K2: Put boolean DFA variables under a feature flag

^KT-62547 Fixed

I also tested the commit by disabling DfaBooleanVariables in all tests
and checking what tests will fail

I didn't find anything that worked in K1, but doesn't work in K2 with
DfaBooleanVariables disabled

Related tests:
- DiagnosticCompilerTestFirTestdataTestGenerated$Resolve$Smartcasts
- LLFirPreresolvedReversedDiagnosticCompilerFirTestDataTestGenerated$Resolve$Smartcasts
- PreFirIdeSpecTestGenerated$NotLinked$Dfa
- DiagnosticCompilerTestFE10TestdataTestGenerated$Tests$SmartCasts
- FirPsiDiagnosticTestGenerated$Resolve$Expresssions.testSyntheticSmartCast

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java

@@ -34962,6 +34962,12 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
                 runTest("compiler/testData/diagnostics/tests/smartCasts/dataFlowInfoForArguments.kt");
             }
 
+            @Test
+            @TestMetadata("disableDfaBooleanVariables.kt")
+            public void testDisableDfaBooleanVariables() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/disableDfaBooleanVariables.kt");
+            }
+
             @Test
             @TestMetadata("doubleLambdaArgument.kt")
             public void testDoubleLambdaArgument() throws Exception {

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java

@@ -34962,6 +34962,12 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
                 runTest("compiler/testData/diagnostics/tests/smartCasts/dataFlowInfoForArguments.kt");
             }
 
+            @Test
+            @TestMetadata("disableDfaBooleanVariables.kt")
+            public void testDisableDfaBooleanVariables() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/disableDfaBooleanVariables.kt");
+            }
+
             @Test
             @TestMetadata("doubleLambdaArgument.kt")
             public void testDoubleLambdaArgument() throws Exception {

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirLightTreeOldFrontendDiagnosticsTestGenerated.java

@@ -32606,6 +32606,12 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
                 runTest("compiler/testData/diagnostics/tests/smartCasts/dataFlowInfoForArguments.kt");
             }
 
+            @Test
+            @TestMetadata("disableDfaBooleanVariables.kt")
+            public void testDisableDfaBooleanVariables() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/disableDfaBooleanVariables.kt");
+            }
+
             @Test
             @TestMetadata("doubleLambdaArgument.kt")
             public void testDoubleLambdaArgument() throws Exception {

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirPsiOldFrontendDiagnosticsTestGenerated.java

@@ -32732,6 +32732,12 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
                 runTest("compiler/testData/diagnostics/tests/smartCasts/dataFlowInfoForArguments.kt");
             }
 
+            @Test
+            @TestMetadata("disableDfaBooleanVariables.kt")
+            public void testDisableDfaBooleanVariables() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/disableDfaBooleanVariables.kt");
+            }
+
             @Test
             @TestMetadata("doubleLambdaArgument.kt")
             public void testDoubleLambdaArgument() throws Exception {

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

@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.fir.resolve.dfa
 
 import kotlinx.collections.immutable.toPersistentSet
+import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.contracts.description.LogicOperationKind
 import org.jetbrains.kotlin.contracts.description.canBeRevisited
 import org.jetbrains.kotlin.descriptors.Modality
@@ -1086,9 +1087,15 @@ abstract class FirDataFlowAnalyzer(
                 // if (b != null) { /* a != null, but a.x could have changed */ }
                 logicSystem.translateVariableFromConditionInStatements(flow, initializerVariable, propertyVariable)
             }
-        } else if (initializerVariable != null && propertyVariable.isStable) {
+        } else if (initializerVariable != null && propertyVariable.isStable &&
+            (components.session.languageVersionSettings.supportsFeature(LanguageFeature.DfaBooleanVariables) ||
+                    !initializer.resolvedType.isBoolean)
+        ) {
             // val b = x is String
             // if (b) { /* x is String */ }
+
+            // val b = x?.foo() // `x?.foo()` is synthetic
+            // if (b != null) { /* x != null */ }
             logicSystem.translateVariableFromConditionInStatements(flow, initializerVariable, propertyVariable)
         }
 

compiler/testData/diagnostics/tests/smartCasts/disableDfaBooleanVariables.kt

@@ -0,0 +1,131 @@
+// FIR_IDENTICAL
+// DIAGNOSTICS: -DEBUG_INFO_SMARTCAST
+// LANGUAGE:-DfaBooleanVariables
+
+import kotlin.contracts.ExperimentalContracts
+import kotlin.contracts.contract
+
+fun test1() {
+    val nullableString: String? = ""
+    val test = nullableString != null
+    if (test) nullableString<!UNSAFE_CALL!>.<!>length
+    if (nullableString != null) nullableString.length
+}
+
+class A {
+    val a: String? = ""
+}
+fun test3(a: A) {
+    val test = a.a != null
+    if (test) a.a<!UNSAFE_CALL!>.<!>length
+    if (a.a != null) {
+        a.a.length
+    }
+}
+
+fun test4() {
+    val nullableAny: Any? = ""
+    val test = nullableAny != null && nullableAny is String?
+    if (test) nullableAny.<!UNRESOLVED_REFERENCE!>length<!>
+    if (nullableAny != null && nullableAny is String?) nullableAny.length
+}
+
+fun test7(bar: Any) {
+    val test = bar is String
+    if (test) bar.<!UNRESOLVED_REFERENCE!>length<!>
+    if (bar is String) bar.length
+}
+
+fun test8(bar: Any) {
+    val test = bar is String
+    when {
+        test -> bar.<!UNRESOLVED_REFERENCE!>length<!>
+    }
+    when {
+        bar is String -> bar.length
+    }
+}
+
+fun test9(bar: Any) {
+    bar is String && bar.length == 0
+    val test = bar is String
+    test && bar.<!UNRESOLVED_REFERENCE!>length<!>
+}
+
+fun test10(bar: Any) {
+    val test = bar !is String
+    run {
+        if (test) return@run
+        bar.<!UNRESOLVED_REFERENCE!>length<!>
+    }
+    run {
+        if (bar !is String) return@run
+        bar.length
+    }
+}
+
+fun test11(bar: Any, baz: Any) {
+    val test = baz is String && baz === bar
+    val test2 = baz is String
+    val test3 = baz === bar
+    val test4 = test2 && test3
+
+    if (test) bar.<!UNRESOLVED_REFERENCE!>length<!>
+    if (test2 && test3) bar.<!UNRESOLVED_REFERENCE!>length<!>
+    if (baz is String && test3) bar.<!UNRESOLVED_REFERENCE!>length<!>
+    if (test2 && baz === bar) bar.<!UNRESOLVED_REFERENCE!>length<!>
+    if (test4) bar.<!UNRESOLVED_REFERENCE!>length<!>
+
+    if (baz is String && baz === bar) bar.length
+}
+
+fun test12(bar: Any) {
+    val test = bar !is String
+    if (!test) bar.<!UNRESOLVED_REFERENCE!>length<!>
+}
+
+fun test13(bar: Any) {
+    val test = bar is String
+    if (test == true) bar.<!UNRESOLVED_REFERENCE!>length<!>
+    when (test) {
+        true -> bar.<!UNRESOLVED_REFERENCE!>length<!>
+        false -> {}
+    }
+
+    val test2: Boolean? = bar is String
+    if (test2 == true) bar.<!UNRESOLVED_REFERENCE!>length<!>
+}
+
+fun test14(bar: Any) {
+    val test = (bar as? String) != null
+    if (test) bar.<!UNRESOLVED_REFERENCE!>length<!>
+}
+
+fun test15(bar: Any) {
+    val test = bar is String
+    val test2 = test
+    if (test2) bar.<!UNRESOLVED_REFERENCE!>length<!>
+}
+
+fun test16(bar: Any) {
+    val x = bar.isString()
+    if (x) bar.<!UNRESOLVED_REFERENCE!>length<!>
+    if (bar.isString()) bar.length
+}
+
+@OptIn(ExperimentalContracts::class)
+fun Any.isString(): Boolean {
+    contract {
+        returns(true) implies (this@isString is String)
+    }
+    return this is String
+}
+
+fun String.isEmpty(): Boolean = length == 0
+fun test17(a: String?) {
+    val x = a?.length
+    if (x != null) a.length
+
+    val y = a?.isEmpty()
+    if (y == true) a.length
+}

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java

@@ -34962,6 +34962,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                 runTest("compiler/testData/diagnostics/tests/smartCasts/dataFlowInfoForArguments.kt");
             }
 
+            @Test
+            @TestMetadata("disableDfaBooleanVariables.kt")
+            public void testDisableDfaBooleanVariables() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/disableDfaBooleanVariables.kt");
+            }
+
             @Test
             @TestMetadata("doubleLambdaArgument.kt")
             public void testDoubleLambdaArgument() throws Exception {

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -308,6 +308,7 @@ enum class LanguageFeature(
     ForbidInferringTypeVariablesIntoEmptyIntersection(KOTLIN_2_0, kind = BUG_FIX), // KT-51221
     ProhibitDefaultArgumentsInExpectActualizedByFakeOverride(KOTLIN_2_0, kind = BUG_FIX), // KT-62036
     DisableCompatibilityModeForNewInference(KOTLIN_2_0, kind = OTHER), // KT-63558 (umbrella), KT-64306, KT-64307, KT-64308
+    DfaBooleanVariables(KOTLIN_2_0), // KT-25747
 
     // 2.1