Fix regression on smart casts in when on a sealed class

See the comment in PatternMatchingTypingVisitor

 #KT-27221 Fixed

compiler/frontend/src/org/jetbrains/kotlin/types/expressions/PatternMatchingTypingVisitor.kt

@@ -410,7 +410,13 @@ class PatternMatchingTypingVisitor internal constructor(facade: ExpressionTyping
         possibleTypesForSubject: Set<KotlinType>
     ) {
         val subjectExpression = expression.subjectExpression ?: return
-        for (possibleCastType in possibleTypesForSubject) {
+        // Using "reversed()" here is a kind of hack to fix KT-27221
+        // KT-27221 was a breaking change introduced after DataFlowImpl optimization that changed the order of possible types to reversed,
+        // and it led to the wrong sealed class being chosen.
+        // But it seems that order of collected types shouldn't matter at all
+        // (at least because the order of relevant checks might change the behavior, see KT-27252)
+        // TODO: Read the comment above, wait for resolution in KT-27252 and get rid of "reversed" call here
+        for (possibleCastType in possibleTypesForSubject.reversed()) {
             val possibleCastClass = possibleCastType.constructor.declarationDescriptor as? ClassDescriptor ?: continue
             if (possibleCastClass.kind == ClassKind.ENUM_CLASS || possibleCastClass.modality == Modality.SEALED) {
                 if (checkSmartCastToExpectedTypeInSubject(

compiler/testData/diagnostics/tests/smartCasts/kt27221.kt

@@ -0,0 +1,28 @@
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+// SKIP_TXT
+
+sealed class A
+sealed class B : A()
+sealed class C : B()
+object BB : B()
+object CC : C()
+
+fun foo(a: A) {
+    if (a is B) {
+        if (a is C) {
+            val t = when (<!DEBUG_INFO_SMARTCAST!>a<!>) {
+                is CC -> "CC"
+            }
+        }
+    }
+}
+
+fun foo2(a: A) {
+    if (a is C) {
+        if (<!USELESS_IS_CHECK!>a is B<!>) {
+            val t = when (<!DEBUG_INFO_SMARTCAST!>a<!>) {
+                is CC -> "CC"
+            }
+        }
+    }
+}

compiler/testData/diagnostics/tests/smartCasts/kt27221_2.kt

@@ -0,0 +1,36 @@
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+// SKIP_TXT
+
+sealed class A
+sealed class B : A()
+sealed class C : B()
+sealed class D : C()
+object BB : B()
+object CC : C()
+object DD : D()
+
+fun foo1(a: A) {
+    if (a is B) {
+        if (a is D) {
+            if (<!USELESS_IS_CHECK!>a is C<!>) {
+                val t =
+                    when (<!DEBUG_INFO_SMARTCAST!>a<!>) {
+                        is DD -> "DD"
+                    }
+            }
+        }
+    }
+}
+
+fun foo2(a: A) {
+    if (a is B) {
+        if (a is D) {
+            if (<!USELESS_IS_CHECK!>a is C<!>) {
+                val t =
+                    when (<!DEBUG_INFO_SMARTCAST!>a<!>) {
+                        is DD -> "DD"
+                    }
+            }
+        }
+    }
+}

compiler/testData/diagnostics/tests/smartCasts/kt27221_irrelevantClasses.kt

@@ -0,0 +1,28 @@
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+// SKIP_TXT
+
+sealed class A
+sealed class B : A()
+sealed class C : A()
+object BB : B()
+object CC : C()
+
+fun foo(a: A) {
+    if (a is B) {
+        if (a is C) {
+            val t = when (<!DEBUG_INFO_SMARTCAST!>a<!>) {
+                is CC -> "CC"
+            }
+        }
+    }
+}
+
+fun foo2(a: A) {
+    if (a is C) {
+        if (a is B) {
+            val t = <!NO_ELSE_IN_WHEN!>when<!> (<!DEBUG_INFO_SMARTCAST!>a<!>) {
+                    is CC -> "CC"
+            }
+        }
+    }
+}

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -19117,6 +19117,21 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 runTest("compiler/testData/diagnostics/tests/smartCasts/kt2422.kt");
             }
 
+            @TestMetadata("kt27221.kt")
+            public void testKt27221() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/kt27221.kt");
+            }
+
+            @TestMetadata("kt27221_2.kt")
+            public void testKt27221_2() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/kt27221_2.kt");
+            }
+
+            @TestMetadata("kt27221_irrelevantClasses.kt")
+            public void testKt27221_irrelevantClasses() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/kt27221_irrelevantClasses.kt");
+            }
+
             @TestMetadata("kt2865.kt")
             public void testKt2865() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/smartCasts/kt2865.kt");

compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java

@@ -19117,6 +19117,21 @@ public class DiagnosticsUsingJavacTestGenerated extends AbstractDiagnosticsUsing
                 runTest("compiler/testData/diagnostics/tests/smartCasts/kt2422.kt");
             }
 
+            @TestMetadata("kt27221.kt")
+            public void testKt27221() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/kt27221.kt");
+            }
+
+            @TestMetadata("kt27221_2.kt")
+            public void testKt27221_2() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/kt27221_2.kt");
+            }
+
+            @TestMetadata("kt27221_irrelevantClasses.kt")
+            public void testKt27221_irrelevantClasses() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/kt27221_irrelevantClasses.kt");
+            }
+
             @TestMetadata("kt2865.kt")
             public void testKt2865() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/smartCasts/kt2865.kt");