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 30ca32112fa..9cc7cb22fb6 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
@@ -24036,6 +24036,12 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
                 runTest("compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguity.kt");
             }
 
+            @Test
+            @TestMetadata("AssignOperatorAmbiguityExtension.kt")
+            public void testAssignOperatorAmbiguityExtension() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguityExtension.kt");
+            }
+
             @Test
             @TestMetadata("AssignmentOperations.kt")
             public void testAssignmentOperations() throws Exception {
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 93e07689fa0..b326a94bc3f 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
@@ -24036,6 +24036,12 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
                 runTest("compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguity.kt");
             }
 
+            @Test
+            @TestMetadata("AssignOperatorAmbiguityExtension.kt")
+            public void testAssignOperatorAmbiguityExtension() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguityExtension.kt");
+            }
+
             @Test
             @TestMetadata("AssignmentOperations.kt")
             public void testAssignmentOperations() throws Exception {
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 6ab77af182f..3e0d16a2679 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
@@ -24036,6 +24036,12 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
                 runTest("compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguity.kt");
             }
 
+            @Test
+            @TestMetadata("AssignOperatorAmbiguityExtension.kt")
+            public void testAssignOperatorAmbiguityExtension() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguityExtension.kt");
+            }
+
             @Test
             @TestMetadata("AssignmentOperations.kt")
             public void testAssignmentOperations() throws Exception {
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 b9419cb86b0..84c9685afaa 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
@@ -24042,6 +24042,12 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
                 runTest("compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguity.kt");
             }
 
+            @Test
+            @TestMetadata("AssignOperatorAmbiguityExtension.kt")
+            public void testAssignOperatorAmbiguityExtension() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguityExtension.kt");
+            }
+
             @Test
             @TestMetadata("AssignmentOperations.kt")
             public void testAssignmentOperations() throws Exception {
diff --git a/compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguityExtension.kt b/compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguityExtension.kt
new file mode 100644
index 00000000000..6eae00734b5
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguityExtension.kt
@@ -0,0 +1,22 @@
+// FIR_IDENTICAL
+// ISSUE: KT-62138
+class HashMap<K, V>(
+    private val defaultValue: V
+) {
+    operator fun get(key: K): V = defaultValue
+    operator fun set(key: K, value: V) { }
+}
+
+private class X
+
+private operator fun X?.plus(p: Int) = X()
+private operator fun X?.plusAssign(p: Int) { }
+
+class C {
+    private val map = HashMap<String, X>(defaultValue = X())
+
+    fun f(): Any? {
+        map[""] <!ASSIGN_OPERATOR_AMBIGUITY!>+=<!> 1
+        return map[""]
+    }
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguityExtension.txt b/compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguityExtension.txt
new file mode 100644
index 00000000000..08556d1a116
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguityExtension.txt
@@ -0,0 +1,30 @@
+package
+
+private operator fun X?.plus(/*0*/ p: kotlin.Int): X
+private operator fun X?.plusAssign(/*0*/ p: kotlin.Int): kotlin.Unit
+
+public final class C {
+    public constructor C()
+    private final val map: HashMap<kotlin.String, X>
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final fun f(): kotlin.Any?
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class HashMap</*0*/ K, /*1*/ V> {
+    public constructor HashMap</*0*/ K, /*1*/ V>(/*0*/ defaultValue: V)
+    private final val defaultValue: V
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final operator fun get(/*0*/ key: K): V
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final operator fun set(/*0*/ key: K, /*1*/ value: V): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+private final class X {
+    public constructor X()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
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 2d74daecf8f..76613b8a9ad 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
@@ -25888,6 +25888,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                 runTest("compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguity.kt");
             }
 
+            @Test
+            @TestMetadata("AssignOperatorAmbiguityExtension.kt")
+            public void testAssignOperatorAmbiguityExtension() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguityExtension.kt");
+            }
+
             @Test
             @TestMetadata("AssignmentOperations.kt")
             public void testAssignmentOperations() throws Exception {