No generic signature generated for type aliases

[hamdiallam]

Compiler version: 3.0.1

Problem

Given

Scala

object Foo {
  type IntGetter = () => Int
  def getFunc(): IntGetter = () => 1
  def getFunc2(): () => Int = () => 1
}

The generic signatures generated (as seen using javap) are:

public static scala.Function0 getFunc();
public static scala.Function0<java.lang.Object> getFunc2();

Whereas in Scala 2 we get:

public static scala.Function0<java.lang.Object> getFunc();
public static scala.Function0<java.lang.Object> getFunc2();

[Kordyjan]

I've tried to replicate your bug on the current master branch and failed. For me both methods look identical in the bytecode and have the same jvm signature (<A:Ljava/lang/Object;>()LBoxed<TA;>;). There are also no warnings when they are called from java. Can you provide the full java file that you see this problem in?

[smarter]

If get was defined as a val or var in a trait, this would be #6350

[hamdiallam]

will work on another repro case

[hamdiallam]

if you clone twitter/util and run ./sbt 'project util-core '++3.0.2-RC1' 'compile', a bunch of unchecked conversion warnings show.

I was able to get a repro working as well. I can publish the project, but I've updated the issue description with it

[smarter]

Thanks, the generic signatures we generate are:

public static scala.Function0 getFunc();
public static scala.Function0<java.lang.Object> getFunc2();

By contrast, Scala 2 generates:

public static scala.Function0<java.lang.Object> getFunc();
public static scala.Function0<java.lang.Object> getFunc2();

(neither compiler will emit anything referencing scala.Int since that erases to int which cannot be used as a generic type parameter, and Integer cannot be used here either for other reasons).

So the issue appears to be that we fail to generate generic signatures for type aliases.

[smarter]

Looks like a one line fix:

diff --git compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala
index 9392d903857..f0405f1f7e9 100644
--- compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala
+++ compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala
@@ -455,7 +455,7 @@ object GenericSignatures {
   private class NeedsSigCollector(using Context) extends TypeAccumulator[Boolean] {
     override def apply(x: Boolean, tp: Type): Boolean =
       if (!x)
-        tp match {
+        tp.dealias match {
           case RefinedType(parent, refinedName, refinedInfo) =>
             val sym = parent.typeSymbol
             if (sym == defn.ArrayClass) foldOver(x, refinedInfo)

I'm on vacation, anyone wants to make the PR ? :)

[dwijnand]

and Integer cannot be used here either for other reasons

Why can't Integer be used? I've mentally built the model "scala.Int erases to int when it can, to Integer when it can't" (ignoring specialisation) - much like value classes.

[smarter]

Why can't Integer be used?

It looks like it would require extra bridges: scala/scala@e42733e (when generic signatures are available, javac uses them to derive the bytecode signatures it will emit in invoke* instructions instead of just reading the actual bytecode signature written in the classfile, this complicates our job a lot).

[SethTisue]

@dwijnand ticket of record on this is scala/bug#4214