[BUG][typescript-axios] Primitive anyOf validation becomes an empty nullable model #25637

Bug Report Checklist

• Provided a minimal standalone spec.
• Validated the OpenAPI 3.0 and 3.1 regression fixtures with the CLI validator.
• Reproduced on the latest master.
• Searched related issues and PRs.
• Included actual and expected output.

Description

With typescript-axios, a primitive schema whose anyOf alternatives contain validation keywords but no type becomes an empty interface and is incorrectly made nullable.

A pattern-only branch is not a null-only schema. The enclosing type: string still restricts the value to a string. See the OpenAPI Schema Object and JSON Schema validation keywords.

Actual output:

export interface Hostname {}
export interface Address {
    'hostname': Hostname | null;
}

This accepts numbers and null as SDK method arguments, and a returned hostname cannot be assigned to a string.

Expected output:

export type Hostname = string;
export interface Address {
    'hostname': Hostname;
}

The original pattern constraints should remain in the normalized OpenAPI document. TypeScript does not express those predicates, but it can preserve the enclosing primitive type and its nullability.

openapi-generator version

Reproduced with 7.27.0 and master at df804ac9802fba0c51a64f8c3cb03191ed3dde2d (7.28.0-SNAPSHOT). The new ModelUtils, normalizer, and Axios regression tests fail against the unmodified master implementation.

OpenAPI declaration file content or url

openapi: 3.0.3
info: {title: Primitive anyOf validation, version: 1.0.0}
paths: {}
components:
  schemas:
    Hostname:
      type: string
      anyOf: [{pattern: '^[a-z]+[.]example$'}, {pattern: '^[*][.][a-z]+[.]example$'}]
    Address: {type: object, required: [hostname], properties: {hostname: {$ref: '#/components/schemas/Hostname'}}}

Generation Details

java -jar openapi-generator-cli.jar generate \
  -g typescript-axios -i primitive-anyof.yaml -o generated

Steps to reproduce

1. Save the spec above as primitive-anyof.yaml.
2. Run the generation command.
3. Inspect Hostname and Address.hostname in generated/api.ts.

Related issues/PRs

#19155 concerns an untyped union of explicit primitive alternatives. #22427 and #24356 concern nullable reference propagation. This report concerns validation-only alternatives under an explicitly typed parent.

Suggest a fix

• Do not classify validation-only schemas as null schemas or discard their predicates during OpenAPI 3.1 normalization.
• Preserve an enclosing type when simplifying a single validation-only alternative.
• Generate primitive aliases for the Axios models, including inline properties and nullable references, while retaining genuine nullability.