Constructors are unnecessarily given Java generic signatures

[dwijnand]

Minimized code

class Foo[T]

Output

  public <T extends java.lang.Object> Foo();
    descriptor: ()V
[..]
    Signature: #9                           // <T:Ljava/lang/Object;>()V

Expectation

No Signature on the constructor. Compare against the equivalent Java:

$ echo 'class Foo<T> {}' > Foo.java && javac Foo.java && javap -v Foo
Classfile /d/mima/Foo.class
  Last modified 17 Dec 2020; size 243 bytes
  MD5 checksum 33fcdfd618f6e8ce83505acaa1f16ed2
  Compiled from "Foo.java"
class Foo<T extends java.lang.Object> extends java.lang.Object
  minor version: 0
  major version: 55
  flags: (0x0020) ACC_SUPER
  this_class: #2                          // Foo
  super_class: #3                         // java/lang/Object
  interfaces: 0, fields: 0, methods: 1, attributes: 2
Constant pool:
   #1 = Methodref          #3.#12         // java/lang/Object."<init>":()V
   #2 = Class              #13            // Foo
   #3 = Class              #14            // java/lang/Object
   #4 = Utf8               <init>
   #5 = Utf8               ()V
   #6 = Utf8               Code
   #7 = Utf8               LineNumberTable
   #8 = Utf8               Signature
   #9 = Utf8               <T:Ljava/lang/Object;>Ljava/lang/Object;
  #10 = Utf8               SourceFile
  #11 = Utf8               Foo.java
  #12 = NameAndType        #4:#5          // "<init>":()V
  #13 = Utf8               Foo
  #14 = Utf8               java/lang/Object
{
  Foo();
    descriptor: ()V
    flags: (0x0000)
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 1: 0
}
Signature: #9                           // <T:Ljava/lang/Object;>Ljava/lang/Object;
SourceFile: "Foo.java"

(Signature #9 is the class Signature.) Scala 2 doesn't emit this, which I perceive as correct.

This came up in the context of trialing MiMa on different versions of a library compiled with the Scala 3 compiler and some changes are resulting in additional, unexpected constructor signature changes - however signature checking is an opt-in feature of MiMa, so this wouldn't affect all MiMa users.

[dwijnand]

Signature is Java's generic method signature, so it's only impactful for Javac users (or MiMa users).

[xuwei-k]

I think some guice DI does not work due to this issue 😢

Main.scala

package example

import com.google.inject.AbstractModule
import com.google.inject.Guice
import com.google.inject.Key
import com.google.inject.TypeLiteral
import javax.inject.Inject

class B[X] @Inject() (val a: A[X])

class A[X](x: X)

class MyModule extends AbstractModule {
  override def configure(): Unit = {
    bind(new TypeLiteral[A[String]]() {}).toInstance(new A[String]("x"))
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val injector = Guice.createInjector(new MyModule())
    injector.getInstance(
      Key.get(new TypeLiteral[B[String]]() {})
    )
    println("success") // success with Scala 2.13.6
  }
}

sbt run log with Scala 3

[error] (run-main-a) com.google.inject.ConfigurationException: Guice configuration errors:
[error] 
[error] 1) example.A<X> cannot be used as a key; It is not fully specified.
[error]   at example.B.<init>(A.scala:9)
[error]   while locating example.B<java.lang.String>
[error] 
[error] 1 error
[error] com.google.inject.ConfigurationException: Guice configuration errors:
[error] 
[error] 1) example.A<X> cannot be used as a key; It is not fully specified.
[error]   at example.B.<init>(A.scala:9)
[error]   while locating example.B<java.lang.String>
[error] 
[error] 1 error
[error] 	at com.google.inject.internal.InjectorImpl.getProvider(InjectorImpl.java:1120)
[error] 	at com.google.inject.internal.InjectorImpl.getInstance(InjectorImpl.java:1126)
[error] 	at example.Main$.main(A.scala:23)
[error] 	at example.Main.main(A.scala)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error] 	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error] 	at java.lang.reflect.Method.invoke(Method.java:498)
[error] stack trace is suppressed; run last Compile / bgRun for the full output
[error] Nonzero exit code: 1
[error] (Compile / run) Nonzero exit code: 

build.sbt

scalaVersion := "3.0.2-RC1-bin-20210706-6011847-NIGHTLY"

libraryDependencies += "com.google.inject" % "guice" % "4.2.3"

[smarter]

It seems that we're missing https://github.com/scala/scala/pull/7975/files#diff-271d576b3a80359cbc5db9e1a8dd2629c2cb1d99bf8a30854c64742f98c7a6b0R78-R87 which ensures that we do not generate signatures for constructors whose parameters don't refer to a class type parameter, but I don't know if this is related to @xuwei-k's issue since in his case the type parameter is used so it's normal to have a signature on the constructor.

[smarter]

For reference, the corresponding code in scala 3 is in https://github.com/lampepfl/dotty/blob/601184787508d8fd655f1dc4e48ed19c418fa710/compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala#L455

[xuwei-k]

another Java Interop problem

Main.java

package example;

public class Main {
  public static void main(String[] args) {
    A<Integer> a = new A<>("x");
    int x = a.x();
  }
}

A.scala

package example

class A[X](val x: X)

Scala 2.13.6

compile error

Main.java:5:1: incompatible types: cannot infer type arguments for example.A<>
[error]     reason: inference variable X has incompatible bounds
[error]       equality constraints: java.lang.Integer
[error]       lower bounds: java.lang.String
[error] A<>
[error] (Compile / compileIncremental) javac returned non-zero exit code

Scala 3.0.2-RC1-bin-20210706-6011847-NIGHTLY

runtime error

[info] running example.Main 
[error] (run-main-0) java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
[error] java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
[error] 	at example.Main.main(Main.java:6)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error] 	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error] 	at java.lang.reflect.Method.invoke(Method.java:498)
[error] stack trace is suppressed; run last Compile / bgRunMain for the full output
[error] Nonzero exit code: 1
[error] (Compile / runMain) Nonzero exit code: 1

[dwijnand]

Nice example, it looks like javac takes the wrong signatures as building a A<Integer> will invoking new A<String>("x") so doesn't fail to compile.

[xuwei-k]

corresponding code in Scala 2

• https://github.com/scala/scala/blob/262c97250a614bf19a0e9e8994ee59abea82a5db/src/compiler/scala/tools/nsc/transform/Erasure.scala#L77-L84
• https://github.com/scala/scala/blob/262c97250a614bf19a0e9e8994ee59abea82a5db/src/compiler/scala/tools/nsc/transform/Erasure.scala#L376-L377

[xuwei-k]

#13047