Fix compile error in generated thunks for throwing functions returning String

Samples/SwiftJavaExtractJNISampleApp/Sources/MySwiftLibrary/Throw.swift

@@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftJava
+
+public func throwString(input: String) throws -> String {
+  return input
+}

Samples/SwiftJavaExtractJNISampleApp/src/test/java/com/example/swift/ThrowTest.java

@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+package com.example.swift;
+
+import com.example.swift.MySwiftLibrary;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class ThrowTest {
+    @Test
+    void throwString() throws Exception {
+        String result = MySwiftLibrary.throwString("hey");
+        assertEquals("hey", result);
+    }
+}

Sources/JExtractSwiftLib/JNI/JNISwift2JavaGenerator+SwiftThunkPrinting.swift

@@ -657,12 +657,14 @@ extension JNISwift2JavaGenerator {
   }
 
   private func dummyReturn(for nativeSignature: NativeFunctionSignature) -> String {
-      return if nativeSignature.result.javaType.isVoid {
-        "return"
-      } else {
-        // We assume it is something that implements JavaValue
-        "return \(nativeSignature.result.javaType.swiftTypeName(resolver: { _ in "" })).jniPlaceholderValue"
-      }
+    return if nativeSignature.result.javaType.isVoid {
+      "return"
+    } else if nativeSignature.result.javaType.isString {
+      "return String.jniPlaceholderValue"
+    } else {
+      // We assume it is something that implements JavaValue
+      "return \(nativeSignature.result.javaType.swiftTypeName(resolver: { _ in "" })).jniPlaceholderValue"
+    }
   }
 
   private func printCDecl(

Tests/JExtractSwiftTests/JNI/JNIModuleTests.swift

@@ -30,6 +30,7 @@ struct JNIModuleTests {
   let globalMethodThrowing = """
     public func methodA() throws
     public func methodB() throws -> Int64
+    public func methodC() throws -> String
   """
 
   @Test
@@ -256,6 +257,17 @@ struct JNIModuleTests {
             }
           }
           """,
+          """
+          @_cdecl("Java_com_example_swift_SwiftModule__00024methodC__")
+          public func Java_com_example_swift_SwiftModule__00024methodC__(environment: UnsafeMutablePointer<JNIEnv?>!, thisClass: jclass) -> jstring? {
+            do {
+              return try SwiftModule.methodC().getJNIValue(in: environment)
+            } catch {
+              environment.throwAsException(error)
+              return String.jniPlaceholderValue
+            }
+          }
+          """
       ]
     )
   }