K2: Fix false-positive RETURN_TYPE_MISMATCH_ON_OVERRIDE

The reason was that `substitutorByMap` ignored the difference between
`T` and `T?`

^KT-57001 Fixed

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java

@@ -12564,6 +12564,12 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
                 runTest("compiler/testData/diagnostics/tests/generics/localClassTypeRefWithTypeParameterFromFunction.kt");
             }
 
+            @Test
+            @TestMetadata("nullableTypeParameterScope.kt")
+            public void testNullableTypeParameterScope() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/generics/nullableTypeParameterScope.kt");
+            }
+
             @Test
             @TestMetadata("outerTypeParametersInNestedClasses.kt")
             public void testOuterTypeParametersInNestedClasses() throws Exception {

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java

@@ -12564,6 +12564,12 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
                 runTest("compiler/testData/diagnostics/tests/generics/localClassTypeRefWithTypeParameterFromFunction.kt");
             }
 
+            @Test
+            @TestMetadata("nullableTypeParameterScope.kt")
+            public void testNullableTypeParameterScope() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/generics/nullableTypeParameterScope.kt");
+            }
+
             @Test
             @TestMetadata("outerTypeParametersInNestedClasses.kt")
             public void testOuterTypeParametersInNestedClasses() throws Exception {

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

@@ -12564,6 +12564,12 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
                 runTest("compiler/testData/diagnostics/tests/generics/localClassTypeRefWithTypeParameterFromFunction.kt");
             }
 
+            @Test
+            @TestMetadata("nullableTypeParameterScope.kt")
+            public void testNullableTypeParameterScope() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/generics/nullableTypeParameterScope.kt");
+            }
+
             @Test
             @TestMetadata("outerTypeParametersInNestedClasses.kt")
             public void testOuterTypeParametersInNestedClasses() throws Exception {

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

@@ -12570,6 +12570,12 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
                 runTest("compiler/testData/diagnostics/tests/generics/localClassTypeRefWithTypeParameterFromFunction.kt");
             }
 
+            @Test
+            @TestMetadata("nullableTypeParameterScope.kt")
+            public void testNullableTypeParameterScope() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/generics/nullableTypeParameterScope.kt");
+            }
+
             @Test
             @TestMetadata("outerTypeParametersInNestedClasses.kt")
             public void testOuterTypeParametersInNestedClasses() throws Exception {

compiler/fir/providers/src/org/jetbrains/kotlin/fir/resolve/substitution/Substitutors.kt

@@ -173,7 +173,7 @@ abstract class AbstractConeSubstitutor(protected val typeContext: ConeTypeContex
 fun substitutorByMap(substitution: Map<FirTypeParameterSymbol, ConeKotlinType>, useSiteSession: FirSession): ConeSubstitutor {
     // If all arguments match parameters, then substitutor isn't needed
     if (substitution.all { (parameterSymbol, argumentType) ->
-            (argumentType as? ConeTypeParameterType)?.lookupTag?.typeParameterSymbol == parameterSymbol
+            (argumentType as? ConeTypeParameterType)?.lookupTag?.typeParameterSymbol == parameterSymbol && !argumentType.isMarkedNullable
         }
     ) return ConeSubstitutor.Empty
     return ConeSubstitutorByMap(substitution, useSiteSession)

compiler/testData/diagnostics/tests/generics/nullableTypeParameterScope.kt

@@ -0,0 +1,11 @@
+// FIR_IDENTICAL
+// ISSUE: KT-57001
+
+interface ConverterFromString<T> {
+
+    fun ofS(s: String): T
+
+    fun nullable(nullText: String): ConverterFromString<T?> = object: ConverterFromString<T?> {
+        override fun ofS(s: String): T? = if (s == nullText) null else this@ConverterFromString.ofS(s)
+    }
+}

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java

@@ -12570,6 +12570,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                 runTest("compiler/testData/diagnostics/tests/generics/localClassTypeRefWithTypeParameterFromFunction.kt");
             }
 
+            @Test
+            @TestMetadata("nullableTypeParameterScope.kt")
+            public void testNullableTypeParameterScope() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/generics/nullableTypeParameterScope.kt");
+            }
+
             @Test
             @TestMetadata("outerTypeParametersInNestedClasses.kt")
             public void testOuterTypeParametersInNestedClasses() throws Exception {