[FE] Check visibilities of expected and actual property setters are compatible

^KT-30905 Fixed

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

@@ -127,6 +127,12 @@ public class FirOldFrontendMPPDiagnosticsWithLightTreeTestGenerated extends Abst
         runTest("compiler/testData/diagnostics/tests/multiplatform/smartcastOnMemberPropertyFromCommonClass.kt");
     }
 
+    @Test
+    @TestMetadata("varSetterVisibility.kt")
+    public void testVarSetterVisibility() throws Exception {
+        runTest("compiler/testData/diagnostics/tests/multiplatform/varSetterVisibility.kt");
+    }
+
     @Nested
     @TestMetadata("compiler/testData/diagnostics/tests/multiplatform/defaultArguments")
     @TestDataPath("$PROJECT_ROOT")

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

@@ -127,6 +127,12 @@ public class FirOldFrontendMPPDiagnosticsWithPsiTestGenerated extends AbstractFi
         runTest("compiler/testData/diagnostics/tests/multiplatform/smartcastOnMemberPropertyFromCommonClass.kt");
     }
 
+    @Test
+    @TestMetadata("varSetterVisibility.kt")
+    public void testVarSetterVisibility() throws Exception {
+        runTest("compiler/testData/diagnostics/tests/multiplatform/varSetterVisibility.kt");
+    }
+
     @Nested
     @TestMetadata("compiler/testData/diagnostics/tests/multiplatform/defaultArguments")
     @TestDataPath("$PROJECT_ROOT")

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/mpp/FirExpectActualResolver.kt

@@ -523,10 +523,23 @@ object FirExpectActualResolver {
             !equalBy(expected, actual) { p -> p.isVar } -> ExpectActualCompatibility.Incompatible.PropertyKind
             !equalBy(expected, actual) { p -> p.isLateInit } -> ExpectActualCompatibility.Incompatible.PropertyLateinitModifier
             expected.isConst && !actual.isConst -> ExpectActualCompatibility.Incompatible.PropertyConstModifier
+            !arePropertySettersWithCompatibleVisibilities(expected, actual) -> ExpectActualCompatibility.Incompatible.PropertySetterVisibility
             else -> ExpectActualCompatibility.Compatible
         }
     }
 
+    private fun arePropertySettersWithCompatibleVisibilities(
+        expected: FirPropertySymbol,
+        actual: FirPropertySymbol,
+    ): Boolean {
+        val expectedSetterStatus = expected.setterSymbol?.resolvedStatus
+        val actualSetterStatus = actual.setterSymbol?.resolvedStatus
+        if (expectedSetterStatus == null || actualSetterStatus == null) {
+            return true
+        }
+        return areDeclarationsWithCompatibleVisibilities(expectedSetterStatus, actualSetterStatus)
+    }
+
     // ---------------------------------------- Utils ----------------------------------------
 
     private fun List<FirValueParameterSymbol>.toTypeList(substitutor: ConeSubstitutor): List<ConeKotlinType> {