Type erased for `by-name` parameters

[jlprat]

When a method has by-name parameters its type it's erased in the generated bytecode.

Compiler version

3.0.2 and 3.1.0-RC2

Minimized code

object ByNameParam {
  def byNameParam(str: => String): Unit = {}
}

You can find a reproducer project here where one can compile with Scala 2.13.6, 3.0.2 and 3.1.0-RC2 and see the difference in bytecode between versions.

Output

$> javap ByNameParam.class

Compiled from "ByNameParam.scala"
public final class ByNameParam {
  public static void byNameParam(scala.Function0);
}

Expectation

This is the output of the same class when compiling against Scala 2.13.6.
$> javap ByNameParam.class

Compiled from "ByNameParam.scala"
public final class ByNameParam {
  public static void byNameParam(scala.Function0<java.lang.String>);
}

Please note the missing type parameter String on Function0.

Impact

Java interoperability will be affected by this bug as the String type is completely lost.

[smarter]

Java generic signatures are generated by https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala, and by-names are represented as ExprType, presumably the issue is that we don't generate a generic signature at all here and we just need to a case ExprType that returns true to https://github.com/lampepfl/dotty/blob/44d907f19a193707920fd0729d1627f472e3bf1d/compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala#L455