[core] keep property type when an allOf part redefines it without a type

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

@@ -2941,7 +2941,7 @@ private void mergeProperties(Map<String, Schema> existingProperties, Map<String,
             Schema existingType = existingProperties.get("type");
             Schema newType = newProperties.get("type");
             newProperties.forEach((key, value) ->
-                    existingProperties.put(key, ModelUtils.cloneSchema(value, specVersionGreaterThanOrEqualTo310(openAPI)))
+                    putProperty(existingProperties, key, ModelUtils.cloneSchema(value, specVersionGreaterThanOrEqualTo310(openAPI)))
             );
             if (null != existingType && null != newType && null != newType.getEnum() && !newType.getEnum().isEmpty()) {
                 for (Object e : newType.getEnum()) {
@@ -3608,7 +3608,7 @@ protected void addProperties(Map<String, Schema> properties, List<String> requir
         if (ModelUtils.isComposedSchema(schema)) {
             // fix issue #16797 and #15796, constructor fail by missing parent required params
             if (ModelUtils.hasProperties(schema)) {
-                properties.putAll(schema.getProperties());
+                putProperties(properties, schema.getProperties());
             }
 
             if (schema.getAllOf() != null) {
@@ -3642,13 +3642,39 @@ protected void addProperties(Map<String, Schema> properties, List<String> requir
             return;
         }
         if (schema.getProperties() != null) {
-            properties.putAll(schema.getProperties());
+            putProperties(properties, schema.getProperties());
         }
         if (schema.getRequired() != null) {
             required.addAll(schema.getRequired());
         }
     }
 
+    /**
+     * Adds each property to the target map. When a property of the same name is already present
+     * with type information and the incoming schema carries no type of its own (for example an
+     * allOf part that only sets 'nullable: true' on an inherited property), the incoming
+     * constraints (nullable, description, validation keywords, format, default, extensions)
+     * are applied on top of the existing schema instead of replacing it, so the type is not
+     * lost. See issue #4128.
+     */
+    private void putProperties(Map<String, Schema> targetProperties, Map<String, Schema> newProperties) {
+        newProperties.forEach((name, incoming) -> putProperty(targetProperties, name, incoming));
+    }
+
+    private void putProperty(Map<String, Schema> targetProperties, String name, Schema incoming) {
+        Schema existing = targetProperties.get(name);
+        if (existing != null && incoming != null
+                && !ModelUtils.isAnyType(existing)
+                && ModelUtils.isMetadataOnlySchema(incoming)
+                && incoming.getEnum() == null && incoming.getConst() == null && incoming.getNot() == null) {
+            Schema merged = ModelUtils.cloneSchema(existing, specVersionGreaterThanOrEqualTo310(openAPI));
+            ModelUtils.copyConstraints(incoming, merged);
+            targetProperties.put(name, merged);
+        } else {
+            targetProperties.put(name, incoming);
+        }
+    }
+
     /**
      * Camelize the method name of the getter and setter
      *

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

@@ -2807,6 +2807,55 @@ public static void copyMetadata(Schema from, Schema to) {
         }
     }
 
+    /**
+     * Copies metadata plus the validation and format keywords that copyMetadata does not cover.
+     * Used when an allOf part only constrains an inherited property (see issue #4128), so no
+     * declared keyword is lost while the type is kept.
+     *
+     * @param from schema to copy from
+     * @param to   schema to copy to
+     */
+    public static void copyConstraints(Schema from, Schema to) {
+        Map<String, Object> targetExtensions = to.getExtensions() == null ? null : new HashMap<>(to.getExtensions());
+        copyMetadata(from, to);
+        // merge extensions per key instead of replacing, the source wins on conflicts
+        if (targetExtensions != null && from.getExtensions() != null) {
+            Map<String, Object> mergedExtensions = new HashMap<>(targetExtensions);
+            mergedExtensions.putAll(from.getExtensions());
+            to.setExtensions(mergedExtensions);
+        }
+        if (from.getFormat() != null) {
+            to.setFormat(from.getFormat());
+        }
+        if (from.getPattern() != null) {
+            to.setPattern(from.getPattern());
+        }
+        if (from.getExclusiveMaximum() != null) {
+            to.setExclusiveMaximum(from.getExclusiveMaximum());
+        }
+        if (from.getExclusiveMinimum() != null) {
+            to.setExclusiveMinimum(from.getExclusiveMinimum());
+        }
+        if (from.getExclusiveMaximumValue() != null) {
+            to.setExclusiveMaximumValue(from.getExclusiveMaximumValue());
+        }
+        if (from.getExclusiveMinimumValue() != null) {
+            to.setExclusiveMinimumValue(from.getExclusiveMinimumValue());
+        }
+        if (from.getMultipleOf() != null) {
+            to.setMultipleOf(from.getMultipleOf());
+        }
+        if (from.getUniqueItems() != null) {
+            to.setUniqueItems(from.getUniqueItems());
+        }
+        if (from.getMaxProperties() != null) {
+            to.setMaxProperties(from.getMaxProperties());
+        }
+        if (from.getMinProperties() != null) {
+            to.setMinProperties(from.getMinProperties());
+        }
+    }
+
     /**
      * Returns true if a schema is only metadata and not an actual type.
      * For example, a schema that only has a `description` without any `properties` or `$ref` defined.

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

@@ -1314,6 +1314,27 @@ public void testAllOfRequired() {
         assertEquals(getRequiredVars(childModel), Collections.singletonList("name"));
     }
 
+    @Test
+    public void testAllOfNullableWithoutTypeKeepsType() {
+        // issue #4128: an allOf part that only sets 'nullable: true' on a property
+        // defined in another part must not erase the property type
+        final OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/allOf-nullable-typeless-override.yaml");
+        DefaultCodegen codegen = new DefaultCodegen();
+        codegen.setOpenAPI(openAPI);
+
+        Schema schema = openAPI.getComponents().getSchemas().get("UpdateFirm");
+        CodegenModel model = codegen.fromModel("UpdateFirm", schema);
+
+        CodegenProperty addressId = model.vars.stream()
+                .filter(v -> "addressId".equals(v.baseName)).findFirst().orElseThrow();
+        assertEquals("String", addressId.dataType);
+        assertTrue(addressId.isNullable);
+        assertEquals(Integer.valueOf(36), addressId.maxLength);
+        // extensions from both parts survive the merge
+        assertEquals("keep", addressId.vendorExtensions.get("x-base-marker"));
+        assertEquals("added", addressId.vendorExtensions.get("x-overlay-marker"));
+    }
+
     @Test
     public void testAllOfSingleAndDoubleRefWithOwnPropsNoDiscriminator() {
         final OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/allOf_composition.yaml");

modules/openapi-generator/src/test/resources/3_0/allOf-nullable-typeless-override.yaml

@@ -0,0 +1,41 @@
+openapi: 3.0.3
+info:
+  title: allOf nullable without type
+  version: 1.0.0
+paths:
+  /firm/{firmId}:
+    patch:
+      operationId: updateFirm
+      parameters:
+        - in: path
+          name: firmId
+          required: true
+          schema:
+            type: integer
+            format: int64
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/UpdateFirm"
+      responses:
+        "204":
+          description: Updated
+components:
+  schemas:
+    FirmProperties:
+      properties:
+        addressId:
+          type: string
+          x-base-marker: keep
+    UpdateFirm:
+      allOf:
+        - $ref: "#/components/schemas/FirmProperties"
+        - properties:
+            firmName:
+              type: string
+              nullable: true
+            addressId:
+              nullable: true
+              maxLength: 36
+              x-overlay-marker: added