Nondeterministic compiler crash on clean compile (ClassCastException: PolyType, CachedTermRef)

[hnaderi]

Compiler version

I tried with these versions
3.2.2 ,3.2.1, 3.2.0

Minimized code

I couldn't reproduce an independent minimized code yet, as it is hard to check whether the issue is gone or is not appearing due to some other change.

Output (click arrow to expand)

[error] java.lang.ClassCastException: dotty.tools.dotc.core.Types$CachedTermRef cannot be cast to dotty.tools.dotc.core.Types$PolyType [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:352) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.app1$1(PostTyper.scala:324) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:337) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform$$anonfun$1(Trees.scala:1507) [error] scala.collection.immutable.List.mapConserve(List.scala:472) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1507) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.app1$1(PostTyper.scala:323) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:337) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1406) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:49) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transformSelect(PostTyper.scala:207) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:356) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.app1$1(PostTyper.scala:324) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:337) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1406) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:49) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transformSelect(PostTyper.scala:207) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:356) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.app1$1(PostTyper.scala:324) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:337) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1434) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:49) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:283) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform$$anonfun$1(Trees.scala:1507) [error] scala.collection.immutable.List.mapConserve(List.scala:472) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1507) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transformSub(Trees.scala:1511) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1432) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:49) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:465) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1478) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:40) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform$$anonfun$5(PostTyper.scala:384) [error] dotty.tools.dotc.transform.SuperAccessors.wrapDefDef(SuperAccessors.scala:223) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:384) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.loop$2(tpd.scala:1225) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1225) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transformStats(PostTyper.scala:474) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformBlock(tpd.scala:1230) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1426) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:49) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:465) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform$$anonfun$1(Trees.scala:1507) [error] scala.collection.immutable.List.mapConserve(List.scala:472) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1507) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.app1$1(PostTyper.scala:323) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:337) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1478) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:40) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform$$anonfun$5(PostTyper.scala:384) [error] dotty.tools.dotc.transform.SuperAccessors.wrapDefDef(SuperAccessors.scala:223) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:384) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.loop$2(tpd.scala:1225) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1225) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transformStats(PostTyper.scala:474) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformBlock(tpd.scala:1230) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1426) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:49) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:465) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform$$anonfun$1(Trees.scala:1507) [error] scala.collection.immutable.List.mapConserve(List.scala:472) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1507) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.app1$1(PostTyper.scala:323) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:337) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1478) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:40) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform$$anonfun$5(PostTyper.scala:384) [error] dotty.tools.dotc.transform.SuperAccessors.wrapDefDef(SuperAccessors.scala:223) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:384) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.loop$2(tpd.scala:1225) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1225) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transformStats(PostTyper.scala:474) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformBlock(tpd.scala:1230) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1426) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:49) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:465) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform$$anonfun$1(Trees.scala:1507) [error] scala.collection.immutable.List.mapConserve(List.scala:472) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1507) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.app1$1(PostTyper.scala:323) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:337) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformBlock$$anonfun$1$$anonfun$1(tpd.scala:1230) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.loop$2(tpd.scala:1212) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1225) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transformStats(PostTyper.scala:474) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformBlock(tpd.scala:1230) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1426) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:49) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:465) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform$$anonfun$1(Trees.scala:1507) [error] scala.collection.immutable.List.mapConserve(List.scala:472) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1507) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.app1$1(PostTyper.scala:323) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:337) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.app1$1(PostTyper.scala:324) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:337) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.loop$2(tpd.scala:1225) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1225) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transformStats(PostTyper.scala:474) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1227) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:47) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform$$anonfun$4$$anonfun$1(PostTyper.scala:371) [error] dotty.tools.dotc.transform.SuperAccessors.wrapTemplate(SuperAccessors.scala:208) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform$$anonfun$4(PostTyper.scala:371) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.withNoCheckNews(PostTyper.scala:104) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:373) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1480) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:40) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:411) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.loop$2(tpd.scala:1225) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1225) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transformStats(PostTyper.scala:474) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1227) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1488) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:40) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:465) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.loop$2(tpd.scala:1225) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1225) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transformStats(PostTyper.scala:474) [error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1227) [error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1488) [error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:40) [error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:465) [error] dotty.tools.dotc.transform.MacroTransform.run(MacroTransform.scala:18) [error] dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:316) [error] scala.collection.immutable.List.map(List.scala:250) [error] dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:320) [error] dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:238) [error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15) [error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10) [error] scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1321) [error] dotty.tools.dotc.Run.runPhases$1(Run.scala:249) [error] dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:257) [error] dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:266) [error] dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68) [error] dotty.tools.dotc.Run.compileUnits(Run.scala:266) [error] dotty.tools.dotc.Run.compileSources(Run.scala:190) [error] dotty.tools.dotc.Run.compile(Run.scala:174) [error] dotty.tools.dotc.Driver.doCompile(Driver.scala:35) [error] dotty.tools.xsbt.CompilerBridgeDriver.run(CompilerBridgeDriver.java:88) [error] dotty.tools.xsbt.CompilerBridge.run(CompilerBridge.java:22) [error] sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:91) [error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$7(MixedAnalyzingCompiler.scala:193) [error] scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23) [error] sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:248) [error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4(MixedAnalyzingCompiler.scala:183) [error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4$adapted(MixedAnalyzingCompiler.scala:163) [error] sbt.internal.inc.JarUtils$.withPreviousJar(JarUtils.scala:239) [error] sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:163) [error] sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:211) [error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:534) [error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:534) [error] sbt.internal.inc.Incremental$.$anonfun$apply$5(Incremental.scala:179) [error] sbt.internal.inc.Incremental$.$anonfun$apply$5$adapted(Incremental.scala:177) [error] sbt.internal.inc.Incremental$$anon$2.run(Incremental.scala:463) [error] sbt.internal.inc.IncrementalCommon$CycleState.next(IncrementalCommon.scala:116) [error] sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:56) [error] sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:52) [error] sbt.internal.inc.IncrementalCommon.cycle(IncrementalCommon.scala:263) [error] sbt.internal.inc.Incremental$.$anonfun$incrementalCompile$8(Incremental.scala:418) [error] sbt.internal.inc.Incremental$.withClassfileManager(Incremental.scala:506) [error] sbt.internal.inc.Incremental$.incrementalCompile(Incremental.scala:405) [error] sbt.internal.inc.Incremental$.apply(Incremental.scala:171) [error] sbt.internal.inc.IncrementalCompilerImpl.compileInternal(IncrementalCompilerImpl.scala:534) [error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileIncrementally$1(IncrementalCompilerImpl.scala:488) [error] sbt.internal.inc.IncrementalCompilerImpl.handleCompilationError(IncrementalCompilerImpl.scala:332) [error] sbt.internal.inc.IncrementalCompilerImpl.compileIncrementally(IncrementalCompilerImpl.scala:425) [error] sbt.internal.inc.IncrementalCompilerImpl.compile(IncrementalCompilerImpl.scala:137) [error] sbt.Defaults$.compileIncrementalTaskImpl(Defaults.scala:2363) [error] sbt.Defaults$.$anonfun$compileIncrementalTask$2(Defaults.scala:2313) [error] sbt.internal.server.BspCompileTask$.$anonfun$compute$1(BspCompileTask.scala:30) [error] sbt.internal.io.Retry$.apply(Retry.scala:46) [error] sbt.internal.io.Retry$.apply(Retry.scala:28) [error] sbt.internal.io.Retry$.apply(Retry.scala:23) [error] sbt.internal.server.BspCompileTask$.compute(BspCompileTask.scala:30) [error] sbt.Defaults$.$anonfun$compileIncrementalTask$1(Defaults.scala:2311) [error] scala.Function1.$anonfun$compose$1(Function1.scala:49) [error] sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:62) [error] sbt.std.Transform$$anon$4.work(Transform.scala:68) [error] sbt.Execute.$anonfun$submit$2(Execute.scala:282) [error] sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:23) [error] sbt.Execute.work(Execute.scala:291) [error] sbt.Execute.$anonfun$submit$1(Execute.scala:282) [error] sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265) [error] sbt.CompletionService$$anon$2.call(CompletionService.scala:64) [error] java.util.concurrent.FutureTask.run(FutureTask.java:266) [error] java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [error] java.util.concurrent.FutureTask.run(FutureTask.java:266) [error] java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [error] java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [error] java.lang.Thread.run(Thread.java:750) [error] (clientJS / Test / compileIncremental) java.lang.ClassCastException: dotty.tools.dotc.core.Types$CachedTermRef cannot be cast to dotty.tools.dotc.core.Types$PolyType

[SethTisue]

"ClassCastException: dotty.tools.dotc.core.Types$CachedTermRef cannot be cast to dotty.tools.dotc.core.Types$PolyType" isn't one I've seen before. at minimum, perhaps others who see the same error will search and land here and perhaps have some insight

you seem to have double-pasted the stack trace; mind cleaning that up?

[SethTisue]

perhaps once #16593 lands you'll be able to include more useful diagnostic information in the report (and that might help with minimizing, too... minimizing it is the usual path towards an actual fix)

[hnaderi]

you seem to have double-pasted the stack trace; mind cleaning that up?

Oops! thanks for pointing out that, just cleaned it up.

[smarter]

Is the crash happening on a public codebase you could link us to? That could help us investigate the issue without a minimization.

[hnaderi]

Yes, it's an opensource project.
I created a separate branch that contains the error here https://github.com/hnaderi/lepus/tree/compiler-crash
This issue was introduced here probably (9bf4a9f8584de525c602e980e3a7eec788f647b5)
The problem seems to be residing in this specific file https://github.com/hnaderi/lepus/blob/compiler-crash/modules/client/src/test/scala/internal/ChannelOutputSuite.scala

How to reproduce:

• [project rootJVM]
• clean
• test

It might take a few retries to see the error happening.
The error can be seen on these CI jobs as well:
https://github.com/hnaderi/lepus/actions/runs/4142125900/jobs/7162428494#step:12:66
https://github.com/hnaderi/lepus/actions/runs/4150091032
https://github.com/hnaderi/lepus/actions/runs/4142125900
https://github.com/hnaderi/lepus/actions/runs/4142131777

[smarter]

Thanks, it seems that the issue is easier to reproduce if I reduce the size of the heap by setting -Xmx1G in the project .jvmopts, then it usually happens after a couple of call to clientJVM/clean;clientJVM/Test/compile.

The crash happens on https://github.com/hnaderi/lepus/blob/fecc7c7e793b98b9f9a85eac89856ee51baf2b6a/modules/client/src/test/scala/internal/ChannelOutputSuite.scala#L97 when PostTyper sees out.writeOne(1).timeout: sometimes the denotation of timeout is an overloaded denotation, which makes no sense since overloading should have been resolved in Typer.

This appears to be due to cache pollution: TermRef(..., timeout) is created/read from cache in https://github.com/lampepfl/dotty/blob/3d251d6fd1bb1e98d66760ee50b824861a87ca57/compiler/src/dotty/tools/dotc/typer/Typer.scala#L619 which delegates to https://github.com/lampepfl/dotty/blob/3d251d6fd1bb1e98d66760ee50b824861a87ca57/compiler/src/dotty/tools/dotc/core/Types.scala#L2921-L2922

Sometimes, the apply pulls a previous TermRef from cache which has a non-overloaded denotation and the call to withDenot overwrites it with an overloaded denotation. There are two overloads of timeout in IO, but only one is accessible. So our type's denotation gets fixed in typedSelect afterwards by:
https://github.com/lampepfl/dotty/blob/3d251d6fd1bb1e98d66760ee50b824861a87ca57/compiler/src/dotty/tools/dotc/typer/Typer.scala#L620
which delegates to:
https://github.com/lampepfl/dotty/blob/3d251d6fd1bb1e98d66760ee50b824861a87ca57/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala#L86
Usually, this second call to withDenot will overwrite the overloaded denotation in the TermRef we previously pulled from the cache, thus masking the issue, but I guess sometimes we instead end up copying the TermRef?

IMO, withDenot should never mutate types, the risk of cache pollution is too high and we end up with impossible to debug issues like this one, but short of changing that I don't know how to proceed /cc @odersky .

[odersky]

IMO, withDenot should never mutate types, the risk of cache pollution is too high and we end up with impossible to debug issues like this one, but short of changing that I don't know how to proceed /cc @odersky .

That must have been hard to spot! But I am not sure it's the mutation itself which is to blame. If withDenot would not mutate the type but create a fresh copy instead, the end result would be the same. The fresh copy is hashed to be the same as the old one and the denotation will be overwritten. So I don't see how that should work.