[BUG][core] Normalize OAS 3.1 type: null map value (additionalProperties) under NORMALIZE_31SPEC

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

@@ -983,7 +983,24 @@ public Schema normalizeSchema(Schema schema, Set<Schema> visitedSchemas) {
             normalizeProperties(schema, visitedSchemas);
         } else if (schema.getAdditionalProperties() instanceof Schema) { // map
             normalizeMapSchema(schema);
-            normalizeSchema((Schema) schema.getAdditionalProperties(), visitedSchemas);
+            Schema additionalProperties = (Schema) schema.getAdditionalProperties();
+            if (getRule(NORMALIZE_31SPEC) && ModelUtils.isNullTypeSchema(openAPI, additionalProperties)) {
+                // OAS 3.1 allows a map value schema of `type: "null"` (e.g.
+                // `additionalProperties: { type: "null" }`). There's no OAS 3.0 equivalent type,
+                // so generators emit a fictional `Null` / `ModelNull` value type that fails to
+                // compile. Normalize it to an any-type nullable schema so the map value is
+                // generated as a normal (nullable) object instead.
+                Schema anyTypeNullable = new Schema();
+                anyTypeNullable.setNullable(true);
+                schema.setAdditionalProperties(anyTypeNullable);
+            } else {
+                Schema normalized = normalizeSchema(additionalProperties, visitedSchemas);
+                if (getRule(NORMALIZE_31SPEC)) {
+                    // capture the normalized value schema (e.g. an OAS 3.1 `type: [array, "null"]`
+                    // value is rewritten to a proper array schema), which would otherwise be lost.
+                    schema.setAdditionalProperties(normalized);
+                }
+            }
         } else if (schema instanceof BooleanSchema) {
             normalizeBooleanSchema(schema, visitedSchemas);
         } else if (schema instanceof IntegerSchema) {

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

@@ -1896,4 +1896,29 @@ public void testLooseNullDefinitions() {
         ModelUtils.looseNullDefinitions = false;
     }
 
+    @Test
+    public void testOpenAPINormalizer31SpecNullMapAdditionalProperties() {
+        OpenAPI openAPI = TestUtils.parseSpec("src/test/resources/3_1/issue_23945.yaml");
+        Map<String, String> inputRules = Map.of("NORMALIZE_31SPEC", "true");
+        OpenAPINormalizer openAPINormalizer = new OpenAPINormalizer(openAPI, inputRules);
+        openAPINormalizer.normalize();
+
+        Schema schema = openAPI.getComponents().getSchemas().get("NullableMaps");
+
+        // `additionalProperties: { type: "null" }` must not keep the OAS 3.1 `null` type (which makes
+        // generators emit a fictional `Null` / `ModelNull` value type); it is normalized to an
+        // any-type nullable schema so the map value generates as a normal (nullable) object.
+        Schema stringMapValue = ModelUtils.getAdditionalProperties((Schema) schema.getProperties().get("stringMap"));
+        assertNull(stringMapValue.getType());
+        assertNull(stringMapValue.getTypes());
+        assertTrue(stringMapValue.getNullable());
+
+        // `additionalProperties: { type: [array, "null"], items: ... }` is normalized to a proper
+        // (nullable) array value schema rather than being left half-converted.
+        Schema errorsValue = ModelUtils.getAdditionalProperties((Schema) schema.getProperties().get("errorsByKey"));
+        assertEquals(errorsValue.getType(), "array");
+        assertTrue(errorsValue.getNullable());
+        assertNotNull(errorsValue.getItems());
+    }
+
 }

modules/openapi-generator/src/test/resources/3_1/issue_23945.yaml

@@ -0,0 +1,22 @@
+openapi: 3.1.1
+info:
+  title: Repro
+  version: 1.0.0
+components:
+  schemas:
+    NullableMaps:
+      type: object
+      properties:
+        stringMap:
+          type: [object, "null"]
+          additionalProperties:
+            type: "null"
+        errorsByKey:
+          type: [object, "null"]
+          additionalProperties:
+            type: [array, "null"]
+            items:
+              type: object
+              properties:
+                code:
+                  type: string