reflect.Selectable does not mangle operator names

[Blaisorblade]

scala>     type T = {val a: Int; def a_=(x: Int): Unit}
// defined alias type T = Object{a: Int; a_=(x: Int): Unit}

scala> import scala.reflect.Selectable.reflectiveSelectable

scala> (new {var a = 10}: T).a = 11
java.lang.NoSuchMethodException: rs$line$3$$anon$1.a_=(int)
	at java.lang.Class.getMethod(Class.java:1786)
	at scala.reflect.Selectable$.selectDynamicMethod$extension(Selectable.scala:20)
	at scala.reflect.Selectable.selectDynamicMethod(Selectable.scala:17)
	at rs$line$3$.<init>(rs$line$3:1)
	at rs$line$3$.<clinit>(rs$line$3)
	at rs$line$3.res0(rs$line$3)

Haven't tried other characters.

Decompiling a bigger example reminds us that the generated method is called a_$eq and not a_=, while the generated code indeed tries to call a_= (per jad decompilation):

        ((Function1)Selectable$.MODULE$.reflectiveSelectable(v).selectDynamicMethod("a_=", Predef$.MODULE$.wrapRefArray(new ClassTag[] {
            ClassTag$.MODULE$.apply(Integer.TYPE)
        }))).apply(BoxesRunTime.boxToInteger(11));

Since structural types are intended to be mapped also to other backends (see applications to databases in #1886), it's not clear the encoding should be done by the compiler; we might want to mangle the name inside reflectiveSelectable.

EDIT: Discovered while testing fix to #4496 and checking all variants.

[smarter]

@sjrd How does Scala.js name mangling interact with scala.Dynamic ?

[sjrd]

Badly.

In scalac, even calls to scala.Dynamic members are mangled before they are turned to strings. So in the back-end we decode it, but it can lead to false positives, where the user wrote obj.$plus$minus but the compiler will emit `obj['+-']. For example: https://scalafiddle.io/sf/zt1UmSh/0

[smarter]

Ouch! So it'd make sense to follow @Blaisorblade suggestion and move the mangling to library code, since Scala.js can overwrite the library code if needed?

[sjrd]

I think so, yes.

[smarter]

OK, that means something like https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/util/NameTransformer.scala (but that operates on String instead of SimpleName) needs to be moved to dotty-library.

[Blaisorblade]

@sjrd BTW, does it make sense to keep writes to a as calls to a_=, or should we have a separate method for writes? I can imagine a scenario where writes and calls to a_= are distinct things and you run into similar problems. Tho it's harder to imagine a scenario where v.a = 5 becomes a "proper write" and v.a_=(5) a method call; that depends on how early writes become setter calls.

[sjrd]

Writes to properties are supposed to call the updateDynamic("a")(rhs), indeed!

[bishabosha]

this is further complicated by @targetName, but then should only the reflection implementation be aware of these details?, and not some other implementation for Selectable?

[odersky]

I have the impression it's too late to change anything here now.

[bishabosha]

I have the impression it's too late to change anything here now.

May I ask why?

[odersky]

I thought the issue would apply to all selectables, where changing the encoding might break existing user-defined implementations. But I see this is just reflect.Selectable, so OK. I was fooled by the needs-spec label. Can someone explain why this needs a spec?

Also I should not be the one assigned to this, I think.

[wjoel]

FWIW, on Scala 3.5.0 the generated code does call a_$eq

//> using scala 3.5.0

type T = {val a: Int; def a_=(x: Int): Unit}
import scala.reflect.Selectable.reflectiveSelectable

@main def run() =
  (new {var a = 10}.asInstanceOf[T]).a = 11

Generates:

      10: invokevirtual #38                 // Method scala/reflect/Selectable$.reflectiveSelectable:(Ljava/lang/Object;)Lscala/reflect/Selectable;
      13: ldc           #40                 // String a_$eq
      15: getstatic     #45                 // Field scala/runtime/ScalaRunTime$.MODULE$:Lscala/runtime/ScalaRunTime$;

Full output:

Compiled from "Main.scala"
public final class Main$package$ implements java.io.Serializable {
  public static final Main$package$ MODULE$;

  public static {};
    Code:
       0: new           #2                  // class Main$package$
       3: dup
       4: invokespecial #18                 // Method "<init>":()V
       7: putstatic     #20                 // Field MODULE$:LMain$package$;
      10: return

  public void run();
    Code:
       0: getstatic     #33                 // Field scala/reflect/Selectable$.MODULE$:Lscala/reflect/Selectable$;
       3: new           #9                  // class Main$package$$anon$1
       6: dup
       7: invokespecial #34                 // Method Main$package$$anon$1."<init>":()V
      10: invokevirtual #38                 // Method scala/reflect/Selectable$.reflectiveSelectable:(Ljava/lang/Object;)Lscala/reflect/Selectable;
      13: ldc           #40                 // String a_$eq
      15: getstatic     #45                 // Field scala/runtime/ScalaRunTime$.MODULE$:Lscala/runtime/ScalaRunTime$;
      18: iconst_1
      19: anewarray     #47                 // class java/lang/Class
      22: dup
      23: iconst_0
      24: getstatic     #53                 // Field java/lang/Integer.TYPE:Ljava/lang/Class;
      27: aastore
      28: invokevirtual #57                 // Method scala/runtime/ScalaRunTime$.wrapRefArray:([Ljava/lang/Object;)Lscala/collection/immutable/ArraySeq;
      31: getstatic     #45                 // Field scala/runtime/ScalaRunTime$.MODULE$:Lscala/runtime/ScalaRunTime$;
      34: iconst_1
      35: anewarray     #4                  // class java/lang/Object
      38: dup
      39: iconst_0
      40: bipush        11
      42: invokestatic  #63                 // Method scala/runtime/BoxesRunTime.boxToInteger:(I)Ljava/lang/Integer;
      45: aastore
      46: invokevirtual #67                 // Method scala/runtime/ScalaRunTime$.genericWrapArray:(Ljava/lang/Object;)Lscala/collection/immutable/ArraySeq;
      49: invokeinterface #73,  4           // InterfaceMethod scala/reflect/Selectable.applyDynamic:(Ljava/lang/String;Lscala/collection/immutable/Seq;Lscala/collection/immutable/Seq;)Ljava/lang/Object;
      54: pop
      55: return
}