scalajs: compiler crash when overriding Promise.then

[rssh]

Compiler version

3.0.0

Minimized code

package x

import scalajs.*

class MyPromise[T]() extends js.Promise[T]( (resolve, reject) => ???) {

    override def `then`[S](
      onFulfilled: js.Function1[T, S | js.Thenable[S]],
      onRejected: js.UndefOr[js.Function1[scala.Any, S | js.Thenable[S]]] = js.undefined): js.Promise[S] =
        js.Promise( (resolve, reject) => ??? )

    override def `then`[S >: T](
      onFulfilled: Unit,
      onRejected: js.UndefOr[js.Function1[scala.Any, S | js.Thenable[S]]]): js.Promise[S] =
        `then`[S]( x => x , onRejected)

    override def `catch`[S >: T](
      onRejected: js.UndefOr[js.Function1[scala.Any, S | js.Thenable[S]]] = js.undefined): js.Promise[S] =
        `then`[S]( x => x , onRejected )

}

Output (click arrow to expand)

[info] welcome to sbt 1.5.2 (N/A Java 15.0.2)
[info] loading global plugins from /Users/rssh/.sbt/1.0/plugins
[info] loading settings for project scalajs-then-override-build from plugins.sbt ...
[info] loading project definition from /Users/rssh/tests/dotty/scalajs-then-override/project
[info] loading settings for project root from build.sbt ...
[info] set current project to test (in build file:/Users/rssh/tests/dotty/scalajs-then-override/)
[info] Executing in batch mode. For better performance use sbt's shell
[info] compiling 1 Scala source to /Users/rssh/tests/dotty/scalajs-then-override/target/scala-3.0.0/classes ...
[info] exception occurred while compiling /Users/rssh/tests/dotty/scalajs-then-override/src/main/scala/x/X.scala
java.lang.AssertionError: assertion failed: found overloaded default getter def then$default$2(): scala.scalajs.js.| <and> 
  def then$default$2(): scala.runtime.BoxedUnit while compiling /Users/rssh/tests/dotty/scalajs-then-override/src/main/scala/x/X.scala
[error] ## Exception when compiling 1 sources to /Users/rssh/tests/dotty/scalajs-then-override/target/scala-3.0.0/classes
[error] java.lang.AssertionError: assertion failed: found overloaded default getter def then$default$2(): scala.scalajs.js.| <and> 
[error]   def then$default$2(): scala.runtime.BoxedUnit
[error] scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
[error] dotty.tools.backend.sjs.JSExportsGen.genCallDefaultGetter(JSExportsGen.scala:744)
[error] dotty.tools.backend.sjs.JSExportsGen.genPrepareArgs$$anonfun$2(JSExportsGen.scala:718)
[error] scala.collection.Iterator$$anon$9.next(Iterator.scala:575)
[error] scala.collection.mutable.Growable.addAll(Growable.scala:62)
[error] scala.collection.mutable.Growable.addAll$(Growable.scala:57)
[error] scala.collection.immutable.VectorBuilder.addAll(Vector.scala:1565)
[error] scala.collection.immutable.Vector$.from(Vector.scala:1349)
[error] scala.collection.immutable.Vector$.from(Vector.scala:34)
[error] scala.collection.IterableOps$WithFilter.map(Iterable.scala:884)
[error] dotty.tools.backend.sjs.JSExportsGen.genPrepareArgs(JSExportsGen.scala:731)
[error] dotty.tools.backend.sjs.JSExportsGen.genApplyForSingleExportedNonJSSuperCall(JSExportsGen.scala:681)
[error] dotty.tools.backend.sjs.JSExportsGen.genApplyForSingleExported(JSExportsGen.scala:636)
[error] dotty.tools.backend.sjs.JSExportsGen.genExportSameArgcRec(JSExportsGen.scala:540)
[error] dotty.tools.backend.sjs.JSExportsGen.genExportSameArgc(JSExportsGen.scala:516)
[error] dotty.tools.backend.sjs.JSExportsGen.$anonfun$28(JSExportsGen.scala:472)
[error] scala.collection.Iterator$$anon$9.next(Iterator.scala:575)
[error] scala.collection.immutable.List.prependedAll(List.scala:153)
[error] scala.collection.immutable.List$.from(List.scala:651)
[error] scala.collection.immutable.List$.from(List.scala:648)
[error] scala.collection.IterableOps$WithFilter.map(Iterable.scala:884)
[error] dotty.tools.backend.sjs.JSExportsGen.genExportMethodMultiAlts(JSExportsGen.scala:474)
[error] dotty.tools.backend.sjs.JSExportsGen.genExportMethod(JSExportsGen.scala:441)
[error] dotty.tools.backend.sjs.JSExportsGen.genMemberExportOrDispatcher$$anonfun$1(JSExportsGen.scala:333)
[error] dotty.tools.backend.sjs.JSCodeGen.withNewLocalNameScope$$anonfun$1(JSCodeGen.scala:117)
[error] dotty.tools.backend.sjs.ScopedVar$.withScopedVars(ScopedVar.scala:35)
[error] dotty.tools.backend.sjs.JSCodeGen.withNewLocalNameScope(JSCodeGen.scala:118)
[error] dotty.tools.backend.sjs.JSExportsGen.genMemberExportOrDispatcher(JSExportsGen.scala:334)
[error] dotty.tools.backend.sjs.JSExportsGen.genJSClassDispatcher(JSExportsGen.scala:323)
[error] dotty.tools.backend.sjs.JSExportsGen.genJSClassDispatchers$$anonfun$1(JSExportsGen.scala:295)
[error] scala.collection.immutable.List.map(List.scala:246)
[error] dotty.tools.backend.sjs.JSExportsGen.genJSClassDispatchers(JSExportsGen.scala:295)
[error] dotty.tools.backend.sjs.JSCodeGen.genNonNativeJSClass(JSCodeGen.scala:537)
[error] dotty.tools.backend.sjs.JSCodeGen.genCompilationUnit$$anonfun$6$$anonfun$1(JSCodeGen.scala:211)
[error] dotty.tools.backend.sjs.ScopedVar$.withScopedVars(ScopedVar.scala:35)
[error] dotty.tools.backend.sjs.JSCodeGen.genCompilationUnit$$anonfun$2(JSCodeGen.scala:221)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.immutable.List.foreach(List.scala:333)
[error] dotty.tools.backend.sjs.JSCodeGen.genCompilationUnit(JSCodeGen.scala:223)
[error] dotty.tools.backend.sjs.JSCodeGen.run(JSCodeGen.scala:152)
[error] dotty.tools.backend.sjs.GenSJSIR.run(GenSJSIR.scala:15)
[error] dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:303)
[error] scala.collection.immutable.List.map(List.scala:246)
[error] dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:304)
[error] dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:205)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
[error] dotty.tools.dotc.Run.runPhases$5(Run.scala:215)
[error] dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:223)
[error] scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
[error] dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
[error] dotty.tools.dotc.Run.compileUnits(Run.scala:230)
[error] dotty.tools.dotc.Run.compileSources(Run.scala:166)
[error] dotty.tools.dotc.Run.compile(Run.scala:150)
[error] dotty.tools.dotc.Driver.doCompile(Driver.scala:39)
[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:186)

[sjrd]

This happens because Scala 3 uses the inferred type of a parameter default value as the result type of the default accessor. This has always been a terrible idea wrt. binary compatibility, but that battle was lost. Now it's biting us because it creates bridges for param accessors in subclasses, and Scala.js handles default accessors in a very special way. We'll need to get rid of those bridges for default accessors in non-native JS classes.

[smarter]

This happens because Scala 3 uses the inferred type of a parameter default value as the result type of the default accessor.

As of 6fde5c1 this should only happen when the type of the method parameter cannot be used (e.g. def foo[T](x: T = 1)), I don't see what else we could do here.

[sjrd]

Hum, then I don't know what causes the bridge to be generated. But the fact is that there is a bridge in this case, and ignoring default getter bridges in JS classes fixes this issue (see PR #12657).