FIR DFA: check for reassignments of LHS when handling ?..

`x?.y != null` does not imply that `x != null` if e.g. an argument to
`y` has reassigned `x` in the meantime.

The same is true for `x == y` and `functionWithContract(x, y)`, but
those are somewhat harder to implement since there is no easy way to
find the last node of a certain argument.

^KT-55096

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

@@ -6,7 +6,7 @@
 package org.jetbrains.kotlin.fir.resolve.dfa
 
 abstract class Flow {
-    abstract val approvedTypeStatements: TypeStatements
+    abstract val knownVariables: Set<RealVariable>
     abstract fun unwrapVariable(variable: RealVariable): RealVariable
     abstract fun getTypeStatement(variable: RealVariable): TypeStatement?
 }

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

@@ -23,6 +23,7 @@ abstract class LogicSystem<FLOW : Flow>(protected val context: ConeInferenceCont
     abstract fun recordNewAssignment(flow: FLOW, variable: RealVariable, index: Int)
     abstract fun removeAllAboutVariable(flow: FLOW, variable: RealVariable)
     abstract fun copyAllInformation(from: FLOW, to: FLOW)
+    abstract fun isSameValueIn(a: FLOW, b: FLOW, variable: RealVariable): Boolean
 
     abstract fun translateVariableFromConditionInStatements(
         flow: FLOW,

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

@@ -22,7 +22,7 @@ typealias PersistentImplications = PersistentMap<DataFlowVariable, PersistentLis
 
 class PersistentFlow : Flow {
     val previousFlow: PersistentFlow?
-    override var approvedTypeStatements: PersistentApprovedTypeStatements
+    var approvedTypeStatements: PersistentApprovedTypeStatements
     var logicStatements: PersistentImplications
     val level: Int
 
@@ -60,6 +60,9 @@ class PersistentFlow : Flow {
         assignmentIndex = persistentMapOf()
     }
 
+    override val knownVariables: Set<RealVariable>
+        get() = approvedTypeStatements.keys + directAliasMap.keys
+
     override fun unwrapVariable(variable: RealVariable): RealVariable =
         directAliasMap[variable] ?: variable
 
@@ -126,8 +129,7 @@ abstract class PersistentLogicSystem(context: ConeInferenceContext) : LogicSyste
         }
 
         val approvedTypeStatements = result.approvedTypeStatements.builder()
-        val allVariablesWithStatements = flows.flatMapTo(mutableSetOf()) { it.approvedTypeStatements.keys + it.directAliasMap.keys }
-        allVariablesWithStatements.forEach computeStatement@{ variable ->
+        flows.flatMapTo(mutableSetOf()) { it.knownVariables }.forEach computeStatement@{ variable ->
             val statement = if (variable in result.directAliasMap) {
                 return@computeStatement // statements about alias == statements about aliased variable
             } else if (!allExecute) {
@@ -321,6 +323,9 @@ abstract class PersistentLogicSystem(context: ConeInferenceContext) : LogicSyste
         removeAllAboutVariable(flow, variable)
         flow.assignmentIndex = flow.assignmentIndex.put(variable, index)
     }
+
+    override fun isSameValueIn(a: PersistentFlow, b: PersistentFlow, variable: RealVariable): Boolean =
+        a.assignmentIndex[variable] == b.assignmentIndex[variable]
 }
 
 private fun lowestCommonFlow(left: PersistentFlow, right: PersistentFlow): PersistentFlow {