SAM extending `Function1` causes anonymous class to be generated

[armanbilge]

Compiler version

3.4.0-RC1-bin-20240104-2746ee8-NIGHTLY

Minimized code

//> using scala 3.4.0-RC1-bin-20240104-2746ee8-NIGHTLY

trait Foo[-A, +B] {
  def apply(a: A): B
}

trait Bar[-A, +B] extends Foo[A, B]

trait Baz[-A, +B] extends Function1[A, B]

class Test {
  def captureFunction1[A, B](f: Function1[A, B]): Unit = ()
  def captureFoo[A, B](f: Foo[A, B]): Unit = ()
  def captureBar[A, B](f: Bar[A, B]): Unit = ()
  def captureBaz[A, B](f: Baz[A, B]): Unit = ()

  def test() = {
    captureFunction1[String, String](_.length.toString)
    captureFoo[String, String](_.length.toString)
    captureBar[String, String](_.length.toString)
    captureBaz[String, String](_.length.toString)
  }
}

Output

/*
 * Decompiled with CFR 0.151.
 */
import java.io.Serializable;
import scala.Function1;
import scala.runtime.BoxesRunTime;

public class Test {
    public <A, B> void captureFunction1(Function1<A, B> f) {
    }

    public <A, B> void captureFoo(Foo<A, B> f) {
    }

    public <A, B> void captureBar(Bar<A, B> f) {
    }

    public <A, B> void captureBaz(Baz<A, B> f) {
    }

    public void test() {
        this.captureFunction1((Function1<String, String> & Serializable)_$1 -> BoxesRunTime.boxToInteger(_$1.length()).toString());
        this.captureFoo(_$2 -> BoxesRunTime.boxToInteger(_$2.length()).toString());
        this.captureBar(_$3 -> BoxesRunTime.boxToInteger(_$3.length()).toString());
        this.captureBaz(new Baz<String, String>(this){
            {
                if ($outer == null) {
                    throw new NullPointerException();
                }
            }

            public final String apply(String _$4) {
                return Test.Test$$_$test$$anonfun$4(_$4);
            }
        });
    }

    public static final /* synthetic */ String Test$$_$test$$anonfun$4(String _$4) {
        return BoxesRunTime.boxToInteger(_$4.length()).toString();
    }
}

Expectation

Implementing a SAM (single abstract method) should avoid generating an anonymous class.

[jchyb]

An interesting part of this is that all scala versions seem to generate anonymous classes like this, including scala 2.

[lrytz]

In Scala 2, every trait with a non-abstract member gets a $init$ method.

trait A { def f = 1 }

turns into

public interface A {
    public static /* synthetic */ int f$(A $this) { return $this.f(); }
    default public int f() { return 1; }
    public static void $init$(A $this) { }
}

Scala 3 behaves differently in this regard, the $init$ method is not added (see #10530).

Function1 has concrete methods (compose), and being part of the standard library it's compiled with 2.13, so it has an $init$ method. That's the reason Baz is not treated as a SAM type, it's parent Function1 has an $init$ method (https://github.com/lampepfl/dotty/blob/3.3.1/compiler/src/dotty/tools/dotc/config/JavaPlatform.scala#L44).

Corresponding code in Scala 2: https://github.com/scala/scala/blob/v2.13.12/src/compiler/scala/tools/nsc/transform/Erasure.scala#L1436.

I haven't checked, but the reason captureFunction1 doesn't expand must be because the compiler takes a different code path for Function1, both in Scala 2 and 3.

Maybe it would be safe to add a special case for Function1 to isSam (Scala 3) / compilesToPureInterface (Scala 2).