"missing argument" for default annotation parameter to Java interface

[olafurpg]

$ cat <<EOT >> bench.scala
package foo

@Fork(value = 16)
class Bar
EOT
$ cat <<EOT >> fork.java
package foo;

public @interface Fork {
    int value() default -1;
    int warmups() default -1;
}
EOT
$ scalac bench.scala fork.java # using 2.12.2
$ dotc bench.scala fork.java
-- Error: bench.scala:3:0 ------------------------------------------------------
3 |@Fork(value = 16)
  |^^^^^^^^^^^^^^^^
  |missing argument for parameter warmups of constructor Fork: (value: Int, warmups: Int)foo.Fork

one error found

[odersky]

It's the Java parser that does not generate the default correctly. If we compile Fork.java separately, it works.

[odersky]

Here's what we get from the Java parser:

package foo {
  object Fork() extends {}
  abstract class Fork(val value: Int, val warmups: Int) extends _root_.scala.
    annotation
  .Annotation with _root_.java.lang.annotation.Annotation with _root_.scala.
    annotation
  .ClassfileAnnotation {}
}

The default arguments are missing. I believe that's no oversight. To do this, we'd need to be able to parse expressions (actually: only constant expressions), and that functionality is currently missing.

[smarter]

To do this, we'd need to be able to parse expressions (actually: only constant expressions), and that functionality is currently missing.

I don't think that's necessary, we already record whether annotations parameters have default values with a special annotation (https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/parsing/JavaParsers.scala#L518) and that should be enough to compile uses of Java annotations. I think we just need to add dummy default getters for these parameters like we do when parsing from a classfile: https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala#L640-L645

[odersky]

We do record an AnnotationDefault attribute, but we don't record what that attribute is. To do so, we have to parse the default value, which is currently done using unimplementedExpr, returning a ???`.

I think this would be a great contribution for someone who tries to get into the compiler code base. It's tricky, but requires only a limited footprint of pre-existing knowledge about the compiler.