Properly handle vararg interpretation for classic frontend

#KT-55108 Fixed

compiler/fir/fir2ir/tests-gen/org/jetbrains/kotlin/test/runners/codegen/FirBlackBoxCodegenTestGenerated.java

@@ -257,6 +257,12 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
             runTest("compiler/testData/codegen/box/annotations/kt25489.kt");
         }
 
+        @Test
+        @TestMetadata("kt55108.kt")
+        public void testKt55108() throws Exception {
+            runTest("compiler/testData/codegen/box/annotations/kt55108.kt");
+        }
+
         @Test
         @TestMetadata("mustBeDocumented.kt")
         public void testMustBeDocumented() throws Exception {

compiler/ir/ir.interpreter/src/org/jetbrains/kotlin/ir/interpreter/checker/IrConstTransformer.kt

@@ -119,14 +119,10 @@ class IrConstTransformer(
         if (elements.isEmpty()) return this
         val newIrVararg = IrVarargImpl(this.startOffset, this.endOffset, this.type, this.varargElementType)
         for (element in this.elements) {
-            when (element) {
-                is IrExpression -> newIrVararg.addElement(element.transformSingleArg(this.varargElementType))
-                is IrSpreadElement -> {
-                    when (val expression = element.expression) {
-                        is IrVararg -> expression.transformVarArg().elements.forEach { newIrVararg.addElement(it) }
-                        else -> newIrVararg.addElement(expression.transformSingleArg(this.varargElementType))
-                    }
-                }
+            when (val arg = (element as? IrSpreadElement)?.expression ?: element) {
+                is IrVararg -> arg.transformVarArg().elements.forEach { newIrVararg.addElement(it) }
+                is IrExpression -> newIrVararg.addElement(arg.transformSingleArg(this.varargElementType))
+                else -> newIrVararg.addElement(arg)
             }
         }
         return newIrVararg

compiler/testData/codegen/box/annotations/kt55108.kt

@@ -0,0 +1,28 @@
+// TARGET_BACKEND: JVM_IR
+// WITH_REFLECT
+
+annotation class A(vararg val strings: String)
+
+@A(*arrayOf("foo", "bar"), "baz")
+class B
+
+@A("baz", *arrayOf("foo", "bar"), "xyz")
+class C
+
+@A(*arrayOf("foo", "bar", "xyz"))
+class D
+
+@A("foo", "baz", "bar")
+class E
+
+@A(*arrayOf("foo", "bar"), *arrayOf("baz", "xyz"))
+class F
+
+fun box(): String {
+    assert((B::class.annotations.single() as A).strings.contentEquals(arrayOf("foo", "bar", "baz"))) { "Fail1" }
+    assert((C::class.annotations.single() as A).strings.contentEquals(arrayOf("baz", "foo", "bar", "xyz"))) { "Fail 2" }
+    assert((D::class.annotations.single() as A).strings.contentEquals(arrayOf("foo", "bar", "xyz"))) { "Fail 3" }
+    assert((E::class.annotations.single() as A).strings.contentEquals(arrayOf("foo", "baz", "bar"))) { "Fail 4" }
+    assert((F::class.annotations.single() as A).strings.contentEquals(arrayOf("foo", "bar", "baz", "xyz"))) { "Fail 5" }
+    return "OK"
+}

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/codegen/IrBlackBoxCodegenTestGenerated.java

@@ -257,6 +257,12 @@ public class IrBlackBoxCodegenTestGenerated extends AbstractIrBlackBoxCodegenTes
             runTest("compiler/testData/codegen/box/annotations/kt25489.kt");
         }
 
+        @Test
+        @TestMetadata("kt55108.kt")
+        public void testKt55108() throws Exception {
+            runTest("compiler/testData/codegen/box/annotations/kt55108.kt");
+        }
+
         @Test
         @TestMetadata("mustBeDocumented.kt")
         public void testMustBeDocumented() throws Exception {