FIR: refactor FirExpressionsResolveTransformer#transformAssignmentOperatorStatement

No logic change. This commit only rearrange stuff to make it cleaner

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

@@ -25,15 +25,18 @@ import org.jetbrains.kotlin.fir.resolve.calls.*
 import org.jetbrains.kotlin.fir.resolve.diagnostics.*
 import org.jetbrains.kotlin.fir.resolve.inference.FirStubInferenceSession
 import org.jetbrains.kotlin.fir.resolve.inference.inferenceComponents
+import org.jetbrains.kotlin.fir.resolve.substitution.ConeSubstitutor
 import org.jetbrains.kotlin.fir.resolve.transformers.InvocationKindTransformer
 import org.jetbrains.kotlin.fir.resolve.transformers.StoreReceiver
 import org.jetbrains.kotlin.fir.resolve.transformers.firClassLike
-import org.jetbrains.kotlin.name.StandardClassIds
 import org.jetbrains.kotlin.fir.symbols.impl.FirVariableSymbol
 import org.jetbrains.kotlin.fir.types.*
-import org.jetbrains.kotlin.fir.types.builder.*
+import org.jetbrains.kotlin.fir.types.builder.buildErrorTypeRef
+import org.jetbrains.kotlin.fir.types.builder.buildResolvedTypeRef
 import org.jetbrains.kotlin.fir.visitors.*
 import org.jetbrains.kotlin.name.Name
+import org.jetbrains.kotlin.name.StandardClassIds
+import org.jetbrains.kotlin.resolve.calls.inference.buildAbstractResultingSubstitutor
 import org.jetbrains.kotlin.resolve.calls.tower.CandidateApplicability
 import org.jetbrains.kotlin.types.AbstractTypeChecker
 import org.jetbrains.kotlin.types.ConstantValueKind
@@ -374,7 +377,8 @@ open class FirExpressionsResolveTransformer(transformer: FirBodyResolveTransform
             assignOperatorCall.transformSingle(this, ResolutionMode.ContextDependent)
         }
         val assignCallReference = resolvedAssignCall.calleeReference as? FirNamedReferenceWithCandidate
-        val assignIsError = assignCallReference?.isError ?: true
+        val assignIsResolvable = assignCallReference?.isError == false
+
         // x = x + y
         val simpleOperatorName = FirOperationNameConventions.ASSIGNMENTS_TO_SIMPLE_OPERATOR.getValue(assignmentOperatorStatement.operation)
         val simpleOperatorCall = createFunctionCall(simpleOperatorName)
@@ -382,57 +386,66 @@ open class FirExpressionsResolveTransformer(transformer: FirBodyResolveTransform
             simpleOperatorCall.transformSingle(this, ResolutionMode.ContextDependent)
         }
         val operatorCallReference = resolvedOperatorCall.calleeReference as? FirNamedReferenceWithCandidate
-        val operatorIsError = operatorCallReference?.isError ?: true
+        val operatorIsResolvable = operatorCallReference?.isError == false
 
         val lhsReference = leftArgument.toResolvedCallableReference()
         val lhsSymbol = lhsReference?.resolvedSymbol as? FirVariableSymbol<*>
         val lhsVariable = lhsSymbol?.fir
         val lhsIsVar = lhsVariable?.isVar == true
-        return when {
-            operatorIsError || (!lhsIsVar && !assignIsError) -> {
-                callCompleter.completeCall(resolvedAssignCall, noExpectedType)
-                dataFlowAnalyzer.exitFunctionCall(resolvedAssignCall, callCompleted = true)
-                resolvedAssignCall
-            }
-            assignIsError -> {
-                callCompleter.completeCall(resolvedOperatorCall, lhsVariable?.returnTypeRef ?: noExpectedType)
-                dataFlowAnalyzer.exitFunctionCall(resolvedOperatorCall, callCompleted = true)
-                val assignment =
-                    buildVariableAssignment {
-                        source = assignmentOperatorStatement.source
-                        rValue = resolvedOperatorCall
-                        calleeReference = when {
-                            lhsIsVar -> lhsReference!!
-                            else -> buildErrorNamedReference {
-                                source = when (leftArgument) {
-                                    is FirFunctionCall -> leftArgument.source
-                                    is FirQualifiedAccess ->
-                                        leftArgument.calleeReference.source
-                                    else -> leftArgument.source
-                                }
-                                diagnostic = if (lhsSymbol == null) ConeVariableExpectedError() else ConeValReassignmentError(lhsSymbol)
+
+        fun chooseAssign(): FirStatement {
+            callCompleter.completeCall(resolvedAssignCall, noExpectedType)
+            dataFlowAnalyzer.exitFunctionCall(resolvedAssignCall, callCompleted = true)
+            return resolvedAssignCall
+        }
+
+        fun chooseOperator(): FirStatement {
+            callCompleter.completeCall(resolvedOperatorCall, lhsVariable?.returnTypeRef ?: noExpectedType)
+            dataFlowAnalyzer.exitFunctionCall(resolvedOperatorCall, callCompleted = true)
+            val assignment =
+                buildVariableAssignment {
+                    source = assignmentOperatorStatement.source
+                    rValue = resolvedOperatorCall
+                    calleeReference = when {
+                        lhsIsVar -> lhsReference!!
+                        else -> buildErrorNamedReference {
+                            source = when (leftArgument) {
+                                is FirFunctionCall -> leftArgument.source
+                                is FirQualifiedAccess ->
+                                    leftArgument.calleeReference.source
+                                else -> leftArgument.source
                             }
-                        }
-                        (leftArgument as? FirQualifiedAccess)?.let {
-                            dispatchReceiver = it.dispatchReceiver
-                            extensionReceiver = it.extensionReceiver
+                            diagnostic = if (lhsSymbol == null) ConeVariableExpectedError() else ConeValReassignmentError(lhsSymbol)
                         }
                     }
-                assignment.transform(transformer, ResolutionMode.ContextIndependent)
-            }
-            else -> {
-                val operatorCallSymbol = operatorCallReference?.candidateSymbol
-                val assignmentCallSymbol = assignCallReference?.candidateSymbol
-
-                requireNotNull(operatorCallSymbol)
-                requireNotNull(assignmentCallSymbol)
-
-                buildErrorExpression {
-                    source = assignmentOperatorStatement.source
-                    diagnostic = ConeOperatorAmbiguityError(listOf(operatorCallSymbol, assignmentCallSymbol))
+                    (leftArgument as? FirQualifiedAccess)?.let {
+                        dispatchReceiver = it.dispatchReceiver
+                        extensionReceiver = it.extensionReceiver
+                    }
                 }
+            return assignment.transform(transformer, ResolutionMode.ContextIndependent)
+        }
+
+        fun reportAmbiguity(): FirStatement {
+            val operatorCallSymbol = operatorCallReference?.candidateSymbol
+            val assignmentCallSymbol = assignCallReference?.candidateSymbol
+
+            requireNotNull(operatorCallSymbol)
+            requireNotNull(assignmentCallSymbol)
+
+            return buildErrorExpression {
+                source = assignmentOperatorStatement.source
+                diagnostic = ConeOperatorAmbiguityError(listOf(operatorCallSymbol, assignmentCallSymbol))
             }
         }
+
+        return when {
+            assignIsResolvable && !lhsIsVar -> chooseAssign()
+            !assignIsResolvable && !operatorIsResolvable -> chooseAssign()
+            !assignIsResolvable && operatorIsResolvable -> chooseOperator()
+            assignIsResolvable && !operatorIsResolvable -> chooseAssign()
+            else -> reportAmbiguity()
+        }
     }
 
     override fun transformEqualityOperatorCall(