[FIR] Fix exhaustiveness check when subject has flexible type

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirWhenExhaustivenessTransformer.kt

@@ -23,6 +23,7 @@ import org.jetbrains.kotlin.fir.symbols.impl.FirVariableSymbol
 import org.jetbrains.kotlin.fir.types.ConeLookupTagBasedType
 import org.jetbrains.kotlin.fir.types.ConeNullability
 import org.jetbrains.kotlin.fir.types.FirResolvedTypeRef
+import org.jetbrains.kotlin.fir.types.lowerBoundIfFlexible
 import org.jetbrains.kotlin.fir.visitors.*
 import org.jetbrains.kotlin.name.ClassId
 
@@ -46,7 +47,9 @@ class FirWhenExhaustivenessTransformer(private val bodyResolveComponents: BodyRe
             ?: (whenExpression.subject as? FirQualifiedAccessExpression)?.typeRef) as? FirResolvedTypeRef
             ?: return null
 
-        val lookupTag = (typeRef.type as? ConeLookupTagBasedType)?.lookupTag ?: return null
+        // TODO: add some report logic about flexible type (see WHEN_ENUM_CAN_BE_NULL_IN_JAVA diagnostic in old frontend)
+        val type = typeRef.type.lowerBoundIfFlexible()
+        val lookupTag = (type as? ConeLookupTagBasedType)?.lookupTag ?: return null
         val nullable = typeRef.type.nullability == ConeNullability.NULLABLE
         val isExhaustive = when {
             ((lookupTag as? ConeClassLikeLookupTag)?.classId == bodyResolveComponents.session.builtinTypes.booleanType.id) -> {

compiler/fir/resolve/testData/resolve/exhaustiveWhenAndFlexibleType.kt

@@ -1,3 +1,5 @@
+// IGNORE_LIGHT_TREE
+
 // FILE: Utils.java
 
 public class Utils {
@@ -12,12 +14,23 @@ enum class E {
     A, B, C
 }
 
-fun test() {
+fun test_1() {
     val e = Utils.getEnum()
     val s = when (e) {
-        E.A, null -> return
+        null -> return
+        E.A -> ""
         E.B -> ""
         E.C -> ""
     }
-    s.<!UNRESOLVED_REFERENCE!>length<!>
+    s.length
+}
+
+fun test_2() {
+    val e = Utils.getEnum()
+    val s = when (e) {
+        E.A -> ""
+        E.B -> ""
+        E.C -> ""
+    }
+    s.length
 }

compiler/fir/resolve/testData/resolve/exhaustiveWhenAndFlexibleType.txt

@@ -32,11 +32,14 @@ FILE: main.kt
         }
 
     }
-    public final fun test(): R|kotlin/Unit| {
+    public final fun test_1(): R|kotlin/Unit| {
         lval e: R|ft<E, E?>!| = Q|Utils|.R|/Utils.getEnum|()
-        lval s: R|kotlin/Unit| = when (R|<local>/e|) {
+        lval s: R|kotlin/String| = when (R|<local>/e|) {
-            ==($subj$, Q|E|.R|/E.A|) || ==($subj$, Null(null)) ->  {
+            ==($subj$, Null(null)) ->  {
-                ^test Unit
+                ^test_1 Unit
+            }
+            ==($subj$, Q|E|.R|/E.A|) ->  {
+                String()
             }
             ==($subj$, Q|E|.R|/E.B|) ->  {
                 String()
@@ -46,5 +49,21 @@ FILE: main.kt
             }
         }
 
-        R|<local>/s|.<Unresolved name: length>#
+        R|<local>/s|.R|kotlin/String.length|
+    }
+    public final fun test_2(): R|kotlin/Unit| {
+        lval e: R|ft<E, E?>!| = Q|Utils|.R|/Utils.getEnum|()
+        lval s: R|kotlin/String| = when (R|<local>/e|) {
+            ==($subj$, Q|E|.R|/E.A|) ->  {
+                String()
+            }
+            ==($subj$, Q|E|.R|/E.B|) ->  {
+                String()
+            }
+            ==($subj$, Q|E|.R|/E.C|) ->  {
+                String()
+            }
+        }
+
+        R|<local>/s|.R|kotlin/String.length|
     }

compiler/fir/resolve/tests/org/jetbrains/kotlin/fir/FirDiagnosticsTestGenerated.java

@@ -78,6 +78,11 @@ public class FirDiagnosticsTestGenerated extends AbstractFirDiagnosticsTest {
         runTest("compiler/fir/resolve/testData/resolve/enumWithCompanion.kt");
     }
 
+    @TestMetadata("exhaustiveWhenAndFlexibleType.kt")
+    public void testExhaustiveWhenAndFlexibleType() throws Exception {
+        runTest("compiler/fir/resolve/testData/resolve/exhaustiveWhenAndFlexibleType.kt");
+    }
+
     @TestMetadata("exhaustiveness_boolean.kt")
     public void testExhaustiveness_boolean() throws Exception {
         runTest("compiler/fir/resolve/testData/resolve/exhaustiveness_boolean.kt");
@@ -1036,11 +1041,6 @@ public class FirDiagnosticsTestGenerated extends AbstractFirDiagnosticsTest {
             runTest("compiler/fir/resolve/testData/resolve/problems/definitelyNotNullAmbiguity.kt");
         }
 
-        @TestMetadata("exhaustiveWhenAndFlexibleType.kt")
-        public void testExhaustiveWhenAndFlexibleType() throws Exception {
-            runTest("compiler/fir/resolve/testData/resolve/problems/exhaustiveWhenAndFlexibleType.kt");
-        }
-
         @TestMetadata("incorrectSuperCall.kt")
         public void testIncorrectSuperCall() throws Exception {
             runTest("compiler/fir/resolve/testData/resolve/problems/incorrectSuperCall.kt");

compiler/fir/resolve/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithLightTreeTestGenerated.java

@@ -78,6 +78,11 @@ public class FirDiagnosticsWithLightTreeTestGenerated extends AbstractFirDiagnos
         runTest("compiler/fir/resolve/testData/resolve/enumWithCompanion.kt");
     }
 
+    @TestMetadata("exhaustiveWhenAndFlexibleType.kt")
+    public void testExhaustiveWhenAndFlexibleType() throws Exception {
+        runTest("compiler/fir/resolve/testData/resolve/exhaustiveWhenAndFlexibleType.kt");
+    }
+
     @TestMetadata("exhaustiveness_boolean.kt")
     public void testExhaustiveness_boolean() throws Exception {
         runTest("compiler/fir/resolve/testData/resolve/exhaustiveness_boolean.kt");
@@ -1036,11 +1041,6 @@ public class FirDiagnosticsWithLightTreeTestGenerated extends AbstractFirDiagnos
             runTest("compiler/fir/resolve/testData/resolve/problems/definitelyNotNullAmbiguity.kt");
         }
 
-        @TestMetadata("exhaustiveWhenAndFlexibleType.kt")
-        public void testExhaustiveWhenAndFlexibleType() throws Exception {
-            runTest("compiler/fir/resolve/testData/resolve/problems/exhaustiveWhenAndFlexibleType.kt");
-        }
-
         @TestMetadata("incorrectSuperCall.kt")
         public void testIncorrectSuperCall() throws Exception {
             runTest("compiler/fir/resolve/testData/resolve/problems/incorrectSuperCall.kt");