FIR: Make LHS of FirVariableAssignment a FirExpression

This way references can even be resolved for erroneous assignments
(e.g. function call, if expression, ... on LHS)

^KT-54648 Fixed

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/VariableStorageImpl.kt

@@ -25,6 +25,7 @@ import org.jetbrains.kotlin.fir.symbols.FirBasedSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.*
 import org.jetbrains.kotlin.fir.types.ConeClassLikeType
 import org.jetbrains.kotlin.fir.types.coneTypeSafe
+import org.jetbrains.kotlin.fir.expressions.explicitReceiver
 
 @OptIn(DfaInternals::class)
 class VariableStorageImpl(private val session: FirSession) : VariableStorage() {
@@ -92,7 +93,7 @@ class VariableStorageImpl(private val session: FirSession) : VariableStorage() {
     }
 
     private fun getIdentifierBySymbol(flow: Flow, symbol: FirBasedSymbol<*>, fir: FirElement): Identifier {
-        val expression = fir as? FirQualifiedAccess
+        val expression = fir as? FirQualifiedAccessExpression ?: (fir as? FirVariableAssignment)?.lValue as? FirQualifiedAccessExpression
         // TODO: don't create receiver variables if not going to create the composed variable either?
         return Identifier(
             symbol,
@@ -104,10 +105,10 @@ class VariableStorageImpl(private val session: FirSession) : VariableStorage() {
     private fun createReal(flow: Flow, identifier: Identifier, originalFir: FirElement, stability: PropertyStability): RealVariable {
         val receiver: FirExpression?
         val isThisReference: Boolean
-        val expression: FirQualifiedAccess? = when (originalFir) {
+        val expression: FirQualifiedAccessExpression? = when (originalFir) {
             is FirQualifiedAccessExpression -> originalFir
             is FirWhenSubjectExpression -> originalFir.whenRef.value.subject as? FirQualifiedAccessExpression
-            is FirVariableAssignment -> originalFir
+            is FirVariableAssignment -> originalFir.unwrapLValue()
             else -> null
         }
 
@@ -162,7 +163,7 @@ class VariableStorageImpl(private val session: FirSession) : VariableStorage() {
             property.receiverParameter != null -> PropertyStability.PROPERTY_WITH_GETTER
             property.getter.let { it != null && it !is FirDefaultPropertyAccessor } -> PropertyStability.PROPERTY_WITH_GETTER
             property.modality != Modality.FINAL -> {
-                val dispatchReceiver = (originalFir.unwrapElement() as? FirQualifiedAccess)?.dispatchReceiver ?: return null
+                val dispatchReceiver = (originalFir.unwrapElement() as? FirQualifiedAccessExpression)?.dispatchReceiver ?: return null
                 val receiverType = dispatchReceiver.typeRef.coneTypeSafe<ConeClassLikeType>()?.fullyExpandedType(session) ?: return null
                 val receiverSymbol = receiverType.lookupTag.toSymbol(session) ?: return null
                 when (val receiverFir = receiverSymbol.fir) {

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/cfg/CFGNodeRenderer.kt

@@ -5,9 +5,7 @@
 
 package org.jetbrains.kotlin.fir.resolve.dfa.cfg
 
-import org.jetbrains.kotlin.fir.expressions.FirDoWhileLoop
-import org.jetbrains.kotlin.fir.expressions.FirLoop
-import org.jetbrains.kotlin.fir.expressions.FirWhileLoop
+import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.expressions.impl.FirElseIfTrueCondition
 import org.jetbrains.kotlin.fir.render
 import org.jetbrains.kotlin.fir.renderer.FirCallNoArgumentsRenderer
@@ -55,7 +53,7 @@ fun CFGNode<*>.render(): String =
                         CfgRenderer.renderAsCallableDeclarationString(fir)
                     }"
 
-                is VariableAssignmentNode -> "Assignment: ${CfgRenderer.renderElementAsString(fir.lValue)}"
+                is VariableAssignmentNode -> "Assignment: ${fir.calleeReference?.let(CfgRenderer::renderElementAsString)}"
                 is FunctionCallNode -> "Function call: ${CfgRenderer.renderElementAsString(fir)}"
                 is DelegatedConstructorCallNode -> "Delegated constructor call: ${CfgRenderer.renderElementAsString(fir)}"
                 is StringConcatenationCallNode -> "String concatenation call: ${CfgRenderer.renderElementAsString(fir)}"

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/util.kt

@@ -40,10 +40,12 @@ val FirExpression.coneType: ConeKotlinType
 val FirElement.symbol: FirBasedSymbol<*>?
     get() = when (this) {
         is FirResolvable -> symbol
+        is FirVariableAssignment -> unwrapLValue()?.symbol
         is FirDeclaration -> symbol
         is FirWhenSubjectExpression -> whenRef.value.subject?.symbol
         is FirSafeCallExpression -> selector.symbol
         is FirSmartCastExpression -> originalExpression.symbol
+        is FirDesugaredAssignmentValueReferenceExpression -> expressionRef.value.symbol
         else -> null
     }?.takeIf {
         (this as? FirExpression)?.unwrapSmartcastExpression() is FirThisReceiverExpression ||

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/diagnostics/ConeDiagnostics.kt

@@ -138,10 +138,6 @@ object ConeVariableExpectedError : ConeDiagnostic {
     override val reason: String get() = "Variable expected"
 }
 
-class ConeValReassignmentError(val variable: FirVariableSymbol<*>) : ConeDiagnostic {
-    override val reason: String get() = "Re-assigning a val variable"
-}
-
 class ConeContractDescriptionError(override val reason: String) : ConeDiagnostic
 
 class ConeIllegalAnnotationError(val name: Name) : ConeDiagnostic {