MatchError in macro expansion on AndType from explicit null flow typing.

[johnynek]

Compiler version

3.8.1

Minimized code

in one file:

package Foo

import scala.quoted.*

object NonNullFold:

  extension [A](inline arg: A | Null)
    /**
     * Extension method to fold over a nullable value.
     * Uses pattern matching to ensure compatibility with strict equality.
     */
    inline def foldNN[B](inline default: => B)(inline fn: A => B): B =
      ${ foldNNImpl('arg, 'default, 'fn) }

    inline def notNull[B]: Boolean =
      foldNN(false)(_ => true)

  private def foldNNImpl[A: Type, B: Type](
      arg: Expr[A | Null],
      default: Expr[B],
      fn: Expr[A => B]
  )(using Quotes): Expr[B] =
    '{
      // 1. Bind the argument to a val to prevent multiple evaluations
      val t = $arg
      
      // 2. 
      // This works even if 'import scala.language.strictEquality' is active.
      given CanEqual[A | Null, Null] = CanEqual.derived
      if (t == null) $default
      else {
        // CRITICAL FIX: 
        // We define a new local 'val' with the explicit type 'A'.
        // This strips away the complex intersection types (flow typing) from 't'
        // that were causing the MatchError in the unpickler.
        //val safe: A = t.asInstanceOf[A]
        // 3. We cast to A safely because we are in the not-null branch.
        // 4. betaReduce inlines the function body directly here.
       //${ Expr.betaReduce('{ $fn(safe) }) }
        ${ Expr.betaReduce('{ $fn(t) }) }
      }
    }

In another:
import NonNullFold.*

package Foo

val x: String | Null = "not null"

val y = x.foldNN("it's null")(x => x)

Output (click arrow to expand)

Details

error] 135 |val y = x.foldNN("it's null")(x => x)
[error]     |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]     |Exception occurred while executing macro expansion.
[error]     |scala.MatchError: AndType(TermRef(NoPrefix,val t),TypeRef(NoPrefix,type evidence$1$1)) (of class dotty.tools.dotc.core.Types$CachedAndType)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readPathTree$1(TreeUnpickler.scala:1285)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readLengthTree$1(TreeUnpickler.scala:1702)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readTree(TreeUnpickler.scala:1708)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.$anonfun$50(TreeUnpickler.scala:1767)
[error]     |   at dotty.tools.tasty.TastyReader.until(TastyReader.scala:135)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readHole(TreeUnpickler.scala:1767)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readLengthTree$1(TreeUnpickler.scala:1700)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readTree(TreeUnpickler.scala:1708)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readLengthTree$1(TreeUnpickler.scala:1531)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readTree(TreeUnpickler.scala:1708)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler.dotty$tools$dotc$core$tasty$TreeUnpickler$TreeReader$$_$_$$anonfun$34(TreeUnpickler.scala:1521)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readIndexedStats(TreeUnpickler.scala:1248)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readStats(TreeUnpickler.scala:1252)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readLengthTree$1(TreeUnpickler.scala:1521)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readTree(TreeUnpickler.scala:1708)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readLengthTree$1(TreeUnpickler.scala:1539)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readTree(TreeUnpickler.scala:1708)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler.dotty$tools$dotc$core$tasty$TreeUnpickler$TreeReader$$_$_$$anonfun$34(TreeUnpickler.scala:1521)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readIndexedStats(TreeUnpickler.scala:1248)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readStats(TreeUnpickler.scala:1252)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readLengthTree$1(TreeUnpickler.scala:1521)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readTree(TreeUnpickler.scala:1708)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler.dotty$tools$dotc$core$tasty$TreeUnpickler$TreeReader$$_$_$$anonfun$34(TreeUnpickler.scala:1521)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readIndexedStats(TreeUnpickler.scala:1248)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readStats(TreeUnpickler.scala:1252)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readLengthTree$1(TreeUnpickler.scala:1521)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readTree(TreeUnpickler.scala:1708)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readLengthTree$1(TreeUnpickler.scala:1531)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readTree(TreeUnpickler.scala:1708)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler$TreeReader.readTree(TreeUnpickler.scala:1273)
[error]     |   at dotty.tools.dotc.core.tasty.TreeUnpickler.unpickle(TreeUnpickler.scala:146)
[error]     |   at dotty.tools.dotc.core.tasty.DottyUnpickler.computeRootTrees(DottyUnpickler.scala:95)
[error]     |   at dotty.tools.dotc.ast.tpd$TreeProvider.rootTrees(tpd.scala:1374)
[error]     |   at dotty.tools.dotc.ast.tpd$TreeProvider.rootTrees$(tpd.scala:1363)
[error]     |   at dotty.tools.dotc.core.tasty.DottyUnpickler.rootTrees(DottyUnpickler.scala:58)
[error]     |   at dotty.tools.dotc.ast.tpd$TreeProvider.tree(tpd.scala:1378)
[error]     |   at dotty.tools.dotc.ast.tpd$TreeProvider.tree$(tpd.scala:1363)
[error]     |   at dotty.tools.dotc.core.tasty.DottyUnpickler.tree(DottyUnpickler.scala:58)
[error]     |   at dotty.tools.dotc.quoted.PickledQuotes$.unpickle(PickledQuotes.scala:278)
[error]     |   at dotty.tools.dotc.quoted.PickledQuotes$.unpickleTerm(PickledQuotes.scala:86)
[error]     |   at scala.quoted.runtime.impl.QuotesImpl.unpickleExprV2(QuotesImpl.scala:3562)
[error]     |   at dev.bosatsu.NonNullFold$.foldNNImpl(NonNullFold.scala:40)
[error]     |   at dev.bosatsu.NonNullFold$.inline$foldNNImpl(NonNullFold.scala:18)
[error]     |
[error]     |---------------------------------------------------------------------------
[error]     |Inline stack trace
[error]     |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]     |This location contains code that was inlined from NonNullFold.scala:13
[error]  13 |      ${ foldNNImpl('arg, 'default, 'fn) }
[error]     |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]      ---------------------------------------------------------------------------
[error] one error found

My work around is to uncomment the safe related variables and cast away the flow typing added by explicit nulls, but I shouldn't have to do that. The compiler shouldn't crash