Calling an extension method on Unit fails assertion

[DoSOfRedRiver]

The minimal example is:

trait Syntax[F[_]] {
  def (a: A) ret[A]: F[A]
}

trait Instance[A]

implicit val instanceSyntax: Syntax[Instance] = new Syntax[Instance] {
  def (a: A) ret[A]: Instance[A] = new Instance[A] {}
}

object Instance {
  def defer[A](a: => A): Instance[A] = {
    ().ret
    new Instance[A] {}
  }
}

Error

[error] assertion failed: dotty.tools.dotc.typer.Applications$ExtMethodApply@91549 [error] dotty.DottyPredef$.assertFail(DottyPredef.scala:16) [error] dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldMoreCases(Trees.scala:1482) [error] dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1477) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1494) [error] dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:385) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply$$anonfun$1(Trees.scala:1379) [error] scala.collection.LinearSeqOptimized.foldLeft(LinearSeqOptimized.scala:126) [error] scala.collection.LinearSeqOptimized.foldLeft$(LinearSeqOptimized.scala:122) [error] scala.collection.immutable.List.foldLeft(List.scala:89) [error] scala.collection.TraversableOnce.$div$colon(TraversableOnce.scala:154) [error] scala.collection.TraversableOnce.$div$colon$(TraversableOnce.scala:154) [error] scala.collection.AbstractTraversable.$div$colon(Traversable.scala:108) [error] dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply(Trees.scala:1379) [error] dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1397) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1494) [error] dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:385) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply$$anonfun$1(Trees.scala:1379) [error] scala.collection.LinearSeqOptimized.foldLeft(LinearSeqOptimized.scala:126) [error] scala.collection.LinearSeqOptimized.foldLeft$(LinearSeqOptimized.scala:122) [error] scala.collection.immutable.List.foldLeft(List.scala:89) [error] scala.collection.TraversableOnce.$div$colon(TraversableOnce.scala:154) [error] scala.collection.TraversableOnce.$div$colon$(TraversableOnce.scala:154) [error] scala.collection.AbstractTraversable.$div$colon(Traversable.scala:108) [error] dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply(Trees.scala:1379) [error] dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1411) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1494) [error] dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:385) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1460) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1494) [error] dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:385) [error] dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse$$anonfun$3(ExtractDependencies.scala:383) [error] dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15) [error] dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10) [error] scala.collection.immutable.List.foreach(List.scala:392) [error] dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:383) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1463) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1494) [error] dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:385) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply$$anonfun$1(Trees.scala:1379) [error] scala.collection.LinearSeqOptimized.foldLeft(LinearSeqOptimized.scala:126) [error] scala.collection.LinearSeqOptimized.foldLeft$(LinearSeqOptimized.scala:122) [error] scala.collection.immutable.List.foldLeft(List.scala:89) [error] scala.collection.TraversableOnce.$div$colon(TraversableOnce.scala:154) [error] scala.collection.TraversableOnce.$div$colon$(TraversableOnce.scala:154) [error] scala.collection.AbstractTraversable.$div$colon(Traversable.scala:108) [error] dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply(Trees.scala:1379) [error] dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1469) [error] dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1494) [error] dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:385) [error] dotty.tools.dotc.sbt.ExtractDependencies.run(ExtractDependencies.scala:68) [error] dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:316) [error] scala.collection.immutable.List.map(List.scala:286) [error] dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:318) [error] dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:158) [error] dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15) [error] dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10) [error] scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:36) [error] scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:33) [error] scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:198) [error] dotty.tools.dotc.Run.runPhases$5(Run.scala:170) [error] dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:178) [error] dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12) [error] dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:102) [error] dotty.tools.dotc.Run.compileUnits(Run.scala:185) [error] dotty.tools.dotc.Run.compileSources(Run.scala:120) [error] dotty.tools.dotc.Run.compile(Run.scala:104) [error] dotty.tools.dotc.Driver.doCompile(Driver.scala:34) [error] dotty.tools.dotc.Driver.process(Driver.scala:172) [error] dotty.tools.dotc.Main.process(Main.scala) [error] xsbt.CachedCompilerImpl.run(CachedCompilerImpl.java:69) [error] xsbt.CompilerInterface.run(CompilerInterface.java:41) [error] jdk.internal.reflect.GeneratedMethodAccessor11.invoke(Unknown Source) [error] java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) [error] java.base/java.lang.reflect.Method.invoke(Method.java:566) [error] sbt.internal.inc.AnalyzingCompiler.call(AnalyzingCompiler.scala:237) [error] sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:111) [error] sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:90) [error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$3(MixedAnalyzingCompiler.scala:82) [error] scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12) [error] sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:133) [error] sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:73) [error] sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:116) [error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:307) [error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:307) [error] sbt.internal.inc.Incremental$.doCompile(Incremental.scala:106) [error] sbt.internal.inc.Incremental$.$anonfun$compile$4(Incremental.scala:87) [error] sbt.internal.inc.IncrementalCommon.recompileClasses(IncrementalCommon.scala:116) [error] sbt.internal.inc.IncrementalCommon.cycle(IncrementalCommon.scala:63) [error] sbt.internal.inc.Incremental$.$anonfun$compile$3(Incremental.scala:89) [error] sbt.internal.inc.Incremental$.manageClassfiles(Incremental.scala:134) [error] sbt.internal.inc.Incremental$.compile(Incremental.scala:80) [error] sbt.internal.inc.IncrementalCompile$.apply(Compile.scala:67) [error] sbt.internal.inc.IncrementalCompilerImpl.compileInternal(IncrementalCompilerImpl.scala:311) [error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileIncrementally$1(IncrementalCompilerImpl.scala:269) [error] sbt.internal.inc.IncrementalCompilerImpl.handleCompilationError(IncrementalCompilerImpl.scala:159) [error] sbt.internal.inc.IncrementalCompilerImpl.compileIncrementally(IncrementalCompilerImpl.scala:238) [error] sbt.internal.inc.IncrementalCompilerImpl.compile(IncrementalCompilerImpl.scala:69) [error] sbt.Defaults$.compileIncrementalTaskImpl(Defaults.scala:1549) [error] sbt.Defaults$.$anonfun$compileIncrementalTask$1(Defaults.scala:1523) [error] scala.Function1.$anonfun$compose$1(Function1.scala:44) [error] sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:40) [error] sbt.std.Transform$$anon$4.work(System.scala:67) [error] sbt.Execute.$anonfun$submit$2(Execute.scala:269) [error] sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:16) [error] sbt.Execute.work(Execute.scala:278) [error] sbt.Execute.$anonfun$submit$1(Execute.scala:269) [error] sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:178) [error] sbt.CompletionService$$anon$2.call(CompletionService.scala:37) [error] java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264) [error] java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515) [error] java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264) [error] java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [error] java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [error] java.base/java.lang.Thread.run(Thread.java:834) [error] sbt:dotty-simple> [error] java.lang.AssertionError: assertion failed: dotty.tools.dotc.typer.Applications$ExtMethodApply@91549 [error] at dotty.DottyPredef$.assertFail(DottyPredef.scala:16) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldMoreCases(Trees.scala:1482) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1477) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1494) [error] at dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:385) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply$$anonfun$1(Trees.scala:1379) [error] at scala.collection.LinearSeqOptimized.foldLeft(LinearSeqOptimized.scala:126) [error] at scala.collection.LinearSeqOptimized.foldLeft$(LinearSeqOptimized.scala:122) [error] at scala.collection.immutable.List.foldLeft(List.scala:89) [error] at scala.collection.TraversableOnce.$div$colon(TraversableOnce.scala:154) [error] at scala.collection.TraversableOnce.$div$colon$(TraversableOnce.scala:154) [error] at scala.collection.AbstractTraversable.$div$colon(Traversable.scala:108) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply(Trees.scala:1379) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1397) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1494) [error] at dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:385) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply$$anonfun$1(Trees.scala:1379) [error] at scala.collection.LinearSeqOptimized.foldLeft(LinearSeqOptimized.scala:126) [error] at scala.collection.LinearSeqOptimized.foldLeft$(LinearSeqOptimized.scala:122) [error] at scala.collection.immutable.List.foldLeft(List.scala:89) [error] at scala.collection.TraversableOnce.$div$colon(TraversableOnce.scala:154) [error] at scala.collection.TraversableOnce.$div$colon$(TraversableOnce.scala:154) [error] at scala.collection.AbstractTraversable.$div$colon(Traversable.scala:108) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply(Trees.scala:1379) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1411) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1494) [error] at dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:385) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1460) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1494) [error] at dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:385) [error] at dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse$$anonfun$3(ExtractDependencies.scala:383) [error] at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15) [error] at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10) [error] at scala.collection.immutable.List.foreach(List.scala:392) [error] at dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:383) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1463) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1494) [error] at dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:385) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1493) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply$$anonfun$1(Trees.scala:1379) [error] at scala.collection.LinearSeqOptimized.foldLeft(LinearSeqOptimized.scala:126) [error] at scala.collection.LinearSeqOptimized.foldLeft$(LinearSeqOptimized.scala:122) [error] at scala.collection.immutable.List.foldLeft(List.scala:89) [error] at scala.collection.TraversableOnce.$div$colon(TraversableOnce.scala:154) [error] at scala.collection.TraversableOnce.$div$colon$(TraversableOnce.scala:154) [error] at scala.collection.AbstractTraversable.$div$colon(Traversable.scala:108) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply(Trees.scala:1379) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1469) [error] at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1494) [error] at dotty.tools.dotc.sbt.ExtractDependenciesCollector.traverse(ExtractDependencies.scala:385) [error] at dotty.tools.dotc.sbt.ExtractDependencies.run(ExtractDependencies.scala:68) [error] at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:316) [error] at scala.collection.immutable.List.map(List.scala:286) [error] at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:318) [error] at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:158) [error] at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15) [error] at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10) [error] at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:36) [error] at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:33) [error] at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:198) [error] at dotty.tools.dotc.Run.runPhases$5(Run.scala:170) [error] at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:178) [error] at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12) [error] at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:102) [error] at dotty.tools.dotc.Run.compileUnits(Run.scala:185) [error] at dotty.tools.dotc.Run.compileSources(Run.scala:120) [error] at dotty.tools.dotc.Run.compile(Run.scala:104) [error] at dotty.tools.dotc.Driver.doCompile(Driver.scala:34) [error] at dotty.tools.dotc.Driver.process(Driver.scala:172) [error] at dotty.tools.dotc.Main.process(Main.scala) [error] at xsbt.CachedCompilerImpl.run(CachedCompilerImpl.java:69) [error] at xsbt.CompilerInterface.run(CompilerInterface.java:41) [error] at jdk.internal.reflect.GeneratedMethodAccessor11.invoke(Unknown Source) [error] at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) [error] at java.base/java.lang.reflect.Method.invoke(Method.java:566) [error] at sbt.internal.inc.AnalyzingCompiler.call(AnalyzingCompiler.scala:237) [error] at sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:111) [error] at sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:90) [error] at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$3(MixedAnalyzingCompiler.scala:82) [error] at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12) [error] at sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:133) [error] at sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:73) [error] at sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:116) [error] at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:307) [error] at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:307) [error] at sbt.internal.inc.Incremental$.doCompile(Incremental.scala:106) [error] at sbt.internal.inc.Incremental$.$anonfun$compile$4(Incremental.scala:87) [error] at sbt.internal.inc.IncrementalCommon.recompileClasses(IncrementalCommon.scala:116) [error] at sbt.internal.inc.IncrementalCommon.cycle(IncrementalCommon.scala:63) [error] at sbt.internal.inc.Incremental$.$anonfun$compile$3(Incremental.scala:89) [error] at sbt.internal.inc.Incremental$.manageClassfiles(Incremental.scala:134) [error] at sbt.internal.inc.Incremental$.compile(Incremental.scala:80) [error] at sbt.internal.inc.IncrementalCompile$.apply(Compile.scala:67) [error] at sbt.internal.inc.IncrementalCompilerImpl.compileInternal(IncrementalCompilerImpl.scala:311) [error] at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileIncrementally$1(IncrementalCompilerImpl.scala:269) [error] at sbt.internal.inc.IncrementalCompilerImpl.handleCompilationError(IncrementalCompilerImpl.scala:159) [error] at sbt.internal.inc.IncrementalCompilerImpl.compileIncrementally(IncrementalCompilerImpl.scala:238) [error] at sbt.internal.inc.IncrementalCompilerImpl.compile(IncrementalCompilerImpl.scala:69) [error] at sbt.Defaults$.compileIncrementalTaskImpl(Defaults.scala:1549) [error] at sbt.Defaults$.$anonfun$compileIncrementalTask$1(Defaults.scala:1523) [error] at scala.Function1.$anonfun$compose$1(Function1.scala:44) [error] at sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:40) [error] at sbt.std.Transform$$anon$4.work(System.scala:67) [error] at sbt.Execute.$anonfun$submit$2(Execute.scala:269) [error] at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:16) [error] at sbt.Execute.work(Execute.scala:278) [error] at sbt.Execute.$anonfun$submit$1(Execute.scala:269) [error] at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:178) [error] at sbt.CompletionService$$anon$2.call(CompletionService.scala:37) [error] at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264) [error] at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515) [error] at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264) [error] at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [error] at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [error] at java.base/java.lang.Thread.run(Thread.java:834) [error] (Compile / compileIncremental) java.lang.AssertionError: assertion failed: dotty.tools.dotc.typer.Applications$ExtMethodApply@91549

To mitigate the problem you have to import syntax:

def defer[A](a: => A): Instance[A] = {
    import instanceSyntax._
    ().ret
    new Instance[A] {}
  }