Add const type inference to 3.1 normalization

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

@@ -1782,6 +1782,21 @@ protected Schema processNormalize31Spec(Schema schema, Set<Schema> visitedSchema
             return null;
         }
 
+        // process const
+        if (schema.getConst() != null) {
+            Object value = schema.getConst();
+            schema.setEnum(Arrays.asList(value));
+            schema.setConst(null);
+
+            if (schema.getTypes() == null) {
+                if (value instanceof String) {
+                    schema.addType("string");
+                } else if (value instanceof Integer) {
+                    schema.addType("integer");
+                }
+            }
+        }
+
         if (schema instanceof JsonSchema &&
                 schema.get$schema() == null &&
                 schema.getTypes() == null && schema.getType() == null) {
@@ -1803,12 +1818,6 @@ protected Schema processNormalize31Spec(Schema schema, Set<Schema> visitedSchema
             schema.getTypes().remove("null");
         }
 
-        // process const
-        if (schema.getConst() != null) {
-            schema.setEnum(Arrays.asList(schema.getConst()));
-            schema.setConst(null);
-        }
-
         // only one item (type) left
         if (schema.getTypes().size() == 1) {
             String type = String.valueOf(schema.getTypes().iterator().next());

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

@@ -2334,6 +2334,10 @@ public static boolean isNullTypeSchema(OpenAPI openAPI, Schema schema) {
             }
         }
 
+        if (schema.getConst() != null) {
+            return false;
+        }
+
         if (schema.getTypes() != null && !schema.getTypes().isEmpty()) {
             // 3.1 spec
             if (schema.getTypes().size() == 1) { // 1 type only

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

@@ -1180,6 +1180,23 @@ public void testOpenAPINormalizerSingleConstEnum31Spec() {
         assertEquals(Arrays.asList(originalConst), normalizedTypeSchema.getEnum());
     }
 
+    @Test
+    public void testOpenAPINormalizerConstTypeInference31Spec() {
+        OpenAPI openAPI = TestUtils.parseSpec("src/test/resources/3_1/const-type-inference.yaml");
+
+        assertNotEquals(
+                openAPI.getComponents().getSchemas().get("ConstStyle"),
+                openAPI.getComponents().getSchemas().get("EnumStyle")
+        );
+
+        new OpenAPINormalizer(openAPI, Map.of("NORMALIZE_31SPEC", "true")).normalize();
+
+        assertEquals(
+                openAPI.getComponents().getSchemas().get("ConstStyle"),
+                openAPI.getComponents().getSchemas().get("EnumStyle")
+        );
+    }
+
     @Test
     public void testOpenAPINormalizerProcessingAllOfSchema31Spec() {
         // to test array schema processing in 3.1 spec

modules/openapi-generator/src/test/resources/3_1/const-type-inference.yaml

@@ -0,0 +1,24 @@
+openapi: 3.1.0
+info:
+  title: "Const Type Inference Test"
+  version: 1.0.0
+components:
+  schemas:
+    ConstStyle:
+      type: object
+      properties:
+        stringValue:
+          const: "hello-world"
+        intValue:
+          const: 42
+    EnumStyle:
+      type: object
+      properties:
+        stringValue:
+          type: string
+          enum:
+            - "hello-world"
+        intValue:
+          type: integer
+          enum:
+            - 42