Keep nullability when inlining ref to primitive

modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java

@@ -1410,7 +1410,11 @@ public static Schema unaliasSchema(OpenAPI openAPI,
                             schemaMappings);
                 }
             } else {
-                return unaliasSchema(openAPI, allSchemas.get(ModelUtils.getSimpleRef(schema.get$ref())), schemaMappings);
+                Schema unaliased = unaliasSchema(openAPI, allSchemas.get(ModelUtils.getSimpleRef(schema.get$ref())), schemaMappings);
+                // Preserve nullable property from the original schema reference
+                Schema newSchema = cloneSchema(unaliased, false);
+                newSchema.setNullable(schema.getNullable());
+                return newSchema;
             }
         }
         return schema;

modules/openapi-generator/src/test/java/org/openapitools/codegen/utils/ModelUtilsTest.java

@@ -242,6 +242,31 @@ public void testAliasedTypeIsNotUnaliasedIfUsedForImportMapping() {
         Assert.assertEquals(stringSchema, ModelUtils.unaliasSchema(openAPI, emailSchema, new HashMap<>()));
     }
 
+    /**
+     * This test verifies that nullable property is preserved when unaliasing a schema reference.
+     * See https://github.com/OpenAPITools/openapi-generator/issues/21612
+     */
+    @Test
+    public void testNullableUnaliasedSchema() {
+        Schema refSchema = new Schema().$ref("#/components/schemas/MyString").nullable(true);
+        StringSchema myStringSchema = new StringSchema();
+        OpenAPI openAPI = TestUtils.createOpenAPIWithOneSchema("MyString", myStringSchema);
+
+        Schema unaliased = ModelUtils.unaliasSchema(openAPI, refSchema, new HashMap<>());
+
+        Assert.assertTrue(unaliased.getNullable(), "Nullable property should be preserved after unaliasing");
+        Assert.assertNotSame(unaliased, refSchema, "Unaliased schema should not be the same instance as the original reference");
+
+        Schema refSchema2 = new Schema().$ref("#/components/schemas/MyString");
+        StringSchema myStringSchema2 = new StringSchema();
+        OpenAPI openAPI2 = TestUtils.createOpenAPIWithOneSchema("MyString", myStringSchema2);
+
+        Schema unaliased2 = ModelUtils.unaliasSchema(openAPI2, refSchema2, new HashMap<>());
+
+        Assert.assertNull(unaliased2.getNullable(), "Nullable property should be preserved after unaliasing");
+        Assert.assertNotSame(unaliased2, refSchema2, "Unaliased schema should not be the same instance as the original reference");
+    }
+
     /**
      * Issue https://github.com/OpenAPITools/openapi-generator/issues/1624.
      * ModelUtils.isFreeFormObject() should not throw an NPE when passed an empty