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?