fix(go): collapse nullable single-member oneOf union to pointer type

[siemendev]

Summary

When a JSON Schema uses oneOf: [{type: T}, {type: null}], the conversion layer already does the right thing: it sets isNullable: true on the resulting ConstrainedUnionModel and strips the null member, leaving a single-item union. But the Go generator ignored this and still rendered the union as a named struct type:

// Before — anonymous wrapper, never used anywhere
type NullableString struct {
    string
}

// After — idiomatic Go pointer, used inline where the field is declared
*string

The wrapper type polluted generated output and was functionally useless; consumers had to manually rewrite it to *string/*int/etc.

Changes

src/generators/go/GoConstrainer.ts — Union handler: detect the collapsed-nullable case and return *T:

Union({ constrainedModel }): string {
  // A oneOf:[T, null] schema becomes a single-member nullable union — render as *T.
  if (constrainedModel.options.isNullable && constrainedModel.union.length === 1) {
    return `*${constrainedModel.union[0].type}`;
  }
  return constrainedModel.name;
},

src/generators/go/renderers/UnionRenderer.ts — defaultSelf: return empty string for the same condition so no standalone Go type definition is emitted:

public async defaultSelf(): Promise<string> {
  // oneOf:[T, null] collapses to *T at the property level; no standalone type needed.
  if (this.model.options.isNullable && this.model.union.length === 1) {
    return '';
  }
  // ...

Multi-member nullable unions (e.g. oneOf: [{type: string}, {type: integer}, {type: null}]) are unaffected — they still render as named union structs.

test/generators/go/GoConstrainer.spec.ts — two new Union test cases:

• 'nullable single-member union renders as pointer to member type' → *string
• 'nullable multi-member union still renders as union type name' → NullableMulti (no regression)

Test plan

• npx jest test/generators/go/GoConstrainer.spec.ts — 2 new tests pass
• npx jest test/generators/go/ — 69 tests pass, no snapshot changes

Related

Companion to #2566 (fix nullable+required fields emitting value types instead of pointer types). Together these two PRs make Go nullable field generation correct for all combinations.

[netlify]

✅ Deploy Preview for modelina canceled.

Name	Link
🔨 Latest commit	821e4cb
🔍 Latest deploy log	https://app.netlify.com/projects/modelina/deploys/6a17e43c2421e90008021b66

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud