Handle Kotlin Function Names Correctly

[jlink]

In "normal" cases Kotlin function names map directly onto Java method names.
However, there are a few cases where the Kotlin source name of a function will be different than the generated method name in byte code:

1. in internal classes
2. with parameters of specific types present - e.g. UInt, UShort etc.
3. There might be more...

As a test engine writer I stumble upon this problem in a few classes:

• org.junit.platform.engine.discovery.MethodSelector which - when called from IntelliJ's test runner - provides the correct Kotlin function name, but cannot resolve the underlying Java method.
• org.junit.platform.engine.support.descriptor.MethodSource which cannot differentiate between methodName and getJavaMethod()

Deliverables

• Implement MethodSelector so that it works correctly with all Kotlin function names
• Implement MethodSource so that it will correctly produce methodName (Kotlin name) and getJavaMethod().

Alternatively, allow a test engine to change the behaviour of MethodSelector and MethodSource.

Addendum

A similar problem may exist with Kotlin class names, e.g. internal or inner classes, but I haven't checked.

[marcphilipp]

@jlink Thanks for raising the issue! Could you please provide concrete examples of such cases or a link to the specification or docs for this behavior?

[jlink]

class KotlinExperiments {

    internal fun internalFunction() {
    }

    fun functionWithUShortParameter(aShort: UShort) {
    }

    @Test
    fun showNames() {
        println(this::internalFunction.javaMethod?.name)
        println(this::functionWithUShortParameter.javaMethod?.name)
    }
}

As for internal functions "name mangling" is mentioned in https://kotlinlang.org/docs/java-to-kotlin-interop.html#visibility
It looks like internal functions get a "$..." postfix. The exact postfix has changed through Kotlin versions

As for functions with unsigned int parameter types I did not find any specification. What I empirically found out is that names of those functions have a "-abcdefg" appendix, ie dash followed by 7 characters.

The problem with both is that you can create the same names using backticks, e.g.

    fun `internalFunction$kotlin`() {}

which will even lead to a runtime error (not a compiler error in Kotlin 1.5.30) since there are two methdos with same name :-(

[jlink]

As a temporary workaround test engine maintainers can handle the MethodSelector issues in each engine.
MethodSource however cannot be subtyped due to missing non-private constructor. That's why a MethodSource factory method that can differentiate between methodName and javaMethodName would also be welcome.

[marcphilipp]

Are there any Kotlin APIs we could call to determine the source method name? Does Kotlin add anything to the bytecode to reverse mangling, e.g. an annotation?

[sbrannen]

The Spring Framework uses APIs in kotlin-reflect to introspect Kotlin code from Java.

[jlink]

If you’re on the Kotlin side you can find out the name through Kotlin‘s own reflection mechanism. The bytecode , however, does not reflect the manipulation, as far as I could find out.