[FIR] Fix exhaustiveness checking for conditions with equals to object

#KT-37488 Fixed

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

@@ -149,8 +149,16 @@ class FirWhenExhaustivenessTransformer(private val bodyResolveComponents: BodyRe
         override fun visitOperatorCall(operatorCall: FirOperatorCall, data: SealedExhaustivenessData) {
             if (operatorCall.operation == FirOperation.EQ) {
                 val argument = operatorCall.arguments[1]
-                if (argument is FirConstExpression<*> && argument.value == null) {
-                    data.containsNull = true
+                when (argument) {
+                    is FirConstExpression<*> -> {
+                        if (argument.value == null) {
+                            data.containsNull = true
+                        }
+                    }
+
+                    is FirResolvedQualifier -> {
+                        argument.typeRef.accept(this, data)
+                    }
                 }
             }
         }

compiler/fir/resolve/testData/resolve/exhaustiveness_sealedObject.kt

@@ -0,0 +1,24 @@
+// ISSUE: KT-37488
+
+sealed class A
+
+class B : A()
+object C : A()
+
+fun takeString(s: String) {}
+
+fun test_1(a: A) {
+    val s = when(a) {
+        is B -> ""
+        is C -> ""
+    }
+    takeString(s)
+}
+
+fun test_2(a: A) {
+    val s = when(a) {
+        is B -> ""
+        C -> ""
+    }
+    takeString(s)
+}

compiler/fir/resolve/testData/resolve/exhaustiveness_sealedObject.txt

@@ -0,0 +1,45 @@
+FILE: exhaustiveness_sealedObject.kt
+    public sealed class A : R|kotlin/Any| {
+        private constructor(): R|A| {
+            super<R|kotlin/Any|>()
+        }
+
+    }
+    public final class B : R|A| {
+        public constructor(): R|B| {
+            super<R|A|>()
+        }
+
+    }
+    public final object C : R|A| {
+        private constructor(): R|C| {
+            super<R|A|>()
+        }
+
+    }
+    public final fun takeString(s: R|kotlin/String|): R|kotlin/Unit| {
+    }
+    public final fun test_1(a: R|A|): R|kotlin/Unit| {
+        lval s: R|kotlin/String| = when (R|<local>/a|) {
+            ($subj$ is R|B|) ->  {
+                String()
+            }
+            ($subj$ is R|C|) ->  {
+                String()
+            }
+        }
+
+        R|/takeString|(R|<local>/s|)
+    }
+    public final fun test_2(a: R|A|): R|kotlin/Unit| {
+        lval s: R|kotlin/String| = when (R|<local>/a|) {
+            ($subj$ is R|B|) ->  {
+                String()
+            }
+            ==($subj$, Q|C|) ->  {
+                String()
+            }
+        }
+
+        R|/takeString|(R|<local>/s|)
+    }

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

@@ -138,6 +138,11 @@ public class FirDiagnosticsTestGenerated extends AbstractFirDiagnosticsTest {
         runTest("compiler/fir/resolve/testData/resolve/exhaustiveness_sealedClass.kt");
     }
 
+    @TestMetadata("exhaustiveness_sealedObject.kt")
+    public void testExhaustiveness_sealedObject() throws Exception {
+        runTest("compiler/fir/resolve/testData/resolve/exhaustiveness_sealedObject.kt");
+    }
+
     @TestMetadata("extension.kt")
     public void testExtension() throws Exception {
         runTest("compiler/fir/resolve/testData/resolve/extension.kt");

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

@@ -138,6 +138,11 @@ public class FirDiagnosticsWithLightTreeTestGenerated extends AbstractFirDiagnos
         runTest("compiler/fir/resolve/testData/resolve/exhaustiveness_sealedClass.kt");
     }
 
+    @TestMetadata("exhaustiveness_sealedObject.kt")
+    public void testExhaustiveness_sealedObject() throws Exception {
+        runTest("compiler/fir/resolve/testData/resolve/exhaustiveness_sealedObject.kt");
+    }
+
     @TestMetadata("extension.kt")
     public void testExtension() throws Exception {
         runTest("compiler/fir/resolve/testData/resolve/extension.kt");

compiler/testData/diagnostics/tests/sealed/ExhaustiveWhenOnNestedSealed.fir.kt

@@ -1,20 +0,0 @@
-sealed class Sealed {
-    object First: Sealed()
-    sealed class NonFirst {
-        object Second: NonFirst()
-        object Third: NonFirst()
-        object Fourth: Sealed()
-    }    
-}
-
-fun foo(s: Sealed, nf: Sealed.NonFirst): Int {
-    val si = when(s) {
-        Sealed.First -> 1
-        Sealed.NonFirst.Fourth -> 4
-    }
-    val nfi = when(nf) {
-        Sealed.NonFirst.Second -> 2
-        Sealed.NonFirst.Third -> 3
-    }
-    return si <!UNRESOLVED_REFERENCE!>+<!> nfi
-}

compiler/testData/diagnostics/tests/sealed/ExhaustiveWhenOnNestedSealed.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 sealed class Sealed {
     object First: Sealed()
     sealed class NonFirst {