[plugin-helpers] fix: guard isUsingTypes against union and interface types without fields (dotansimha/graphql-code-generator-community#934)

[agustin18]

Description

In isUsingTypes, when an operation contains subselections on a field whose parent type is a GraphQLUnionType (or any non-object/non-interface type), lastType.getFields() throws:

TypeError: lastType.getFields is not a function

This unhandled error is caught by presets such as near-operation-file and masked as a generic Unable to validate GraphQL document!, preventing real GraphQL validation errors from reaching the user.

Additionally, when resolving fields on GraphQLInterfaceType instances:

• In Field.enter, isObjectType(lastType) returned false, skipping nullability checks and potentially undercounting foundFields.

This PR:

1. Types typesStack as Array<GraphQLNamedType | undefined> instead of GraphQLObjectType[], removing the as any cast on typesStack.push(...).
2. In the Kind.FIELD branch of SelectionSet.enter, guards lastType with isObjectType(lastType) || isInterfaceType(lastType) before accessing .getFields(), safely returning null when the parent cannot define fields.
3. In Kind.INLINE_FRAGMENT, safely resolves typesStack[typesStack.length - 1]?.name || null.
4. In Field.enter, updates the check to isObjectType(lastType) || isInterfaceType(lastType) so interface fields are properly recognized.

Related dotansimha/graphql-code-generator-community#934

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Added regression unit tests in packages/utils/plugins-helpers/tests/is-using-types.spec.ts:
  • community #934 - should not crash when selecting fields on union type
  • should correctly detect types and nullable fields on interface types
  • should handle untyped inline fragments on unions without error
• Ran targeted vitest suite (pnpm vitest run packages/utils/plugins-helpers/tests/is-using-types.spec.ts) - 11/11 tests passing.
• Ran full pnpm types:check and pnpm prettier:check.

Test Environment:

• OS: Linux (Ubuntu 24.04)
• @graphql-codegen/plugin-helpers: 7.4.2
• NodeJS: 24.12.0

Checklist:

• I have followed the CONTRIBUTING doc and the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[changeset-bot]

🦋 Changeset detected

Latest commit: aa670b0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@graphql-codegen/plugin-helpers	Patch

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