[FIR] Keep vararg argument order in resolved calls (KT-17691).

compiler/fir/fir2ir/tests/org/jetbrains/kotlin/codegen/ir/FirBlackBoxCodegenTestGenerated.java

@@ -13016,6 +13016,11 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
             runTest("compiler/testData/codegen/box/increment/classVarargGetSet.kt");
         }
 
+        @TestMetadata("classVarargGetSetEvaluationOrder.kt")
+        public void testClassVarargGetSetEvaluationOrder() throws Exception {
+            runTest("compiler/testData/codegen/box/increment/classVarargGetSetEvaluationOrder.kt");
+        }
+
         @TestMetadata("classWithGetSet.kt")
         public void testClassWithGetSet() throws Exception {
             runTest("compiler/testData/codegen/box/increment/classWithGetSet.kt");
@@ -17861,6 +17866,11 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
         public void testVarargs() throws Exception {
             runTest("compiler/testData/codegen/box/mixedNamedPosition/varargs.kt");
         }
+
+        @TestMetadata("varargsEvaluationOrder.kt")
+        public void testVarargsEvaluationOrder() throws Exception {
+            runTest("compiler/testData/codegen/box/mixedNamedPosition/varargsEvaluationOrder.kt");
+        }
     }
 
     @TestMetadata("compiler/testData/codegen/box/multiDecl")
@@ -31536,6 +31546,11 @@ public class FirBlackBoxCodegenTestGenerated extends AbstractFirBlackBoxCodegenT
             runTest("compiler/testData/codegen/box/vararg/emptyVarargOfBoxedPrimitiveType.kt");
         }
 
+        @TestMetadata("evaluationOrder.kt")
+        public void testEvaluationOrder() throws Exception {
+            runTest("compiler/testData/codegen/box/vararg/evaluationOrder.kt");
+        }
+
         @TestMetadata("kt1978.kt")
         public void testKt1978() throws Exception {
             runTest("compiler/testData/codegen/box/vararg/kt1978.kt");

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/FirArgumentsToParametersMapper.kt

@@ -29,6 +29,8 @@ data class ArgumentMapping(
     //      fun foo(a: Int, b: Int) {}
     //      foo(b = bar(), a = qux())
     // parameterToCallArgumentMap.values() should be [ 'bar()', 'foo()' ]
+    // TODO: Consider changing this (and other similar declarations like Candidate.argumentMapping) to LinkedHashMap to signify that
+    // order is important. Right now we're assuming that mutableMapOf() will always return a LinkedHashMap.
     val parameterToCallArgumentMap: Map<FirValueParameter, ResolvedCallArgument>,
     val oldToNewArgumentMap: Map<FirExpression, FirExpression>,
     val diagnostics: List<ResolutionDiagnostic>

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/BodyResolveUtils.kt

@@ -40,10 +40,12 @@ internal fun remapArgumentsWithVararg(
     argumentList: FirArgumentList,
     argumentMapping: Map<FirExpression, FirValueParameter>
 ): Map<FirExpression, FirValueParameter> {
-    // Create a FirVarargArgumentExpression for the vararg arguments
+    // Create a FirVarargArgumentExpression for the vararg arguments.
+    // The order of arguments in the mapping must be preserved for FIR2IR, hence we have to find where the vararg arguments end.
+    // FIR2IR uses the mapping order to determine if arguments need to be reordered.
     val varargParameterTypeRef = varargParameter.returnTypeRef
     val varargElementType = varargArrayType.arrayElementType()
-    var firstIndex = argumentList.arguments.size
+    var indexAfterVarargs = argumentList.arguments.size
     val newArgumentMapping = mutableMapOf<FirExpression, FirValueParameter>()
     val varargArgument = buildVarargArgumentsExpression {
         this.varargElementType = varargParameterTypeRef.withReplacedConeType(varargElementType)
@@ -51,14 +53,26 @@ internal fun remapArgumentsWithVararg(
         for ((i, arg) in argumentList.arguments.withIndex()) {
             val valueParameter = argumentMapping[arg] ?: continue
             if (valueParameter.isVararg) {
-                firstIndex = min(firstIndex, i)
+                // `arg` is a vararg argument.
                 arguments += arg
-            } else {
+            } else if (arguments.isEmpty()) {
+                // `arg` is BEFORE the vararg arguments.
                 newArgumentMapping[arg] = valueParameter
+            } else {
+                // `arg` is AFTER the vararg arguments.
+                indexAfterVarargs = i
+                break
             }
         }
     }
     newArgumentMapping[varargArgument] = varargParameter
+
+    // Add mapping for arguments after the vararg arguments, if any.
+    for (i in indexAfterVarargs until argumentList.arguments.size) {
+        val arg = argumentList.arguments[i]
+        val valueParameter = argumentMapping[arg] ?: continue
+        newArgumentMapping[arg] = valueParameter
+    }
     return newArgumentMapping
 }