[BUG][CORE] Use intersection of required fields across anyOf/oneOf members

The merged composed model from anyOf/oneOf added every member's required
fields to the resulting `required` set. Per OAS / JSON Schema semantics,
a value is only guaranteed present if EVERY member of an anyOf/oneOf
requires it, so the merged set should be the intersection.

Example:

    response:
      anyOf:
        - $ref: '#/components/schemas/VoteResponse'   # required: [status, voteId]
        - $ref: '#/components/schemas/APIError'       # required: [status, reason, code]

Previously: status, voteId, reason, code all marked required.
Now:        only status is required (the only field present in both).

allOf semantics are unchanged (still union).

Affects every generator that consumes CodegenModel.required for non-null
types or constructor signatures (Swift, Kotlin, Java POJOs, Dart, etc.).
Sample regeneration may flip previously-required fields to optional in
generated code where this pattern is in use; that is the intended
behavior.

Test: anyOfRequiredFieldsIntersectionTest in Swift6ClientCodegenModelTest
covers the contract via swift6_anyof_required.yaml (chosen because
swift6 is one of the affected generators; the change itself lives in
DefaultCodegen and applies everywhere).

Closes #23667

Author: winrid

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

@@ -2756,6 +2756,13 @@ protected void updateModelForComposedSchema(CodegenModel m, Schema schema, Map<S
 
         // interfaces (schemas defined in allOf, anyOf, oneOf)
         List<Schema> interfaces = ModelUtils.getInterfaces(composed);
+
+        // For anyOf/oneOf, required fields should be the intersection across members,
+        // not the union. A field is only guaranteed present if ALL members require it.
+        boolean isAnyOfOrOneOf = (composed.getAnyOf() != null && !composed.getAnyOf().isEmpty())
+                || (composed.getOneOf() != null && !composed.getOneOf().isEmpty());
+        List<Set<String>> perMemberRequiredSets = isAnyOfOrOneOf ? new ArrayList<>() : null;
+
         if (!interfaces.isEmpty()) {
             // m.interfaces is for backward compatibility
             if (m.interfaces == null)
@@ -2816,7 +2823,14 @@ protected void updateModelForComposedSchema(CodegenModel m, Schema schema, Map<S
                     } else {
                         // composition
                         Map<String, Schema> newProperties = new LinkedHashMap<>();
-                        addProperties(newProperties, required, refSchema, new HashSet<>());
+                        if (isAnyOfOrOneOf) {
+                            // Collect required fields per-member for later intersection
+                            List<String> memberRequired = new ArrayList<>();
+                            addProperties(newProperties, memberRequired, refSchema, new HashSet<>());
+                            perMemberRequiredSets.add(new HashSet<>(memberRequired));
+                        } else {
+                            addProperties(newProperties, required, refSchema, new HashSet<>());
+                        }
                         mergeProperties(properties, newProperties);
                         addProperties(allProperties, allRequired, refSchema, new HashSet<>());
                     }
@@ -2857,8 +2871,15 @@ protected void updateModelForComposedSchema(CodegenModel m, Schema schema, Map<S
         for (Schema component : interfaces) {
             if (component.get$ref() == null) {
                 if (component != null) {
-                    // component is the child schema
-                    addProperties(properties, required, component, new HashSet<>());
+                    if (isAnyOfOrOneOf) {
+                        // Collect required fields per-member for later intersection
+                        List<String> memberRequired = new ArrayList<>();
+                        addProperties(properties, memberRequired, component, new HashSet<>());
+                        perMemberRequiredSets.add(new HashSet<>(memberRequired));
+                    } else {
+                        // component is the child schema
+                        addProperties(properties, required, component, new HashSet<>());
+                    }
 
                     // includes child's properties (all, required) in allProperties, allRequired
                     addProperties(allProperties, allRequired, component, new HashSet<>());
@@ -2868,6 +2889,16 @@ protected void updateModelForComposedSchema(CodegenModel m, Schema schema, Map<S
             }
         }
 
+        // For anyOf/oneOf, compute the intersection of required fields across all members.
+        // A field is only required in the merged model if ALL members require it.
+        if (isAnyOfOrOneOf && !perMemberRequiredSets.isEmpty()) {
+            Set<String> intersected = new HashSet<>(perMemberRequiredSets.get(0));
+            for (int i = 1; i < perMemberRequiredSets.size(); i++) {
+                intersected.retainAll(perMemberRequiredSets.get(i));
+            }
+            required.addAll(intersected);
+        }
+
         if (composed.getRequired() != null) {
             required.addAll(composed.getRequired());
             allRequired.addAll(composed.getRequired());

modules/openapi-generator/src/test/java/org/openapitools/codegen/swift6/Swift6ClientCodegenModelTest.java

@@ -25,9 +25,12 @@
 import org.openapitools.codegen.DefaultCodegen;
 import org.openapitools.codegen.TestUtils;
 import org.openapitools.codegen.languages.Swift6ClientCodegen;
+import org.openapitools.codegen.utils.ModelUtils;
 import org.testng.Assert;
 import org.testng.annotations.Test;
 
+import java.util.Map;
+
 @SuppressWarnings("static-method")
 public class Swift6ClientCodegenModelTest {
 
@@ -163,4 +166,74 @@ public void useCustomDateTimeTest() {
         Assert.assertFalse(property7.isContainer);
     }
 
+    @Test(description = "anyOf with different required fields should use intersection for required", enabled = true)
+    public void anyOfRequiredFieldsIntersectionTest() {
+        // VoteResponse requires: status, voteId
+        // APIError requires: status, reason, code
+        // Intersection: only "status" should be required in the merged model
+        final OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/swift6_anyof_required.yaml");
+        final Swift6ClientCodegen codegen = new Swift6ClientCodegen();
+        codegen.setOpenAPI(openAPI);
+        codegen.processOpts();
+
+        // The inline model resolver creates a model for the anyOf response.
+        // Find the composed schema that merges VoteResponse and APIError.
+        Map<String, Schema> schemas = ModelUtils.getSchemas(openAPI);
+        Schema composedSchema = null;
+        String composedName = null;
+        for (Map.Entry<String, Schema> entry : schemas.entrySet()) {
+            Schema s = entry.getValue();
+            if (s.getAnyOf() != null && !s.getAnyOf().isEmpty()) {
+                composedSchema = s;
+                composedName = entry.getKey();
+                break;
+            }
+        }
+        Assert.assertNotNull(composedSchema, "Should find an anyOf composed schema");
+
+        final CodegenModel cm = codegen.fromModel(composedName, composedSchema);
+
+        // "status" is required in BOTH VoteResponse and APIError -> should be required
+        CodegenProperty statusProp = cm.vars.stream()
+                .filter(p -> p.baseName.equals("status"))
+                .findFirst().orElse(null);
+        Assert.assertNotNull(statusProp, "status property should exist");
+        Assert.assertTrue(statusProp.required, "status should be required (present in all anyOf members)");
+
+        // "voteId" is required only in VoteResponse, not in APIError -> should NOT be required
+        CodegenProperty voteIdProp = cm.vars.stream()
+                .filter(p -> p.baseName.equals("voteId"))
+                .findFirst().orElse(null);
+        Assert.assertNotNull(voteIdProp, "voteId property should exist");
+        Assert.assertFalse(voteIdProp.required, "voteId should NOT be required (only in VoteResponse, not APIError)");
+
+        // "reason" is required only in APIError, not in VoteResponse -> should NOT be required
+        CodegenProperty reasonProp = cm.vars.stream()
+                .filter(p -> p.baseName.equals("reason"))
+                .findFirst().orElse(null);
+        Assert.assertNotNull(reasonProp, "reason property should exist");
+        Assert.assertFalse(reasonProp.required, "reason should NOT be required (only in APIError, not VoteResponse)");
+
+        // "code" is required only in APIError, not in VoteResponse -> should NOT be required
+        CodegenProperty codeProp = cm.vars.stream()
+                .filter(p -> p.baseName.equals("code"))
+                .findFirst().orElse(null);
+        Assert.assertNotNull(codeProp, "code property should exist");
+        Assert.assertFalse(codeProp.required, "code should NOT be required (only in APIError, not VoteResponse)");
+
+        // "isVerified" is optional in VoteResponse, not in APIError -> should NOT be required
+        CodegenProperty isVerifiedProp = cm.vars.stream()
+                .filter(p -> p.baseName.equals("isVerified"))
+                .findFirst().orElse(null);
+        Assert.assertNotNull(isVerifiedProp, "isVerified property should exist");
+        Assert.assertFalse(isVerifiedProp.required, "isVerified should NOT be required");
+
+        // "secondaryCode" is optional in APIError -> should NOT be required
+        CodegenProperty secondaryCodeProp = cm.vars.stream()
+                .filter(p -> p.baseName.equals("secondaryCode"))
+                .findFirst().orElse(null);
+        Assert.assertNotNull(secondaryCodeProp, "secondaryCode property should exist");
+        Assert.assertFalse(secondaryCodeProp.required, "secondaryCode should NOT be required");
+    }
+
 }

modules/openapi-generator/src/test/resources/3_0/swift6_anyof_required.yaml

@@ -0,0 +1,51 @@
+openapi: 3.0.0
+info:
+  title: anyOf Required Fields Test
+  version: 1.0.0
+paths:
+  /vote:
+    post:
+      operationId: voteOnItem
+      responses:
+        '200':
+          description: Success or error response
+          content:
+            application/json:
+              schema:
+                anyOf:
+                  - $ref: '#/components/schemas/VoteResponse'
+                  - $ref: '#/components/schemas/APIError'
+components:
+  schemas:
+    APIStatus:
+      type: string
+      enum:
+        - success
+        - failed
+    VoteResponse:
+      type: object
+      required:
+        - status
+        - voteId
+      properties:
+        status:
+          $ref: '#/components/schemas/APIStatus'
+        voteId:
+          type: string
+        isVerified:
+          type: boolean
+    APIError:
+      type: object
+      required:
+        - status
+        - reason
+        - code
+      properties:
+        status:
+          $ref: '#/components/schemas/APIStatus'
+        reason:
+          type: string
+        code:
+          type: string
+        secondaryCode:
+          type: string