[FIR] Unwrap definitely not null when matching overrides

#KT-41984 Fixed

compiler/fir/analysis-tests/testData/resolve/kt41984.kt

@@ -0,0 +1,31 @@
+// ISSUE: KT-41984
+
+// FILE: A.java
+
+import org.jetbrains.annotations.NotNull;
+
+public abstract class A<T, V> {
+    @NotNull
+    public abstract String take(@NotNull V value);
+
+    @NotNull
+    public abstract String takeInv(@NotNull Inv<@NotNull V> value);
+}
+
+// FILE: main.kt
+
+class Inv<T>
+
+open class B<V> : A<Any, V>() {
+    override fun take(value: V): String {
+        return ""
+    }
+
+    override fun takeInv(value: Inv<V>): String = ""
+}
+
+fun test_1(b: B<Int>, x: Int, inv: Inv<Int>) {
+    b.take(x)
+    b.<!INAPPLICABLE_CANDIDATE!>take<!>(null)
+    b.takeInv(inv)
+}

compiler/fir/analysis-tests/testData/resolve/kt41984.txt

@@ -0,0 +1,26 @@
+FILE: main.kt
+    public final class Inv<T> : R|kotlin/Any| {
+        public constructor<T>(): R|Inv<T>| {
+            super<R|kotlin/Any|>()
+        }
+
+    }
+    public open class B<V> : R|A<kotlin/Any, V>| {
+        public constructor<V>(): R|B<V>| {
+            super<R|A<kotlin/Any, V>|>()
+        }
+
+        public open override fun take(value: R|V|): R|kotlin/String| {
+            ^take String()
+        }
+
+        public open override fun takeInv(value: R|Inv<V>|): R|kotlin/String| {
+            ^takeInv String()
+        }
+
+    }
+    public final fun test_1(b: R|B<kotlin/Int>|, x: R|kotlin/Int|, inv: R|Inv<kotlin/Int>|): R|kotlin/Unit| {
+        R|<local>/b|.R|FakeOverride</B.take: R|kotlin/String|>|(R|<local>/x|)
+        R|<local>/b|.<Inapplicable(INAPPLICABLE): /B.take>#(Null(null))
+        R|<local>/b|.R|FakeOverride</B.takeInv: R|kotlin/String|>|(R|<local>/inv|)
+    }

compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsTestGenerated.java

@@ -243,6 +243,11 @@ public class FirDiagnosticsTestGenerated extends AbstractFirDiagnosticsTest {
         runTest("compiler/fir/analysis-tests/testData/resolve/javaStaticScopeInheritance.kt");
     }
 
+    @TestMetadata("kt41984.kt")
+    public void testKt41984() throws Exception {
+        runTest("compiler/fir/analysis-tests/testData/resolve/kt41984.kt");
+    }
+
     @TestMetadata("kt41990.kt")
     public void testKt41990() throws Exception {
         runTest("compiler/fir/analysis-tests/testData/resolve/kt41990.kt");

compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithLightTreeTestGenerated.java

@@ -243,6 +243,11 @@ public class FirDiagnosticsWithLightTreeTestGenerated extends AbstractFirDiagnos
         runTest("compiler/fir/analysis-tests/testData/resolve/javaStaticScopeInheritance.kt");
     }
 
+    @TestMetadata("kt41984.kt")
+    public void testKt41984() throws Exception {
+        runTest("compiler/fir/analysis-tests/testData/resolve/kt41984.kt");
+    }
+
     @TestMetadata("kt41990.kt")
     public void testKt41990() throws Exception {
         runTest("compiler/fir/analysis-tests/testData/resolve/kt41990.kt");

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/scopes/impl/FirStandardOverrideChecker.kt

@@ -41,11 +41,16 @@ class FirStandardOverrideChecker(session: FirSession) : FirAbstractOverrideCheck
     }
 
     private fun isEqualTypes(candidateType: ConeKotlinType, baseType: ConeKotlinType, substitutor: ConeSubstitutor): Boolean {
-        val substitutedCandidateType = substitutor.substituteOrSelf(candidateType)
-        val substitutedBaseType = substitutor.substituteOrSelf(baseType)
+        val substitutedCandidateType = substitutor.substituteOrSelf(candidateType).unwrapDefinitelyNotNullType()
+        val substitutedBaseType = substitutor.substituteOrSelf(baseType).unwrapDefinitelyNotNullType()
         return isEqualTypes(substitutedCandidateType, substitutedBaseType)
     }
 
+    private fun ConeKotlinType.unwrapDefinitelyNotNullType(): ConeKotlinType = when (this) {
+        is ConeDefinitelyNotNullType -> original
+        else -> this
+    }
+
     override fun isEqualTypes(candidateTypeRef: FirTypeRef, baseTypeRef: FirTypeRef, substitutor: ConeSubstitutor) =
         isEqualTypes(candidateTypeRef.coneType, baseTypeRef.coneType, substitutor)