Imported From: https://issues.scala-lang.org/browse/SI-8931?orig=1
Reporter: Eric Lafortune (lafortune)
Affected Versions: 2.11.2
See #5278

@soc said:
Hi Eric, thanks a lot for your report!

@gourlaysama said:
Hmm, I don't get it, isn't it the same from Java? If you directly implement an interface (scala.Product here), and also indirectly implement it through something else (here scala.Product1[T] extends scala.Product), then it does appear in the resulting classfile.

As in:

==> A.java <==
import java.io.Serializable;

public class A<T> implements Product1<T>, Product, Serializable {}

==> Product.java <==
public interface Product {}

==> Product1.java <==
public interface Product1<T> extends Product {}

% javap -v A.class
[...]
#12 = Utf8               <T:Ljava/lang/Object;>Ljava/lang/Object;LProduct1<TT;>;LProduct;Ljava/io/Serializable;

Note that Tuple1 directly implements Product because it is a case class:

case class Tuple1[+T](_1: T) extends Product1[T]

is syntactic sugar for this:

class Tuple1[+T] extends AnyRef with Product1[T] with Product with Serializable { <autogenerated members> }

Eric Lafortune (lafortune) said:
Looking at the compiled bytecode, Product really isn't listed as a direct interface. I presume the scalac compiler has removed it (but not from the Signature). The tool javap confusingly lists the generic direct interfaces, not the plain direct interfaces.

With a lower-level tool, you should see the actual interfaces. E.g. with ProGuard's -dump option:

java -jar proguard5.0/lib/proguard.jar \
  -dontshrink \
  -dontoptimize \
  -dontobfuscate \
  -dontpreverify \
  -dump \
  -injars lib/scala-library.jar'(scala/Tuple1.class)' \
| less

You'll then see

- Program class: scala/Tuple1
  Superclass:    java/lang/Object
.....
Interfaces (count = 2):
  - Class [scala/Product1]
  - Class [scala/Serializable]

.....
  - Signature attribute:
    - Utf8 [<T1:Ljava/lang/Object;>Ljava/lang/Object;Lscala/Product1<TT1;>;Lscala/Product;Lscala/Serializable;]

Tools like asm or jad may provide the same information. Other tell-tale sign: scala/Product isn't present as a class constant in the bytecode.

@gourlaysama said:
bq. scala/Product isn't present as a class constant in the bytecode.
Oh, I had completely missed that; that's very wrong. And it happens for any interface:

trait Foo

trait Foo1 extends Foo

class A extends Foo1 with Foo

% javap -v A.class | grep '= Class.*Foo' # where is Foo...
#6 = Class              #5             // Foo1

This seems to have regressed in Scala 2.9.2:

% java -jar ~/Downloads/proguard5.0/lib/proguard.jar \
  -dontshrink \
  -dontoptimize \
  -dontobfuscate \
  -dontpreverify \
  -dump \
  -injars ~/scaladev/packs/scala-v2.9.1-1/pack/lib/scala-library.jar'(scala/Tuple1.class)' \
  | grep 'Interfaces' -A 4
Interfaces (count = 4):
  - Class [scala/Product1]
  - Class [scala/ScalaObject]
  - Class [scala/Product]
  - Class [scala/Serializable]
% java -jar ~/Downloads/proguard5.0/lib/proguard.jar \
  -dontshrink \
  -dontoptimize \
  -dontobfuscate \
  -dontpreverify \
  -dump \
  -injars ~/scaladev/packs/scala-v2.9.2/pack/lib/scala-library.jar'(scala/Tuple1.class)' \
  | grep 'Interfaces' -A 4
Interfaces (count = 2):
  - Class [scala/Product1]
  - Class [scala/Serializable]

Constant Pool (count = 184):

@gourlaysama said:
The jvms does say:

bq. Oracle's Java Virtual Machine implementation does not check the well-formedness of Signature attributes during class loading or linking. Instead, Signature attributes are checked by methods of the Java SE platform class libraries which expose generic signatures of classes, interfaces, constructors, methods, and fields. Examples include getGenericSuperclass in Class and toGenericString in java.lang.reflect.Executable.
(§4.7.9 in jdk8, and something similar at $4.3.4 in jdk7)

But even java reflection seems fine with it:

scala> val c = classOf[Tuple1[String]]
c: Class[(String,)] = class scala.Tuple1

scala> c.getInterfaces
res0: Array[Class[_]] = Array(interface scala.Product1, interface scala.Serializable)

scala> c.getGenericInterfaces
res1: Array[java.lang.reflect.Type] = Array(scala.Product1<T1>, interface scala.Product, interface scala.Serializable)

@gourlaysama said:
So apparently this was done on purpose, in 7a99c03 (#5278) to "ease trouble on android".

So either it shouldn't do it at all, or it should do it properly (for both the signature and the interface list).

@gourlaysama said:
scala/scala#4080