`publicTypeUsage` reports a file facade whose members are all `internal`

[eygraber]

Plugin version
3.18.0

Gradle version
9.6.0

JDK version
26

Kotlin and Kotlin Gradle Plugin (KGP) version
2.4.10

Describe the bug

When an internal top-level function has a default argument whose type is an inline value class wrapping a nullable reference, the function's synthetic $default bridge is treated as public API. That keeps the file facade in the ABI, so publicTypeUsage reports a FooKt class even though every top-level member in the file is internal or private. There's no Kotlin syntax to restrict a facade, so the finding is unactionable.

To Reproduce

lib:

@JvmInline
value class Path(val id: String)

@JvmInline
value class PathParent(private val path: Path?) {   // nullable → stays boxed
  companion object { val Empty = PathParent(null) }
}

consumer (depends on lib), where every top-level member is internal:

internal fun sat(p: PathParent = PathParent.Empty): Boolean = p != PathParent.Empty
internal class User { fun go() = sat() }

Expected behavior

consumer/build/reports/dependency-analysis/main/abi-dump.txt is empty.

Actual behavior

@Lkotlin/Metadata;
public final class com/example/UseKt {
        public static synthetic fun sat-WUG17-E$default (Lcom/example/PathParent;ILjava/lang/Object;)Z
}

The function itself is correctly excluded; only the bridge leaks.

Additional context

MethodBinarySignature.alternateDefaultSignature reconstructs the non-bridge signature to look up the real function's visibility in Kotlin metadata, but the bridge's descriptor carries the boxed wrapper type while metadata records the unboxed underlying type:

reconstructed:       sat-WUG17-E (Lcom/example/PathParent;)Z
in kotlin.Metadata:  sat-WUG17-E (Ljava/lang/String;)Z

A value class over a non-nullable reference erases to its underlying type so this works. Over a nullable reference it must stay boxed, so they diverge, the lookup misses, and isEffectivelyPublic's ?: true fallback treats the bridge as public.

Note the facade handling itself is working. isFileOrMultipartFacade() / isNotUsedWhenEmpty correctly drop facades with no public members.

A non-nullable value class, or a plain internal fun f(a: Int, b: Int = 0), both produce an empty dump.

Failing test

eygraber@51db957

expected to be empty
but was: [..., public static synthetic fun sat-WUG17-E$default (Lcom/example/PathParent;ILjava/lang/Object;)Z, }]

[autonomousapps]

Thanks for the report.

I've been aware of the issue with "synthetic $default bridge" functions for a while now. I'd like to filter those out of the public API, in general.