fix(gql.tada): Fix recursive optional fragment types causing exponential complexity

[kitten]

Summary

Patterns that require a T | {} optionality being merged into selections are causing exponential complexity. This is because all fields in a selection are merged using the obj utility. However, T | {} causes 2^n complexity in evaluating and flattening types in TypeScript.

This is because for each A & (B | {}) merge two variants are created, (A & B) | (A & {}). For each subsequent addition (e.g. A & (B | {}) & (C | {})) the complexity doubles ((A & B) | (A & C) | (A & B & C) | A. This can quickly cause a recursion complexity error in TypeScript once we reach the limit of 16 variants.

This fix addresses this by limiting the flattening of fields to just the fields and not optional sub-selections.

This fix however does have a few problems:

• We can probably add a shortcut for fragment masks, since those are represented by a single field consistently
• This can maybe still occur with unions/interfaces that create multiple different variants of types
• The implementation of this fix makes getPossibleTypeSelectionRec just slightly too hard to follow for my taste
• The output type shape quality is sometimes now compromised on

As such, post-merge, we're not planning to release this fix as another stable release straightaway and are still looking for improvements.

Set of changes

• Update tests for new output type shapes
• Separate field flattening from "non-flatteneable" types

[changeset-bot]

🦋 Changeset detected

Latest commit: c25848c

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

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR