Allow overloading a method with a signature which matches before erasure but not after

[smarter]

This works in scala but not in dotty:

trait A
trait B

class Test {
  def foo(x: List[A]): Function1[A, A] = ???
  def foo(x: List[B]): Function2[B, B, B] = ???
}

It fails with:

[error] eras.scala:6: error: method foo is already defined as method foo: (x: scala.collection.immutable.List[A])A => A
[error]  (both definitions have the same erased type signature)
[error]   def foo(x: List[B]): Function2[B, B, B] = ???
[error]         ^
[error] one error found
j

Even though they don't have the same erased type signature.

The problem is that the following code in Checking#checkNoDoubleDefs:

if (decl.signature matches other.signature) {

Is run before erasure, and matches is defined as:

  /** The meaning of `matches` depends on the phase. If types are not erased,
   *  it means `sameParams`. Once types are erased, it means `==`, comparing parameter as
   *  well as result type parts.
   */
  final def matches(that: Signature)(implicit ctx: Context) =
    if (ctx.erasedTypes) equals(that) else sameParams(that)

Simply changing the code in Checking#checkNoDoubleDefs is probably not enough to fix that since the compiler complains:

warning: cannot merge (x: scala.collection.immutable.List[B])(B, B) => B with (x: scala.collection.immutable.List[A])A => A as members of one type; keeping only (x: scala.collection.immutable.List[B])(B, B) => B
warning: cannot merge (x: scala.collection.immutable.List[B])(B, B) => B with (x: scala.collection.immutable.List[A])A => A as members of one type; keeping only (x: scala.collection.immutable.List[B])(B, B) => B

And changing Signature#matches to always match the whole signature and not just the params breaks everything in a way I don't understand:

[info] applied type mismatch: TypeRef(TermRef(TermRef(TermRef(TermRef(TermRef(ThisType(TypeRef(NoPrefix,<root>)),scala),reflect),runtime),package),universe),TypeTag) List(TypeRef(NoPrefix,dotty$DottyPredef$typeTag$$T)), typeParams = List(), tsym = val <none>#1
[info] precomplete decls = 
[error] java.util.NoSuchElementException: head of empty list
[error]         at scala.collection.immutable.Nil$.head(List.scala:420)
[error]         at scala.collection.immutable.Nil$.head(List.scala:417)
[error]         at dotty.tools.dotc.core.TypeApplications$$anonfun$appliedTo$extension0$1.matchParams$1(TypeApplications.scala:146)
[error]         at dotty.tools.dotc.core.TypeApplications$$anonfun$appliedTo$extension0$1.instantiate$1(TypeApplications.scala:173)
[error]         at dotty.tools.dotc.core.TypeApplications$$anonfun$appliedTo$extension0$1.apply(TypeApplications.scala:190)
[error]         at dotty.tools.dotc.core.TypeApplications$$anonfun$appliedTo$extension0$1.apply(TypeApplications.scala:139)
[error]         at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)
[error]         at dotty.tools.dotc.core.TypeApplications$.appliedTo$extension0(TypeApplications.scala:139)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.readType(Scala2Unpickler.scala:679)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler$$anonfun$readTypeRef$1.apply(Scala2Unpickler.scala:795)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler$$anonfun$readTypeRef$1.apply(Scala2Unpickler.scala:795)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.atReadPos(Scala2Unpickler.scala:326)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.at(Scala2Unpickler.scala:316)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.readTypeRef(Scala2Unpickler.scala:795)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.readType(Scala2Unpickler.scala:710)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler$$anonfun$readTypeRef$1.apply(Scala2Unpickler.scala:795)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler$$anonfun$readTypeRef$1.apply(Scala2Unpickler.scala:795)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.atReadPos(Scala2Unpickler.scala:326)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.at(Scala2Unpickler.scala:316)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.readTypeRef(Scala2Unpickler.scala:795)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.readType(Scala2Unpickler.scala:710)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler$LocalUnpickler$$anonfun$9.apply(Scala2Unpickler.scala:537)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler$LocalUnpickler$$anonfun$9.apply(Scala2Unpickler.scala:537)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.atReadPos(Scala2Unpickler.scala:326)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.at(Scala2Unpickler.scala:316)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler$LocalUnpickler.dotty$tools$dotc$core$unpickleScala2$Scala2Unpickler$LocalUnpickler$$parseToCompletion$1(Scala2Unpickler.scala:537)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler$LocalUnpickler$$anonfun$complete$1.apply$mcV$sp(Scala2Unpickler.scala:566)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler$LocalUnpickler$$anonfun$complete$1.apply(Scala2Unpickler.scala:566)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler$LocalUnpickler$$anonfun$complete$1.apply(Scala2Unpickler.scala:566)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.atReadPos(Scala2Unpickler.scala:326)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler$LocalUnpickler.complete(Scala2Unpickler.scala:566)
[error]         at dotty.tools.dotc.core.SymDenotations$SymDenotation.completeFrom(SymDenotations.scala:161)
[error]         at dotty.tools.dotc.core.SymDenotations$SymDenotation.info(SymDenotations.scala:143)
[error]         at dotty.tools.dotc.transform.TreeTransforms$AnnotationTransformer$class.transform(TreeTransform.scala:183)
[error]         at dotty.tools.dotc.transform.FirstTransform.transform(FirstTransform.scala:30)
[error]         at dotty.tools.dotc.core.Denotations$SingleDenotation.currentIfExists(Denotations.scala:580)
[error]         at dotty.tools.dotc.core.Denotations$SingleDenotation.current(Denotations.scala:616)
[error]         at dotty.tools.dotc.core.Denotations$SingleDenotation.currentIfExists(Denotations.scala:598)
[error]         at dotty.tools.dotc.core.Denotations$SingleDenotation.current(Denotations.scala:616)
[error]         at dotty.tools.dotc.core.Symbols$Symbol.denot(Symbols.scala:394)
[error]         at dotty.tools.dotc.core.Scopes$Scope.denotsNamed(Scopes.scala:119)
[error]         at dotty.tools.dotc.core.SymDenotations$ClassDenotation.membersNamed(SymDenotations.scala:1412)
[error]         at dotty.tools.dotc.core.SymDenotations$ClassDenotation.findMember(SymDenotations.scala:1465)
[error]         at dotty.tools.dotc.core.Types$Type.go$1(Types.scala:441)
[error]         at dotty.tools.dotc.core.Types$Type.findMember(Types.scala:491)
[error]         at dotty.tools.dotc.core.Types$Type$$anonfun$member$1.apply(Types.scala:407)
[error]         at dotty.tools.dotc.core.Types$Type$$anonfun$member$1.apply(Types.scala:407)
[error]         at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)
[error]         at dotty.tools.dotc.core.Types$Type.member(Types.scala:406)
[error]         at dotty.tools.dotc.core.Symbols$class.requiredMethod(Symbols.scala:362)
[error]         at dotty.tools.dotc.core.Contexts$Context.requiredMethod(Contexts.scala:51)
[error]         at dotty.tools.dotc.transform.ClassTags.prepareForUnit(ClassTags.scala:36)
[error]         at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$34.apply(TreeTransform.scala:549)
[error]         at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$34.apply(TreeTransform.scala:549)
[error]         at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.mutateTransformers(TreeTransform.scala:501)
[error]         at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.macroTransform(TreeTransform.scala:561)
[error]         at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.run(TreeTransform.scala:480)
[error]         at dotty.tools.dotc.core.Phases$Phase$$anonfun$runOn$1.apply(Phases.scala:270)
[error]         at dotty.tools.dotc.core.Phases$Phase$$anonfun$runOn$1.apply(Phases.scala:268)
[error]         at scala.collection.immutable.List.map(List.scala:273)
[error]         at dotty.tools.dotc.core.Phases$Phase$class.runOn(Phases.scala:268)
[error]         at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.runOn(TreeTransform.scala:474)
[error]         at dotty.tools.dotc.Run$$anonfun$compileUnits$1$$anonfun$apply$mcV$sp$1.apply(Run.scala:59)
[error]         at dotty.tools.dotc.Run$$anonfun$compileUnits$1$$anonfun$apply$mcV$sp$1.apply(Run.scala:56)
[error]         at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
[error]         at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:186)
[error]         at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply$mcV$sp(Run.scala:56)
[error]         at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply(Run.scala:52)
[error]         at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply(Run.scala:52)
[error]         at dotty.tools.dotc.util.Stats$.monitorHeartBeat(Stats.scala:68)
[error]         at dotty.tools.dotc.Run.compileUnits(Run.scala:52)
[error]         at dotty.tools.dotc.Run.compileSources(Run.scala:49)
[error]         at dotty.tools.dotc.Run.compile(Run.scala:33)
[error]         at dotty.tools.dotc.Driver.doCompile(Driver.scala:20)
[error]         at dotty.tools.dotc.Main$.doCompile(Main.scala:26)
[error]         at dotty.tools.dotc.Driver.process(Driver.scala:31)
[error]         at dotty.tools.dotc.Driver.main(Driver.scala:40)
[error]         at dotty.tools.dotc.Main.main(Main.scala)
[info] findMember exception for ClassInfo(ThisType(TypeRef(NoPrefix,dotty)), module class DottyPredef$) member typeTag
[info] exception occurred while compiling try/eras.scala
[error] Exception in thread "main" dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler$BadSignature: error reading Scala signature of class DottyPredef from /home/smarter/opt/dotty/target/scala-2.11/dotty_2.11-0.1-SNAPSHOT.jar(dotty/DottyPredef.class):
[error] error occurred at position 1181: a runtime exception occurred: java.util.NoSuchElementException: head of empty list
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.errorBadSignature(Scala2Unpickler.scala:182)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler.handleRuntimeException(Scala2Unpickler.scala:191)
[error]         at dotty.tools.dotc.core.unpickleScala2.Scala2Unpickler$LocalUnpickler.complete(Scala2Unpickler.scala:568)
[error]         at dotty.tools.dotc.core.SymDenotations$SymDenotation.completeFrom(SymDenotations.scala:161)
[error]         at dotty.tools.dotc.core.SymDenotations$SymDenotation.info(SymDenotations.scala:143)
[error]         at dotty.tools.dotc.transform.TreeTransforms$AnnotationTransformer$class.transform(TreeTransform.scala:183)
[error]         at dotty.tools.dotc.transform.FirstTransform.transform(FirstTransform.scala:30)
[error]         at dotty.tools.dotc.core.Denotations$SingleDenotation.currentIfExists(Denotations.scala:580)
[error]         at dotty.tools.dotc.core.Denotations$SingleDenotation.current(Denotations.scala:616)
[error]         at dotty.tools.dotc.core.Denotations$SingleDenotation.currentIfExists(Denotations.scala:598)
[error]         at dotty.tools.dotc.core.Denotations$SingleDenotation.current(Denotations.scala:616)
[error]         at dotty.tools.dotc.core.Symbols$Symbol.denot(Symbols.scala:394)
[error]         at dotty.tools.dotc.core.Scopes$Scope.denotsNamed(Scopes.scala:119)
[error]         at dotty.tools.dotc.core.SymDenotations$ClassDenotation.membersNamed(SymDenotations.scala:1412)
[error]         at dotty.tools.dotc.core.SymDenotations$ClassDenotation.findMember(SymDenotations.scala:1465)
[error]         at dotty.tools.dotc.core.Types$Type.go$1(Types.scala:441)
[error]         at dotty.tools.dotc.core.Types$Type.findMember(Types.scala:491)
[error]         at dotty.tools.dotc.core.Types$Type$$anonfun$member$1.apply(Types.scala:407)
[error]         at dotty.tools.dotc.core.Types$Type$$anonfun$member$1.apply(Types.scala:407)
[error]         at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)
[error]         at dotty.tools.dotc.core.Types$Type.member(Types.scala:406)
[error]         at dotty.tools.dotc.core.Symbols$class.requiredMethod(Symbols.scala:362)
[error]         at dotty.tools.dotc.core.Contexts$Context.requiredMethod(Contexts.scala:51)
[error]         at dotty.tools.dotc.transform.ClassTags.prepareForUnit(ClassTags.scala:36)
[error]         at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$34.apply(TreeTransform.scala:549)
[error]         at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$34.apply(TreeTransform.scala:549)
[error]         at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.mutateTransformers(TreeTransform.scala:501)
[error]         at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.macroTransform(TreeTransform.scala:561)
[error]         at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.run(TreeTransform.scala:480)
[error]         at dotty.tools.dotc.core.Phases$Phase$$anonfun$runOn$1.apply(Phases.scala:270)
[error]         at dotty.tools.dotc.core.Phases$Phase$$anonfun$runOn$1.apply(Phases.scala:268)
[error]         at scala.collection.immutable.List.map(List.scala:273)
[error]         at dotty.tools.dotc.core.Phases$Phase$class.runOn(Phases.scala:268)
[error]         at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.runOn(TreeTransform.scala:474)
[error]         at dotty.tools.dotc.Run$$anonfun$compileUnits$1$$anonfun$apply$mcV$sp$1.apply(Run.scala:59)
[error]         at dotty.tools.dotc.Run$$anonfun$compileUnits$1$$anonfun$apply$mcV$sp$1.apply(Run.scala:56)
[error]         at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
[error]         at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:186)
[error]         at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply$mcV$sp(Run.scala:56)
[error]         at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply(Run.scala:52)
[error]         at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply(Run.scala:52)
[error]         at dotty.tools.dotc.util.Stats$.monitorHeartBeat(Stats.scala:68)
[error]         at dotty.tools.dotc.Run.compileUnits(Run.scala:52)
[error]         at dotty.tools.dotc.Run.compileSources(Run.scala:49)
[error]         at dotty.tools.dotc.Run.compile(Run.scala:33)
[error]         at dotty.tools.dotc.Driver.doCompile(Driver.scala:20)
[error]         at dotty.tools.dotc.Main$.doCompile(Main.scala:26)
[error]         at dotty.tools.dotc.Driver.process(Driver.scala:31)
[error]         at dotty.tools.dotc.Driver.main(Driver.scala:40)
[error]         at dotty.tools.dotc.Main.main(Main.scala)

We could also choose to not support this kind of overloading, but it is needed to compile Function.scala:

  /** Uncurrying for functions of arity 2. This transforms a unary function
   *  returning another unary function into a function of arity 2.
   */
  def uncurried[a1, a2, b](f: a1 => a2 => b): (a1, a2) => b = {
    (x1, x2) => f(x1)(x2)
  }

  /** Uncurrying for functions of arity 3.
   */
  def uncurried[a1, a2, a3, b](f: a1 => a2 => a3 => b): (a1, a2, a3) => b = {
    (x1, x2, x3) => f(x1)(x2)(x3)
  }

  ...

@odersky : What do you think?

[DarkDimius]

Even though they don't have the same erased type signature.

They do, given current defition of

final def matches(that: Signature)(implicit ctx: Context) =
    if (ctx.erasedTypes) equals(that) else sameParams(that)

AFAIK that's intentional. We stopped assuming that return type is part of signature for duplicate detection

[smarter]

No, the erased("if ctx.erasedTypes") signature is different (because it looks at the return value), the non-erased signature is the same (because it only look at the parameters). But Checking#checkNoDoubleDefs is run before erasure.

[DarkDimius]

@smarter after erasure we need to treat those methods as different, due to bridge generation for example.
But before erasure, we intentionally disallow them, users should not define such methods.

[odersky]

Indeed several parts in the frontend rely on this behavior of signatures. E.g. when looking things up, we merge ethods (decide they override instead of overload) based on their parameter signature. Since it is not a big loss to disallow these things I beleive we should do that. @smarter can you do a PR that changes the error message from (both definitions have the same erased type signature) to (the definitions have matching type signatures)?