JVM_IR: do not set ACC_VARARGS if vararg is not the last parameter

Fun fact: this is not actually validated when loading the class; and
even when compiling a java file against a class file that does this,
only javac 9 has a check (1.8 and before simply crashes with
NullPointerException somewhere deep inside the compiler).

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/codegen/FunctionCodegen.kt

@@ -108,7 +108,7 @@ open class FunctionCodegen(
 
         val visibility = irFunction.getVisibilityAccessFlag()
         val staticFlag = if (isStatic) Opcodes.ACC_STATIC else 0
-        val varargFlag = if (irFunction.valueParameters.any { it.varargElementType != null }) Opcodes.ACC_VARARGS else 0
+        val varargFlag = if (irFunction.valueParameters.lastOrNull()?.varargElementType != null) Opcodes.ACC_VARARGS else 0
         val deprecation = irFunction.deprecationFlags
         val bridgeFlag = if (
             irFunction.origin == IrDeclarationOrigin.BRIDGE ||

compiler/testData/codegen/box/vararg/notInLastPosition.kt

@@ -0,0 +1,15 @@
+// TARGET_BACKEND: JVM
+// FILE: A.kt
+class A {
+    fun f(vararg xs: String, y: String) = xs[0] + y
+}
+
+// FILE: B.java
+public class B {
+    public static String f() {
+        return new A().f(new String[]{"O"}, "K");
+    }
+}
+
+// FILE: C.kt
+fun box() = B.f()

compiler/tests/org/jetbrains/kotlin/codegen/BlackBoxCodegenTestGenerated.java

@@ -27773,6 +27773,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             runTest("compiler/testData/codegen/box/vararg/kt796_797.kt");
         }
 
+        @TestMetadata("notInLastPosition.kt")
+        public void testNotInLastPosition() throws Exception {
+            runTest("compiler/testData/codegen/box/vararg/notInLastPosition.kt");
+        }
+
         @TestMetadata("singleAssignmentToVarargsInFunction.kt")
         public void testSingleAssignmentToVarargsInFunction() throws Exception {
             runTest("compiler/testData/codegen/box/vararg/singleAssignmentToVarargsInFunction.kt");

compiler/tests/org/jetbrains/kotlin/codegen/LightAnalysisModeTestGenerated.java

@@ -26590,6 +26590,11 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
             runTest("compiler/testData/codegen/box/vararg/kt796_797.kt");
         }
 
+        @TestMetadata("notInLastPosition.kt")
+        public void testNotInLastPosition() throws Exception {
+            runTest("compiler/testData/codegen/box/vararg/notInLastPosition.kt");
+        }
+
         @TestMetadata("singleAssignmentToVarargsInFunction.kt")
         public void testSingleAssignmentToVarargsInFunction() throws Exception {
             runTest("compiler/testData/codegen/box/vararg/singleAssignmentToVarargsInFunction.kt");

compiler/tests/org/jetbrains/kotlin/codegen/ir/FirBlackBoxCodegenTestGenerated.java

@@ -26323,6 +26323,11 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
             runTest("compiler/testData/codegen/box/vararg/kt796_797.kt");
         }
 
+        @TestMetadata("notInLastPosition.kt")
+        public void testNotInLastPosition() throws Exception {
+            runTest("compiler/testData/codegen/box/vararg/notInLastPosition.kt");
+        }
+
         @TestMetadata("singleAssignmentToVarargsInFunction.kt")
         public void testSingleAssignmentToVarargsInFunction() throws Exception {
             runTest("compiler/testData/codegen/box/vararg/singleAssignmentToVarargsInFunction.kt");

compiler/tests/org/jetbrains/kotlin/codegen/ir/IrBlackBoxCodegenTestGenerated.java

@@ -26323,6 +26323,11 @@ public class IrBlackBoxCodegenTestGenerated extends AbstractIrBlackBoxCodegenTes
             runTest("compiler/testData/codegen/box/vararg/kt796_797.kt");
         }
 
+        @TestMetadata("notInLastPosition.kt")
+        public void testNotInLastPosition() throws Exception {
+            runTest("compiler/testData/codegen/box/vararg/notInLastPosition.kt");
+        }
+
         @TestMetadata("singleAssignmentToVarargsInFunction.kt")
         public void testSingleAssignmentToVarargsInFunction() throws Exception {
             runTest("compiler/testData/codegen/box/vararg/singleAssignmentToVarargsInFunction.kt");