Better isAlias detection (#16943)

* better isAlias check for allOf with single item

* better code format

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

@@ -5987,21 +5987,31 @@ public class DefaultCodegen implements CodegenConfig {
         for (Map.Entry<String, Schema> entry : schemas.entrySet()) {
             Schema schema = entry.getValue();
             if (isAliasOfSimpleTypes(schema)) {
-                String oasName = entry.getKey();
-                String schemaType = getPrimitiveType(schema);
-                aliases.put(oasName, schemaType);
+                if (schema.getAllOf() != null && schema.getAllOf().size() == 1) { // allOf with a single item
+                    Schema unaliasSchema = unaliasSchema(schema);
+                    unaliasSchema = ModelUtils.getReferencedSchema(this.openAPI, unaliasSchema);
+                    aliases.put(entry.getKey() /* schema name, e.g. Pet */, getPrimitiveType(unaliasSchema));
+                } else {
+                    aliases.put(entry.getKey() /* schema name, e.g. Pet */, getPrimitiveType(schema));
+                }
             }
-
         }
 
         return aliases;
     }
 
-    private static Boolean isAliasOfSimpleTypes(Schema schema) {
+    private Boolean isAliasOfSimpleTypes(Schema schema) {
         if (schema == null) {
             return false;
         }
 
+        // allOf with a single item
+        if (schema.getAllOf() != null && schema.getAllOf().size() == 1
+                && schema.getAllOf().get(0) instanceof Schema) {
+            schema = unaliasSchema((Schema) schema.getAllOf().get(0));
+            schema = ModelUtils.getReferencedSchema(this.openAPI, schema);
+        }
+
         return (!ModelUtils.isObjectSchema(schema)
                 && !ModelUtils.isArraySchema(schema)
                 && !ModelUtils.isMapSchema(schema)

modules/openapi-generator/src/test/resources/3_0/allof_primitive.yaml

@@ -0,0 +1,36 @@
+openapi: 3.0.0
+info:
+  title: Foo
+  version: 1.0.0
+paths:
+  /foo:
+    get:
+      responses:
+        '200':
+          description: example
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                 $ref: '#/components/schemas/Payment'
+components:
+  schemas:
+    DateTime:
+      type: string
+      format: date-time
+      example: "2019-01-01T00:00:00"
+    # the following schema shouldn't be generated as model
+    AllOfDatetime:
+      allOf:
+        - $ref: '#/components/schemas/DateTime'
+      example: "2019-01-01 00:00:00" 
+    Payment:
+      type: object
+      properties:
+        date:
+          $ref: '#/components/schemas/AllOfDatetime'
+        amount:
+          type: number
+          format: float
+          example: 625.0