Regression: Macro usage compilation error (related to closure?)

[Eduardo D'Avila]

Hi!

I may have found a compiler bug related to macros.

The macro definition:

import scala.language.experimental.macros

import scala.reflect.macros.blackbox.Context

object Y {

  def m(g: () => Int): Unit = macro impl

  def impl(c: Context)(g: c.Tree): c.Tree = {

    import c.universe._

    q"{ () => $g() }.apply()"

  }

}

And the usage:

object X {

  def f(): Unit = {

    val thisIsTheVal = 0

    Y.m { () => thisIsTheVal }

  }

}

On 2.11.8, the compilation works without any error. But on 2.12.0 and 2.12.1, the compilation of the code using the macro issues a "java.util.NoSuchElementException: key not found: value thisIsTheVal" exception. See the attached file for the full stack trace.

Is this a known issue?

Regards,

Eduardo

[Jason Zaugg]

In general, it is not safe to splice a macro argument into the body of a synthesised closure, class, method, or val. The result can be an incoherent "owner chain". 

A common fix (albeit with its own corner cases) it to call `c.untypecheck` on the tree before splicing. 

Changes to the way that lambdas are desugared in 2.12. might have changed this particular example from benign to a crasher.

-jason

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[Eduardo D'Avila]

Thank you, Jason! 

With the proposed fix, now the code compiles successfully:

  def impl(c: Context)(g: c.Tree): c.Tree = {

    import c.universe._

    val ug = c.untypecheck(g)

    q"{ () => $ug() }.apply()"

  }

Anyways, should I report an issue?

Eduardo

[Jason Zaugg]

I think in this case the macro is at fault, and it only worked accidentally in 2.11, so no need for an issue.

-jason

[Eduardo R. D'Avila]

Hmmm... OK

Thanks!

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