[KT-76629] FIR: Fix wrong implicit visibility on Sealed classes.

compiler/fir/analysis-tests/testData/resolve/extraCheckers/RedundantVisibilityInSealedConstructorOverride.kt

@@ -0,0 +1,43 @@
+// FIR_IDENTICAL
+// RUN_PIPELINE_TILL: FRONTEND
+// KT-76629
+
+sealed class A(val x: Int) {
+    <!REDUNDANT_VISIBILITY_MODIFIER!>protected<!> constructor(s: String) : this(s.length)
+
+    class A1 : A(100)
+    class A2 : A("String")
+}
+
+
+sealed class B <!REDUNDANT_VISIBILITY_MODIFIER!>protected<!> constructor(val x: Int) {
+    <!REDUNDANT_VISIBILITY_MODIFIER!>protected<!> constructor(s: String) : this(s.length)
+
+    class B1 : B(100)
+    class B2 : B("String")
+}
+
+sealed class C private constructor(val x: Int) {
+    <!REDUNDANT_VISIBILITY_MODIFIER!>protected<!> constructor(s: String) : this(s.length)
+
+    class C1 : C(100)
+    class C2 : C("String")
+}
+
+sealed class C3 : <!INVISIBLE_REFERENCE!>C<!>(200)
+
+sealed class D {
+    <!REDUNDANT_VISIBILITY_MODIFIER!>protected<!> constructor(x: Int)
+    <!REDUNDANT_VISIBILITY_MODIFIER!>protected<!> constructor(s: String) : this(s.length)
+
+    class D1 : D(100)
+    class D2 : D("String")
+}
+
+sealed class E() {
+    private constructor(x: Int) : this()
+    <!REDUNDANT_VISIBILITY_MODIFIER!>protected<!> constructor(x: Byte) : this()
+    <!NON_PRIVATE_OR_PROTECTED_CONSTRUCTOR_IN_SEALED!>internal constructor(x: Short) : this()<!>
+    <!NON_PRIVATE_OR_PROTECTED_CONSTRUCTOR_IN_SEALED!>public constructor(x: Long) : this()<!>
+    constructor(x: Double) : this()
+}

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/extra/RedundantVisibilityModifierSyntaxChecker.kt

@@ -210,13 +210,11 @@ object RedundantVisibilityModifierSyntaxChecker : FirDeclarationSyntaxChecker<Fi
 
             this is FirConstructor -> {
                 val classSymbol = this.getContainingClassSymbol()
-                if (
-                    classSymbol is FirRegularClassSymbol
-                    && (classSymbol.isEnumClass || classSymbol.isSealed)
-                ) {
-                    Visibilities.Private
-                } else {
-                    defaultVisibility
+                when {
+                    classSymbol !is FirRegularClassSymbol -> defaultVisibility
+                    classSymbol.isEnumClass -> Visibilities.Private
+                    classSymbol.isSealed -> Visibilities.Protected
+                    else -> defaultVisibility
                 }
             }