FIR: add test for KT-50969

compiler/fir/analysis-tests/testData/resolveWithStdlib/problems/immutableName.fir.txt

@@ -0,0 +1,28 @@
+FILE: Impl.kt
+    public abstract class Impl : R|Base| {
+        public constructor(): R|Impl| {
+            super<R|kotlin/Any|>()
+        }
+
+        public open override fun foo(name: R|kotlin/String|): R|Base| {
+            ^foo this@R|/Impl|
+        }
+
+    }
+FILE: test.kt
+    public abstract class ImplDerived : R|Impl|, R|Derived| {
+        public constructor(): R|ImplDerived| {
+            super<R|Impl|>()
+        }
+
+        public abstract override fun foo(name: R|kotlin/String?|): R|Derived|
+
+    }
+    public abstract class DerivedImpl : R|Derived|, R|Impl| {
+        public constructor(): R|DerivedImpl| {
+            super<R|Impl|>()
+        }
+
+        public abstract override fun foo(name: R|kotlin/String?|): R|Derived|
+
+    }

compiler/fir/analysis-tests/testData/resolveWithStdlib/problems/immutableName.kt

@@ -0,0 +1,36 @@
+// SCOPE_DUMP: ImplDerived:foo, DerivedImpl:foo
+// FILE: Base.java
+
+import org.jetbrains.annotations.NotNull;
+
+public interface Base {
+    Base foo(@NotNull String name);
+}
+
+// FILE: Derived.java
+
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+public interface Derived extends Base {
+    @Override
+    @NotNull Derived foo(@Nullable String name);
+}
+
+// FILE: Impl.kt
+
+abstract class Impl : Base {
+    override fun foo(name: String): Base {
+        return this
+    }
+}
+
+// FILE: test.kt
+
+abstract <!RETURN_TYPE_MISMATCH_ON_INHERITANCE!>class ImplDerived<!> : Impl(), Derived {
+    abstract <!NOTHING_TO_OVERRIDE!>override<!> fun foo(name: String?): Derived
+}
+
+abstract class DerivedImpl : Derived, Impl() {
+    abstract override fun foo(name: String?): Derived
+}

compiler/fir/analysis-tests/testData/resolveWithStdlib/problems/immutableName.overrides.txt

@@ -0,0 +1,14 @@
+ImplDerived:
+  [Source]: public abstract override fun foo(name: R|kotlin/String?|): R|Derived| from Use site scope of /ImplDerived [id: 0]
+  [IntersectionOverride]: public open fun foo(name: R|ft<kotlin/String, kotlin/String?>|): R|@EnhancedNullability Derived| from Use site scope of /ImplDerived [id: 0]
+    [Source]: public open override fun foo(name: R|kotlin/String|): R|Base| from Use site scope of /Impl [id: 1]
+      [Enhancement]: public abstract fun foo(name: R|@EnhancedNullability kotlin/String|): R|ft<Base, Base?>| from Java enhancement scope for /Base [id: 2]
+    [Enhancement]: public abstract fun foo(name: R|ft<kotlin/String, kotlin/String?>|): R|@EnhancedNullability Derived| from Java enhancement scope for /Derived [id: 3]
+      [Enhancement]: public abstract fun foo(name: R|@EnhancedNullability kotlin/String|): R|ft<Base, Base?>| from Java enhancement scope for /Base [id: 2]
+
+DerivedImpl:
+  [Source]: public abstract override fun foo(name: R|kotlin/String?|): R|Derived| from Use site scope of /DerivedImpl [id: 0]
+    [Enhancement]: public abstract fun foo(name: R|ft<kotlin/String, kotlin/String?>|): R|@EnhancedNullability Derived| from Java enhancement scope for /Derived [id: 1]
+      [Enhancement]: public abstract fun foo(name: R|@EnhancedNullability kotlin/String|): R|ft<Base, Base?>| from Java enhancement scope for /Base [id: 2]
+    [Source]: public open override fun foo(name: R|kotlin/String|): R|Base| from Use site scope of /Impl [id: 3]
+      [Enhancement]: public abstract fun foo(name: R|@EnhancedNullability kotlin/String|): R|ft<Base, Base?>| from Java enhancement scope for /Base [id: 2]