[FIR] KT-57195: Don't approximate ft when checking override return types

Why does this check even exist? K1 doesn't do it.

^KT-57195 Fixed

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosisCompilerTestFE10TestdataTestGenerated.java

@@ -657,6 +657,18 @@ public class DiagnosisCompilerTestFE10TestdataTestGenerated extends AbstractDiag
             runTest("compiler/testData/diagnostics/tests/OverrideFunctionWithParamDefaultValue.kt");
         }
 
+        @Test
+        @TestMetadata("overrideNotNull_Fail.kt")
+        public void testOverrideNotNull_Fail() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/overrideNotNull_Fail.kt");
+        }
+
+        @Test
+        @TestMetadata("overrideNotNull_Ok.kt")
+        public void testOverrideNotNull_Ok() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/overrideNotNull_Ok.kt");
+        }
+
         @Test
         @TestMetadata("OverridenFunctionAndSpecifiedTypeParameter.kt")
         public void testOverridenFunctionAndSpecifiedTypeParameter() throws Exception {

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

@@ -657,6 +657,18 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
             runTest("compiler/testData/diagnostics/tests/OverrideFunctionWithParamDefaultValue.kt");
         }
 
+        @Test
+        @TestMetadata("overrideNotNull_Fail.kt")
+        public void testOverrideNotNull_Fail() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/overrideNotNull_Fail.kt");
+        }
+
+        @Test
+        @TestMetadata("overrideNotNull_Ok.kt")
+        public void testOverrideNotNull_Ok() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/overrideNotNull_Ok.kt");
+        }
+
         @Test
         @TestMetadata("OverridenFunctionAndSpecifiedTypeParameter.kt")
         public void testOverridenFunctionAndSpecifiedTypeParameter() throws Exception {

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

@@ -657,6 +657,18 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
             runTest("compiler/testData/diagnostics/tests/OverrideFunctionWithParamDefaultValue.kt");
         }
 
+        @Test
+        @TestMetadata("overrideNotNull_Fail.kt")
+        public void testOverrideNotNull_Fail() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/overrideNotNull_Fail.kt");
+        }
+
+        @Test
+        @TestMetadata("overrideNotNull_Ok.kt")
+        public void testOverrideNotNull_Ok() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/overrideNotNull_Ok.kt");
+        }
+
         @Test
         @TestMetadata("OverridenFunctionAndSpecifiedTypeParameter.kt")
         public void testOverridenFunctionAndSpecifiedTypeParameter() throws Exception {

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirOverrideChecker.kt

@@ -213,7 +213,7 @@ object FirOverrideChecker : FirClassChecker() {
             return null
         }
 
-        val bounds = overriddenSymbols.map { context.returnTypeCalculator.tryCalculateReturnType(it).coneType.upperBoundIfFlexible() }
+        val bounds = overriddenSymbols.map { context.returnTypeCalculator.tryCalculateReturnType(it).coneType }
 
         for (it in bounds.indices) {
             val overriddenDeclaration = overriddenSymbols[it]

compiler/testData/diagnostics/tests/overrideNotNull_Fail.kt

@@ -0,0 +1,12 @@
+// FIR_IDENTICAL
+// ISSUE: KT-57195
+
+import java.io.File
+
+open class AbstractFE1UastTest {
+    open var testDataDir = File("").parentFile
+}
+
+class Legacy: AbstractFE1UastTest() {
+    override var testDataDir: File = File("").parentFile // K1: ok, K2: VAR_TYPE_MISMATCH_ON_OVERRIDE
+}

compiler/testData/diagnostics/tests/overrideNotNull_Ok.kt

@@ -0,0 +1,12 @@
+// FIR_IDENTICAL
+// ISSUE: KT-57195
+
+import java.io.File
+
+open class AbstractFE1UastTest {
+    open var testDataDir = File("").parentFile
+}
+
+class Legacy: AbstractFE1UastTest() {
+    override var testDataDir: File? = File("").parentFile // K1 & K2: ok
+}

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

@@ -657,6 +657,18 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
             runTest("compiler/testData/diagnostics/tests/OverrideFunctionWithParamDefaultValue.kt");
         }
 
+        @Test
+        @TestMetadata("overrideNotNull_Fail.kt")
+        public void testOverrideNotNull_Fail() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/overrideNotNull_Fail.kt");
+        }
+
+        @Test
+        @TestMetadata("overrideNotNull_Ok.kt")
+        public void testOverrideNotNull_Ok() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/overrideNotNull_Ok.kt");
+        }
+
         @Test
         @TestMetadata("OverridenFunctionAndSpecifiedTypeParameter.kt")
         public void testOverridenFunctionAndSpecifiedTypeParameter() throws Exception {