[FIR] Unconditionally enable BooleanElvisBoundSmartCasts feature in K2

^KT-54694 Fixed
^KT-26357

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

@@ -5,7 +5,6 @@
 
 package org.jetbrains.kotlin.fir.resolve.dfa
 
-import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.contracts.description.EventOccurrencesRange
 import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.fir.*
@@ -1179,11 +1178,7 @@ abstract class FirDataFlowAnalyzer<FLOW : Flow>(
         val lhsVariable = variableStorage.getOrCreateIfReal(flow, elvisExpression.lhs) ?: return
         if (isLhsNotNull) {
             flow.commitOperationStatement(lhsVariable notEq null)
-        } else if (
-            components.session.languageVersionSettings.supportsFeature(LanguageFeature.BooleanElvisBoundSmartCasts) &&
-            rhs is FirConstExpression<*> &&
-            rhs.kind == ConstantValueKind.Boolean
-        ) {
+        } else if (rhs is FirConstExpression<*> && rhs.kind == ConstantValueKind.Boolean) {
             // (a ?: x) != x -> a != null. The logic system can only handle cases where x is Boolean.
             // Or null...but nobody would write that, right?
             val elvisVariable = variableStorage.createSynthetic(elvisExpression)

compiler/testData/diagnostics/tests/smartCasts/elvis/basicOff.fir.kt

@@ -0,0 +1,50 @@
+// !LANGUAGE: -BooleanElvisBoundSmartCasts
+
+interface Order {
+    val expired: Boolean?
+
+    fun notExpired(): Boolean
+
+    fun doSomething()
+}
+
+fun foo(o: Any) {
+    val order = o as? Order
+    if (order?.expired ?: false) {
+        order.doSomething()
+    }
+    else {
+
+    }
+    if (order?.notExpired() ?: false) {
+        order.doSomething()
+    }
+}
+
+fun bar(o: Any) {
+    val order = o as? Order
+    if (order?.expired ?: true) {
+
+    }
+    else {
+        order<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.doSomething()
+    }
+    if (order?.notExpired() ?: true) {
+
+    }
+    else {
+        order<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.doSomething()
+    }
+}
+
+fun baz(o: Boolean?) {
+    if (o ?: false) {
+        o.hashCode()
+    }
+    if (o ?: true) {
+
+    }
+    else {
+        o.hashCode()
+    }
+}

compiler/testData/diagnostics/tests/smartCasts/elvis/basicOff.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 // !LANGUAGE: -BooleanElvisBoundSmartCasts
 
 interface Order {
@@ -48,4 +47,4 @@ fun baz(o: Boolean?) {
     else {
         o<!UNSAFE_CALL!>.<!>hashCode()
     }
-}
+}