-Ycc-log crash

## Compiler version

3.10.1-RC1-bin-SNAPSHOT (main @ 297c812, built 2026-09-03)

Unreleased regression on main: introducing commit a6365f765c (2026-08-19, PR #26854).

All released versions <= 3.10.0-RC1 are unaffected (checked 3.8.1 / 3.8.2 / 3.8.4 / 3.9.0 / 3.10.0-RC1).

## Minimized code

`a.scala`
```Scala
import language.experimental.captureChecking
import caps.*

class Cap1 extends caps.ExclusiveCapability

def test(): Unit =
  val v1 = (x0: Cap1^) => { val u2 = new Cap1
    def d3(): Cap1^{u2} = u2
    d3()
    ()
  }

@main def main(): Unit = ()
```
Run with (`-Ycc-log` crucial)
```bash
scala-cli compile --scala-version 3.10.1-RC1-bin-SNAPSHOT --repository ivy2Local   -O "-language:experimental.captureChecking" -O "-Ycc-log" --server=false a.scala
```

## Output (just the error)

```Scala
  unhandled exception while running cc on /tmp/a.scala

  An unhandled exception was thrown in the compiler.
  Please file a crash report here:
  https://github.com/scala/scala3/issues/new/choose
  For non-enriched exceptions, compile with -Xno-enrich-error-messages.


     while compiling: /tmp/a.scala
        during phase: cc
                mode: Mode(ImplicitsEnabled)
     library version: version 3.10.1-RC1-bin-SNAPSHOT
    compiler version: version 3.10.1-RC1-bin-SNAPSHOT-git-297c812
            settings: -Ycc-log true -classpath /home/julian/.ivy2/local/org.scala-lang/scala3-library_3/3.10.1-RC1-bin-SNAPSHOT/jars/scala3-library_3.jar:/home/julian/.ivy2/local/org.scala-lang/scala-library/3.10.1-RC1-bin-SNAPSHOT/jars/scala-library.jar -d /home/julian/.cache/scalacli/virtual-projects/ff/project-4e60d9b9/.scala-build/tmp_a2d2d53713/classes/main -language List(experimental.captureChecking) -sourceroot /home/julian/.cache/scalacli/virtual-projects/ff/project-4e60d9b9

Exception in thread "main" dotty.tools.dotc.core.CyclicReference: Cyclic reference involving method d3

 Run with -explain-cyclic for more details.
Compilation failed
```

## Output (full)
<details>

```
cc check /tmp/a.scala
expanded explicit in class Cap1: scala.caps.ExclusiveCapability  -->  scala.caps.ExclusiveCapability^
expanded explicit in class Cap1: scala.caps.ExclusiveCapability  -->  scala.caps.ExclusiveCapability^
update class info of class Cap1 with parents [Object, scala.caps.ExclusiveCapability] selfinfo <notype> to Object with (scala.caps.ExclusiveCapability^) {...}
expanded inferred in package object a$package: a$package  -->  a$package^'s1
expanded explicit in value v1: Cap1^  -->  Cap1^
expanded inferred in value v1: (x0: (Cap1^) @declared) -> Unit  -->  (x0: Cap1^) ->'s2 Unit
expanded explicit in method $anonfun: Cap1^  -->  Cap1^
expanded inferred in value u2: Cap1  -->  Cap1^'s3
expanded explicit in method d3: Cap1^{u2}  -->  Cap1^{u2}
update class info of the top-level definitions in package <empty> with parents [Object] selfinfo a$package.type to Object {...}
update class info of class main with parents [Object] selfinfo <notype> to Object {...}
expanded explicit in trait ExclusiveCapability: Object with scala.caps.Capability {...}  -->  Object with (scala.caps.Capability^) {...}
Complete method $anonfun in [method test, the top-level definitions in package <empty>]
constr type (): Cap1 with  in class Cap1 = Cap1^
Include call or box capture 's4 from 's5 in method $anonfun --> 's6
Include call or box capture 's7 from 's4 in method test --> 's8
Include call or box capture 's9 from 's7 in the top-level definitions in package <empty> --> 's10
solved vars for value u2 in Cap1^
error while rechecking def d3(): Cap1^{u2} = u2
error while rechecking ... (caught cyclic reference) ...
==> rechecking ... (caught cyclic reference) ... with pt = <?>?
  ==> rechecking class Cap1() extends Object(), scala.caps.ExclusiveCapability {} with pt = <?>?
    ==> rechecking def <init>(): Unit with pt = <?>?
      ==> rechecking Unit with pt = <?>?
      <== rechecking Unit with pt = <?> = Unit
solved vars for constructor Cap1 in Unit
    <== rechecking def <init>(): Unit with pt = <?> = (Cap1.this.<init> : (): Cap1)
    ==> rechecking new Object() with pt = <?>?
      ==> rechecking new Object with pt = ?.{ constructor Object}?
      <== rechecking new Object with pt = ?.{ constructor Object} = Object
constr type (): Object with  in class Object = Object
    <== rechecking new Object() with pt = <?> = Object
    ==> rechecking scala.caps.ExclusiveCapability with pt = <?>?
    <== rechecking scala.caps.ExclusiveCapability with pt = <?> = scala.caps.ExclusiveCapability^
    ==> rechecking val _ with pt = <?>?
    <== rechecking val _ with pt = <?> = <notype>
Use set of class Cap1 = 's11, self type: Cap1^
  <== rechecking class Cap1() extends Object(), scala.caps.ExclusiveCapability {} with pt = <?> = Cap1
  ==> rechecking final lazy module val a$package: a$package = new a$package() with pt = <?>?
    ==> rechecking a$package with pt = <?>?
    <== rechecking a$package with pt = <?> = a$package^'s1
    ==> rechecking new a$package() with pt = a$package^'s1?
      ==> rechecking new a$package with pt = ?.{ constructor a$package}?
      <== rechecking new a$package with pt = ?.{ constructor a$package} = a$package
constr type (): a$package with  in the top-level definitions in package <empty> = a$package^'s10
Include call or box capture 's12 from 's13 in package object a$package --> 's14
    <== rechecking new a$package() with pt = a$package^'s1 = a$package^'s10
  <== rechecking final lazy module val a$package: a$package = new a$package() with pt = <?> = a$package.type
  ==> rechecking ... (caught cyclic reference) ... with pt = <?>?
    ==> rechecking def <init>(): Unit with pt = <?>?
      ==> rechecking Unit with pt = <?>?
      <== rechecking Unit with pt = <?> = Unit
solved vars for constructor a$package in Unit
    <== rechecking def <init>(): Unit with pt = <?> = (<init> : (): a$package)
    ==> rechecking new Object() with pt = <?>?
      ==> rechecking new Object with pt = ?.{ constructor Object}?
      <== rechecking new Object with pt = ?.{ constructor Object} = Object
constr type (): Object with  in class Object = Object
    <== rechecking new Object() with pt = <?> = Object
    ==> rechecking val _ with pt = <?>?
    <== rechecking val _ with pt = <?> = <notype>
    ==> rechecking private[this] type $this = a$package.type with pt = <?>?
      ==> rechecking a$package.type with pt = <?>?
      <== rechecking a$package.type with pt = <?> = a$package.type
    <== rechecking private[this] type $this = a$package.type with pt = <?> = $this
    ==> rechecking private def writeReplace(): AnyRef =
  new scala.runtime.ModuleSerializationProxy(classOf[a$package.type]) with pt = <?>?
      ==> rechecking AnyRef with pt = <?>?
      <== rechecking AnyRef with pt = <?> = AnyRef
      ==> rechecking new scala.runtime.ModuleSerializationProxy(classOf[a$package.type]) with pt = AnyRef?
        ==> rechecking new scala.runtime.ModuleSerializationProxy with pt = ?.{ constructor ModuleSerializationProxy}?
        <== rechecking new scala.runtime.ModuleSerializationProxy with pt = ?.{ constructor ModuleSerializationProxy} = scala.runtime.ModuleSerializationProxy
        ==> rechecking classOf[a$package.type] with pt = Class[?]^<fluid>?
        <== rechecking classOf[a$package.type] with pt = Class[?]^<fluid> = (classOf[a$package.type] : Class[a$package.type])
recheck arg classOf[a$package.type] vs Class[?]^<fluid> = (classOf[a$package.type] : Class[a$package.type])
constr type (moduleClass: Class[?]^<fluid>): scala.runtime.ModuleSerializationProxy with (classOf[a$package.type] : Class[a$package.type]) in class ModuleSerializationProxy = scala.runtime.ModuleSerializationProxy^'s15
Include call or box capture 's16 from 's17 in method writeReplace --> 's18
Include call or box capture 's19 from 's16 in the top-level definitions in package <empty> --> 's10
      <== rechecking new scala.runtime.ModuleSerializationProxy(classOf[a$package.type]) with pt = AnyRef = scala.runtime.ModuleSerializationProxy^'s15
    <== rechecking private def writeReplace(): AnyRef =
  new scala.runtime.ModuleSerializationProxy(classOf[a$package.type]) with pt = <?> = (writeReplace : (): AnyRef)
    ==> rechecking ... (caught cyclic reference) ... with pt = <?>?
      ==> rechecking Unit with pt = <?>?
      <== rechecking Unit with pt = <?> = Unit
      ==> rechecking ... (caught cyclic reference) ... with pt = Unit?
        ==> rechecking ... (caught cyclic reference) ... with pt = <?>?
Complete value v1 in [method test, the top-level definitions in package <empty>]
          ==> rechecking (x0: (Cap1^) @declared) -> Unit with pt = <?>?
          <== rechecking (x0: (Cap1^) @declared) -> Unit with pt = <?> = (x0: Cap1^) ->'s2 Unit
          ==> rechecking ... (caught cyclic reference) ... with pt = (x0: Cap1^) ->'s2 Unit?
recheck closure block ... (caught cyclic reference) ...: (x0: Cap1^): Unit
            ==> rechecking Unit with pt = <?>?
            <== rechecking Unit with pt = <?> = Unit
            ==> rechecking ... (caught cyclic reference) ... with pt = Unit?
              ==> rechecking val u2: Cap1 = new Cap1() with pt = <?>?
                ==> rechecking Cap1 with pt = <?>?
                <== rechecking Cap1 with pt = <?> = Cap1^
                ==> rechecking new Cap1() with pt = Cap1^?
                  ==> rechecking new Cap1 with pt = ?.{ constructor Cap1}?
                  <== rechecking new Cap1 with pt = ?.{ constructor Cap1} = Cap1
constr type (): Cap1 with  in class Cap1 = Cap1^
Include call or box capture 's20 from 's21 in method $anonfun --> 's6
Include call or box capture 's22 from 's20 in method test --> 's8
Include call or box capture 's23 from 's22 in the top-level definitions in package <empty> --> 's10
                <== rechecking new Cap1() with pt = Cap1^ = Cap1^
solved vars for value u2 in Cap1^
              <== rechecking val u2: Cap1 = new Cap1() with pt = <?> = (u2 : Cap1)
              ==> rechecking def d3(): Cap1^{u2} = u2 with pt = <?>?
error while rechecking def d3(): Cap1^{u2} = u2
              <== rechecking def d3(): Cap1^{u2} = u2 with pt = <?> = <missing> (with exception dotty.tools.dotc.core.CyclicReference: Cyclic reference involving method d3

 Run with -explain-cyclic for more details.)
error while rechecking ... (caught cyclic reference) ...
            <== rechecking ... (caught cyclic reference) ... with pt = Unit = <missing> (with exception dotty.tools.dotc.core.CyclicReference: Cyclic reference involving method d3

 Run with -explain-cyclic for more details.)
error while rechecking ... (caught cyclic reference) ...
          <== rechecking ... (caught cyclic reference) ... with pt = (x0: Cap1^) ->'s2 Unit = <missing> (with exception dotty.tools.dotc.core.CyclicReference: Cyclic reference involving method d3

 Run with -explain-cyclic for more details.)
solved vars for value v1 in (x0: Cap1^) -> Unit
error while rechecking ... (caught cyclic reference) ...
        <== rechecking ... (caught cyclic reference) ... with pt = <?> = <missing> (with exception dotty.tools.dotc.core.CyclicReference: Cyclic reference involving method d3

 Run with -explain-cyclic for more details.)
error while rechecking ... (caught cyclic reference) ...
      <== rechecking ... (caught cyclic reference) ... with pt = Unit = <missing> (with exception dotty.tools.dotc.core.CyclicReference: Cyclic reference involving method d3

 Run with -explain-cyclic for more details.)
error while rechecking ... (caught cyclic reference) ...
    <== rechecking ... (caught cyclic reference) ... with pt = <?> = <missing> (with exception dotty.tools.dotc.core.CyclicReference: Cyclic reference involving method d3

 Run with -explain-cyclic for more details.)
error while rechecking ... (caught cyclic reference) ...
  <== rechecking ... (caught cyclic reference) ... with pt = <?> = <missing> (with exception dotty.tools.dotc.core.CyclicReference: Cyclic reference involving method d3

 Run with -explain-cyclic for more details.)
error while rechecking ... (caught cyclic reference) ...
<== rechecking ... (caught cyclic reference) ... with pt = <?> = <missing> (with exception dotty.tools.dotc.core.CyclicReference: Cyclic reference involving method d3

 Run with -explain-cyclic for more details.)

  unhandled exception while running cc on /tmp/a.scala

  An unhandled exception was thrown in the compiler.
  Please file a crash report here:
  https://github.com/scala/scala3/issues/new/choose
  For non-enriched exceptions, compile with -Xno-enrich-error-messages.


     while compiling: /tmp/a.scala
        during phase: cc
                mode: Mode(ImplicitsEnabled)
     library version: version 3.10.1-RC1-bin-SNAPSHOT
    compiler version: version 3.10.1-RC1-bin-SNAPSHOT-git-297c812
            settings: -Ycc-log true -classpath /home/julian/.ivy2/local/org.scala-lang/scala3-library_3/3.10.1-RC1-bin-SNAPSHOT/jars/scala3-library_3.jar:/home/julian/.ivy2/local/org.scala-lang/scala-library/3.10.1-RC1-bin-SNAPSHOT/jars/scala-library.jar -d /home/julian/.cache/scalacli/virtual-projects/ff/project-4e60d9b9/.scala-build/tmp_a2d2d53713/classes/main -language List(experimental.captureChecking) -sourceroot /home/julian/.cache/scalacli/virtual-projects/ff/project-4e60d9b9

Exception in thread "main" dotty.tools.dotc.core.CyclicReference: Cyclic reference involving method d3

 Run with -explain-cyclic for more details.
Compilation failed
```

</details>


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Uh oh!

-Ycc-log crash #26986

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Uh oh!

-Ycc-log crash #26986

Description

Compiler version

Minimized code

Output (just the error)

Output (full)

Activity

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions