Compilation error for default protocol function implementation: "invalid redeclaration of..."

[vgritsenko]

Commit hash: 0dd772b

Context:
While testing swiftly's support of existing Swift code, noticed that a protocol with default implementation results in generated Java code that is not compiling due to a duplicate Java_com_example_swiftexamplelib_TestBox__00024action_1__JJ symbol.

import Foundation

public protocol Test {
    func action()
}
public extension Test {
    func action() { }
}

Results in the following compilation error:

.../.build/plugins/outputs/lib/SwiftTestLib/destination/JExtractSwiftPlugin/Sources/Sample+SwiftJava.swift:54:13: error: invalid redeclaration of 'Java_com_example_swiftexamplelib_TestBox__00024action_1__JJ(environment:thisClass:selfPointer:selfTypePointer:)'
26 | #endif
27 | @_cdecl("Java_com_example_swiftexamplelib_TestBox__00024action_1__JJ")
28 | public func Java_com_example_swiftexamplelib_TestBox__00024action_1__JJ(environment: UnsafeMutablePointer<JNIEnv?>!, thisClass: jclass, selfPointer: jlong, selfTypePointer: jlong) {
   |             `- note: 'Java_com_example_swiftexamplelib_TestBox__00024action_1__JJ(environment:thisClass:selfPointer:selfTypePointer:)' previously declared here
29 |   guard let selfPointerTypeMetadataPointer$ = UnsafeRawPointer(bitPattern: Int(Int64(fromJNI: selfTypePointer, in: environment))) else {
30 |     fatalError("selfTypePointer memory address was null")
   :
52 | #endif
53 | @_cdecl("Java_com_example_swiftexamplelib_TestBox__00024action_1__JJ")
54 | public func Java_com_example_swiftexamplelib_TestBox__00024action_1__JJ(environment: UnsafeMutablePointer<JNIEnv?>!, thisClass: jclass, selfPointer: jlong, selfTypePointer: jlong) {
   |             `- error: invalid redeclaration of 'Java_com_example_swiftexamplelib_TestBox__00024action_1__JJ(environment:thisClass:selfPointer:selfTypePointer:)'
55 |   guard let selfPointerTypeMetadataPointer$ = UnsafeRawPointer(bitPattern: Int(Int64(fromJNI: selfTypePointer, in: environment))) else {
56 |     fatalError("selfTypePointer memory address was null")

Generated Java file:

// ==== --------------------------------------------------
// Thunks for Test

// Generated by swift-java

#if hasFeature(InternalImportsByDefault)
public import SwiftJava
public import SwiftJavaJNICore
public import SwiftJavaRuntimeSupport
#else
import SwiftJava
import SwiftJavaJNICore
import SwiftJavaRuntimeSupport
#endif
import Foundation
import SwiftJava
import SwiftJavaRuntimeSupport


#if compiler(>=6.3)
@used
#endif
#if compiler(>=6.4)
@diagnose(DeprecatedDeclaration, as: ignored)
#endif
@_cdecl("Java_com_example_swiftexamplelib_TestBox__00024action_1__JJ")
public func Java_com_example_swiftexamplelib_TestBox__00024action_1__JJ(environment: UnsafeMutablePointer<JNIEnv?>!, thisClass: jclass, selfPointer: jlong, selfTypePointer: jlong) {
  guard let selfPointerTypeMetadataPointer$ = UnsafeRawPointer(bitPattern: Int(Int64(fromJNI: selfTypePointer, in: environment))) else {
    fatalError("selfTypePointer memory address was null")
  }
  let selfPointerDynamicType$: Any.Type = unsafeBitCast(selfPointerTypeMetadataPointer$, to: Any.Type.self)
  guard let selfPointerRawPointer$ = UnsafeMutableRawPointer(bitPattern: Int(Int64(fromJNI: selfPointer, in: environment))) else {
    fatalError("selfPointer memory address was null")
  }
  #if hasFeature(ImplicitOpenExistentials)
  let selfPointerExistential$ = selfPointerRawPointer$.load(as: selfPointerDynamicType$) as! (any Test)
  #else
  func selfPointerDoLoad<Ty>(_ ty: Ty.Type) -> (any Test) {
    selfPointerRawPointer$.load(as: ty) as! (any Test)
  }
  let selfPointerExistential$ = _openExistential(selfPointerDynamicType$, do: selfPointerDoLoad)
  #endif
  selfPointerExistential$.action()
} // printCDecl(_:javaMethodName:parentName:parameters:resultType:_:) @ JExtractSwiftLib/JNISwift2JavaGenerator+SwiftThunkPrinting.swift:893

#if compiler(>=6.3)
@used
#endif
#if compiler(>=6.4)
@diagnose(DeprecatedDeclaration, as: ignored)
#endif
@_cdecl("Java_com_example_swiftexamplelib_TestBox__00024action_1__JJ")
public func Java_com_example_swiftexamplelib_TestBox__00024action_1__JJ(environment: UnsafeMutablePointer<JNIEnv?>!, thisClass: jclass, selfPointer: jlong, selfTypePointer: jlong) {
  guard let selfPointerTypeMetadataPointer$ = UnsafeRawPointer(bitPattern: Int(Int64(fromJNI: selfTypePointer, in: environment))) else {
    fatalError("selfTypePointer memory address was null")
  }
  let selfPointerDynamicType$: Any.Type = unsafeBitCast(selfPointerTypeMetadataPointer$, to: Any.Type.self)
  guard let selfPointerRawPointer$ = UnsafeMutableRawPointer(bitPattern: Int(Int64(fromJNI: selfPointer, in: environment))) else {
    fatalError("selfPointer memory address was null")
  }
  #if hasFeature(ImplicitOpenExistentials)
  let selfPointerExistential$ = selfPointerRawPointer$.load(as: selfPointerDynamicType$) as! (any Test)
  #else
  func selfPointerDoLoad<Ty>(_ ty: Ty.Type) -> (any Test) {
    selfPointerRawPointer$.load(as: ty) as! (any Test)
  }
  let selfPointerExistential$ = _openExistential(selfPointerDynamicType$, do: selfPointerDoLoad)
  #endif
  selfPointerExistential$.action()
} // printCDecl(_:javaMethodName:parentName:parameters:resultType:_:) @ JExtractSwiftLib/JNISwift2JavaGenerator+SwiftThunkPrinting.swift:893

Environment: Swift 6.3.3 on macOS.

$ swiftly run swift --version
Apple Swift version 6.3.3 (swift-6.3.3-RELEASE)
Target: arm64-apple-macosx26.0

$ uname -a
Darwin it-C01241 25.6.0 Darwin Kernel Version 25.6.0: Sat Jul 11 15:26:21 PDT 2026; root:xnu-12377.161.13~4/RELEASE_ARM64_T6000 arm64

[ktoso]

Thanks for the report, I’ll give it a look