FIR CFG: handle reassignments with explicit receivers more precisely

When a looping control structure has a reassignment with an explicit
receiver, we don't need to erase its smartcast info as it has to be a
non-local property and is thus automatically unstable anyway. However,
we should still do that upon reaching the assignment itself for slightly
more precise error messages (as the property not just "could be"
reassigned, but *has been* reassigned already).

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/dfa/FirDataFlowAnalyzer.kt

@@ -1216,9 +1216,14 @@ abstract class FirDataFlowAnalyzer<FLOW : Flow>(
     fun exitVariableAssignment(assignment: FirVariableAssignment) {
         val node = graphBuilder.exitVariableAssignment(assignment).mergeIncomingFlow()
         val property = assignment.lValue.resolvedSymbol?.fir as? FirProperty ?: return
-        // TODO: add unstable smartcast
-        if (property.isLocal || !property.isVar) {
+        if (property.isLocal || property.isVal) {
             exitVariableInitialization(node, assignment.rValue, property, assignment, hasExplicitType = false)
+        } else {
+            // TODO: add unstable smartcast for non-local var
+            val variable = variableStorage.getRealVariableWithoutUnwrappingAlias(node.flow, property.symbol, assignment)
+            if (variable != null) {
+                logicSystem.removeAllAboutVariable(node.flow, variable)
+            }
         }
         processConditionalContract(assignment)
     }

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/dfa/FirLocalVariableAssignmentAnalyzer.kt

@@ -12,6 +12,7 @@ import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.utils.referredPropertySymbol
 import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.references.FirNamedReference
+import org.jetbrains.kotlin.fir.references.FirReference
 import org.jetbrains.kotlin.fir.resolve.dfa.FirLocalVariableAssignmentAnalyzer.Companion.MiniFlow.Companion.join
 import org.jetbrains.kotlin.fir.symbols.impl.FirFunctionSymbol
 import org.jetbrains.kotlin.fir.visitors.FirVisitor
@@ -369,23 +370,21 @@ internal class FirLocalVariableAssignmentAnalyzer(
 
             override fun visitVariableAssignment(variableAssignment: FirVariableAssignment, data: MiniCfgData) {
                 super.visitVariableAssignment(variableAssignment, data)
-                val flow = data.flow ?: return
-                val name = (variableAssignment.lValue as? FirNamedReference)?.name ?: return
-                flow.recordAssignment(name, data)
+                if (variableAssignment.explicitReceiver != null) return
+                data.recordAssignment(variableAssignment.lValue)
             }
 
             override fun visitAssignmentOperatorStatement(assignmentOperatorStatement: FirAssignmentOperatorStatement, data: MiniCfgData) {
                 super.visitAssignmentOperatorStatement(assignmentOperatorStatement, data)
-                val flow = data.flow ?: return
                 val lhs = assignmentOperatorStatement.leftArgument as? FirQualifiedAccessExpression ?: return
                 if (lhs.explicitReceiver != null) return
-                val name = (lhs.calleeReference as? FirNamedReference)?.name ?: return
-                flow.recordAssignment(name, data)
+                data.recordAssignment(lhs.calleeReference)
             }
 
-            fun MiniFlow.recordAssignment(name: Name, data: MiniCfgData) {
-                val property = data.resolveLocalVariable(name) ?: return
-                recordAssignment(property, mutableSetOf())
+            fun MiniCfgData.recordAssignment(reference: FirReference) {
+                val name = (reference as? FirNamedReference)?.name ?: return
+                val property = resolveLocalVariable(name) ?: return
+                flow?.recordAssignment(property, mutableSetOf())
             }
 
             private fun MiniFlow.recordAssignment(property: FirProperty, visited: MutableSet<MiniFlow>) {

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/dfa/PreliminaryLoopVisitor.kt

@@ -44,6 +44,10 @@ class PreliminaryLoopVisitor {
         override fun visitVariableAssignment(variableAssignment: FirVariableAssignment, data: FirStatement?) {
             if (variableAssignment.source?.kind == KtFakeSourceElementKind.DesugaredIncrementOrDecrement) return
 
+            // Only care about local variable assignments, which never have explicit receivers. If this is a `var`
+            // property assignment, the smart cast will be unstable anyway.
+            if (variableAssignment.explicitReceiver != null) return
+
             val reference = variableAssignment.lValue as? FirNamedReference
             if (reference != null) {
                 requireNotNull(data)

compiler/testData/diagnostics/tests/smartCasts/implicitThisOrLocalVar.fir.kt

@@ -10,7 +10,7 @@ fun Box.test() {
             other.item = null
             take<String>(<!SMARTCAST_IMPOSSIBLE!>item<!>)
             this.item = null
-            take<String>(<!SMARTCAST_IMPOSSIBLE!>item<!>)
+            take<String>(<!ARGUMENT_TYPE_MISMATCH!>item<!>)
         }
     }
 
@@ -18,7 +18,7 @@ fun Box.test() {
     myRun {
         if (item != null) {
             this.item = null
-            take<String>(<!SMARTCAST_IMPOSSIBLE!>item<!>)
+            take<String>(item)
         }
     }
 }