Trying to splice an empty list of trees into a quasiquote to instantiate a trait does not work as expected.

For example, this works as expected:

object Bar {
  import language.experimental.macros
  import scala.reflect.macros.Context

  trait Foo
  def foo: Bar.Foo = macro fooImpl
  def fooImpl(c: Context): c.Expr[Bar.Foo] = {
    import c.universe._
    c.Expr[Foo](q"new Bar.Foo {}")
  }
}

And so is this:

object Bar {
  import language.experimental.macros
  import scala.reflect.macros.Context

  trait Foo
  def foo: Bar.Foo = macro fooImpl
  def fooImpl(c: Context): c.Expr[Bar.Foo] = {
    import c.universe._
    val trees = List(q"")
    c.Expr[Foo](q"new Bar.Foo {..$trees}")
  }
}

Notice that the spliced list is not empty.

But this fails to compile:

object Bar {
  import language.experimental.macros
  import scala.reflect.macros.Context

  trait Foo
  def foo: Bar.Foo = macro fooImpl
  def fooImpl(c: Context): c.Expr[Bar.Foo] = {
    import c.universe._
    val trees = List()
    c.Expr[Foo](q"new Bar.Foo {..$trees}")
  }
}

With this trace:

Bar.scala:10: exception during macro expansion:
java.util.NoSuchElementException: head of empty list
        at scala.collection.immutable.Nil$.head(List.scala:337)
        at scala.collection.immutable.Nil$.head(List.scala:334)
        at org.scalalang.macroparadise.quasiquotes.Holes$class.parseCardinality(Holes.scala:193)
        at org.scalalang.macroparadise.quasiquotes.Quasiquotes.parseCardinality(Quasiquotes.scala:7)
        at org.scalalang.macroparadise.quasiquotes.Holes$class.parseCardinality(Holes.scala:193)
        at org.scalalang.macroparadise.quasiquotes.Quasiquotes.parseCardinality(Quasiquotes.scala:7)
        at org.scalalang.macroparadise.quasiquotes.Holes$Hole$.apply(Holes.scala:168)
        at org.scalalang.macroparadise.quasiquotes.Placeholders$class.appendHole$1(Placeholders.scala:35)
        at org.scalalang.macroparadise.quasiquotes.Placeholders$$anonfun$codeAndPosMap$1.apply(Placeholders.scala:41)
        at org.scalalang.macroparadise.quasiquotes.Placeholders$$anonfun$codeAndPosMap$1.apply(Placeholders.scala:38)
        at scala.reflect.internal.util.Collections$class.foreach2(Collections.scala:150)
        at scala.reflect.internal.SymbolTable.foreach2(SymbolTable.scala:13)
        at org.scalalang.macroparadise.quasiquotes.Placeholders$class.codeAndPosMap(Placeholders.scala:38)
        at org.scalalang.macroparadise.quasiquotes.Quasiquotes.codeAndPosMap$lzycompute(Quasiquotes.scala:7)
        at org.scalalang.macroparadise.quasiquotes.Quasiquotes.codeAndPosMap(Quasiquotes.scala:7)
        at org.scalalang.macroparadise.quasiquotes.Placeholders$class.code(Placeholders.scala:48)
        at org.scalalang.macroparadise.quasiquotes.Quasiquotes.code$lzycompute(Quasiquotes.scala:7)
        at org.scalalang.macroparadise.quasiquotes.Quasiquotes.code(Quasiquotes.scala:7)
        at org.scalalang.macroparadise.quasiquotes.Quasiquotes.expandQuasiquote(Quasiquotes.scala:46)
        at org.scalalang.macroparadise.typechecker.FastTrack$ParadiseFastTrack$$anonfun$hijack$1$$anonfun$apply$1.apply(FastTrack.scala:22)
        at org.scalalang.macroparadise.typechecker.FastTrack$ParadiseFastTrack$$anonfun$hijack$1$$anonfun$apply$1.apply(FastTrack.scala:22)
        at org.scalalang.macroparadise.typechecker.FastTrack$$anonfun$1.applyOrElse(FastTrack.scala:14)
        at org.scalalang.macroparadise.typechecker.FastTrack$$anonfun$1.applyOrElse(FastTrack.scala:14)
        at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:33)
        at scala.tools.reflect.FastTrack$FastTrackEntry.run(FastTrack.scala:25)

But what's more strange, is that when making an empty list in a less apparent way, such as:

object Bar {
  import language.experimental.macros
  import scala.reflect.macros.Context

  trait Foo
  def foo: Bar.Foo = macro fooImpl
  def fooImpl(c: Context): c.Expr[Bar.Foo] = {
    import c.universe._
    val trees = for { x <- 0 to -1} yield x
    c.Expr[Foo](q"new Bar.Foo {..$trees}")
  }
}

The code compiles, but fails to instantiate the trait at runtime, with this error:

error: trait Foo is abstract; cannot be instantiated
Bar.foo

Which boils down to the fact, that for some reason, the quasiquote produces the tree for {code}new Bar.Foo() {code} instead of {code} new Bar.Foo{} {code}

scabug commented 10 years ago

Imported From: https://issues.scala-lang.org/browse/SI-7945?orig=1 Reporter: Daniel Beskin (n_creep) Affected Versions: macro-paradise, 2.10.3

SethTisue commented 6 years ago

closing all quasiquotes tickets; see #10755

scala / bug

Quasiquote splicing for empty lists misbehaves during trait instantiation #7945