Staging: Substituting values for mutable variables generates invalid code

[b-studios]

When using the staging API one can extract a mutable variable into a HOAS function and then substitute a value. This will generate an assignment to the substituted value, which is clearly wrong.

Minimized code

import scala.quoted._
import scala.quoted.staging._

object Test extends App {

  // make available the necessary toolbox for runtime code generation
  given Toolbox = Toolbox.make(getClass.getClassLoader)

  run {
    val expr: Expr[Int] = '{ var x = 1; x = 2; 42 }

    expr match {
      case '{ var x: Int = $binding; $body(x): Int } =>
    val res = Expr.betaReduce('{ $body(4) })
    println(res.show)
    res
      case _ => println(expr.show); '{0}
    }
  }
}

Output (click arrow to expand)

[warn] -- Warning: /Users/jonathan/playground/staging-test/src/main/scala/Main.scala:14:4
[warn] 14 |    val res = Expr.betaReduce('{ $body(4) })
[warn]    |    ^
[warn]    |    Line is indented too far to the left, or a `}` is missing
[warn] -- Warning: /Users/jonathan/playground/staging-test/src/main/scala/Main.scala:15:4
[warn] 15 |    println(res.show)
[warn]    |    ^
[warn]    |    Line is indented too far to the left, or a `}` is missing
[warn] -- Warning: /Users/jonathan/playground/staging-test/src/main/scala/Main.scala:16:4
[warn] 16 |    res
[warn]    |    ^
[warn]    |    Line is indented too far to the left, or a `}` is missing
[warn] three warnings found
[info] running Test
{
  4 = 2
  42
}
[error] (run-main-3) java.lang.AssertionError: assertion failed: asTerm called on not-a-Term val <none>
[error] java.lang.AssertionError: assertion failed: asTerm called on not-a-Term val <none>
[error] 	at dotty.DottyPredef$.assertFail(DottyPredef.scala:17)
[error] 	at dotty.tools.dotc.core.Symbols$Symbol.asTerm(Symbols.scala:156)
[error] 	at dotty.tools.dotc.transform.Getters.transformAssign(Getters.scala:114)
[error] 	at dotty.tools.dotc.transform.MegaPhase.goAssign(MegaPhase.scala:718)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:344)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:428)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:424)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:438)
[error] 	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:441)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:441)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:298)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:428)
[error] 	at dotty.tools.dotc.transform.MegaPhase.mapDefDef$1(MegaPhase.scala:248)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:251)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:426)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:436)
[error] 	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:441)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:441)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:361)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:428)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:255)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:426)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:436)
[error] 	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:441)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:441)
[error] 	at dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:381)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:384)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:428)
[error] 	at dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:447)
[error] 	at dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:459)
[error] 	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:296)
[error] 	at scala.collection.immutable.List.map(List.scala:246)
[error] 	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:297)
[error] 	at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:180)
[error] 	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] 	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] 	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
[error] 	at dotty.tools.dotc.Run.runPhases$5(Run.scala:190)
[error] 	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:198)
[error] 	at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
[error] 	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:64)
[error] 	at dotty.tools.dotc.Run.compileUnits(Run.scala:205)
[error] 	at dotty.tools.dotc.Run.compileUnits(Run.scala:147)
[error] 	at scala.quoted.staging.QuoteCompiler$ExprRun.compileExpr(QuoteCompiler.scala:107)
[error] 	at scala.quoted.staging.QuoteDriver.run(QuoteDriver.scala:39)
[error] 	at scala.quoted.staging.Toolbox$$anon$1.run(Toolbox.scala:36)
[error] 	at scala.quoted.staging.package$.run(staging.scala:19)
[error] 	at Test$.<clinit>(Main.scala:19)
[error] 	at Test.main(Main.scala)
[error] 	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error] 	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error] 	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error] 	at java.base/java.lang.reflect.Method.invoke(Method.java:566)