diff --git a/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java b/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java
index 7f4c37e509c..ac1904811a0 100644
--- a/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java
+++ b/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java
@@ -13921,6 +13921,12 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
             public void testSuspendFunInterfaceConversion() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/funInterface/suspendFunInterfaceConversion.kt");
             }
+
+            @Test
+            @TestMetadata("unitInContravariantPosition.kt")
+            public void testUnitInContravariantPosition() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/funInterface/unitInContravariantPosition.kt");
+            }
         }
 
         @Nested
diff --git a/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java b/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java
index 27c46aa40fd..d90bfbf5363 100644
--- a/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java
+++ b/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java
@@ -13921,6 +13921,12 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
             public void testSuspendFunInterfaceConversion() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/funInterface/suspendFunInterfaceConversion.kt");
             }
+
+            @Test
+            @TestMetadata("unitInContravariantPosition.kt")
+            public void testUnitInContravariantPosition() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/funInterface/unitInContravariantPosition.kt");
+            }
         }
 
         @Nested
diff --git a/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirLightTreeOldFrontendDiagnosticsTestGenerated.java b/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirLightTreeOldFrontendDiagnosticsTestGenerated.java
index 8befaff6917..f1748246244 100644
--- a/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirLightTreeOldFrontendDiagnosticsTestGenerated.java
+++ b/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirLightTreeOldFrontendDiagnosticsTestGenerated.java
@@ -13915,6 +13915,12 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
             public void testSuspendFunInterfaceConversion() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/funInterface/suspendFunInterfaceConversion.kt");
             }
+
+            @Test
+            @TestMetadata("unitInContravariantPosition.kt")
+            public void testUnitInContravariantPosition() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/funInterface/unitInContravariantPosition.kt");
+            }
         }
 
         @Nested
diff --git a/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirPsiOldFrontendDiagnosticsTestGenerated.java b/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirPsiOldFrontendDiagnosticsTestGenerated.java
index c37c3cd9169..3ee12df7cef 100644
--- a/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirPsiOldFrontendDiagnosticsTestGenerated.java
+++ b/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirPsiOldFrontendDiagnosticsTestGenerated.java
@@ -13921,6 +13921,12 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
             public void testSuspendFunInterfaceConversion() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/funInterface/suspendFunInterfaceConversion.kt");
             }
+
+            @Test
+            @TestMetadata("unitInContravariantPosition.kt")
+            public void testUnitInContravariantPosition() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/funInterface/unitInContravariantPosition.kt");
+            }
         }
 
         @Nested
diff --git a/compiler/testData/diagnostics/tests/funInterface/unitInContravariantPosition.fir.kt b/compiler/testData/diagnostics/tests/funInterface/unitInContravariantPosition.fir.kt
new file mode 100644
index 00000000000..985d190f726
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/funInterface/unitInContravariantPosition.fir.kt
@@ -0,0 +1,17 @@
+// ISSUE: KT-63379
+
+class TestDummyClass<T> {
+    fun testFun(t: T) {}
+}
+
+fun more(t: TestDummyClass<in CustomRunnable>) {
+    t.testFun <!ARGUMENT_TYPE_MISMATCH!>{ Unit }<!>
+    t.testFun(CustomRunnable { Unit })
+    t.testFun(object : CustomRunnable {
+        override fun run() {}
+    })
+}
+
+fun interface CustomRunnable {
+    fun run()
+}
diff --git a/compiler/testData/diagnostics/tests/funInterface/unitInContravariantPosition.kt b/compiler/testData/diagnostics/tests/funInterface/unitInContravariantPosition.kt
new file mode 100644
index 00000000000..a09f01e73f8
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/funInterface/unitInContravariantPosition.kt
@@ -0,0 +1,17 @@
+// ISSUE: KT-63379
+
+class TestDummyClass<T> {
+    fun testFun(t: T) {}
+}
+
+fun more(t: TestDummyClass<in CustomRunnable>) {
+    t.testFun { Unit }
+    t.testFun(CustomRunnable { Unit })
+    t.testFun(object : CustomRunnable {
+        override fun run() {}
+    })
+}
+
+fun interface CustomRunnable {
+    fun run()
+}
diff --git a/compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java b/compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java
index 689eee4ed9f..e2704383dd9 100644
--- a/compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java
+++ b/compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java
@@ -13921,6 +13921,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
             public void testSuspendFunInterfaceConversion() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/funInterface/suspendFunInterfaceConversion.kt");
             }
+
+            @Test
+            @TestMetadata("unitInContravariantPosition.kt")
+            public void testUnitInContravariantPosition() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/funInterface/unitInContravariantPosition.kt");
+            }
         }
 
         @Nested