[lucasbraum]

Pack

react, >=18 <19 → >=19 <20. Companions move with it: react-dom, @types/react and @types/react-dom, as the engine's companion rules decide (not listed by the pack). Supersedes #45: same pack and ground truth, brought onto current main (merge of main at 0de525c, no conflict), with the changeset, the README table and a test fixed up.

Sources

• React 19 Upgrade Guide: https://react.dev/blog/2024/04/25/react-19-upgrade-guide
• React 19.0.0 changelog: https://github.com/facebook/react/blob/main/CHANGELOG.md#1900-december-5-2024
• types-react-codemod (preset-19): https://github.com/eps1lon/types-react-codemod

Rules and behavior notes

Rewrites by rule:

• use-ref-argument: useRef<T>() → useRef<T>(undefined).
• ref-callback-return: ref={(el) => (x = el)} → a block body (sites from detect).
• mutable-ref-object: React.MutableRefObject → React.RefObject.

To the agent, with a guide:

• ref-object-nullable: a parameter or prop typed RefObject<T> once useRef returns RefObject<T | null>.
• global-jsx-namespace: the removed global JSX namespace (one finding at the tsconfig's jsx line, the elements as evidence).
• element-props-unknown: untyped element.props.
• removed-react-dom: ReactDOM.render, hydrate, unmountComponentAtNode, findDOMNode.
• props-with-ref.

Behavior notes, listed for review (the compiler cannot see them): errors-not-rethrown, removed-legacy-apis, act-moved-to-react, element-ref-deprecated, strict-mode-memoization, suspense-fallback-first, umd-removed, javascript-urls-blocked, use-reducer-types.

Ground-truth repositories

Repository	Pinned commit (before the upgrade)	Upgrade	With	Expected findings
excalidraw/excalidraw	9ee0b8ffcbd3664a47748a93262860321a203821	react 18.2.0 → 19.0.0	react-dom 19.0.0, @types/react 19.0.10, @types/react-dom 19.0.4	73
tldraw/tldraw	5957867a9c5c205a0b0c3665be443138b8014a79	react 18.3.1 → 19.2.1	react-dom 19.2.1, @types/react 19.2.7, @types/react-dom 19.2.3	60

Expected findings were drafted with pnpm packs:truth from each repository's own TypeScript and its upgrade commit, and read by hand; tldraw's 1,745 JSX element diagnostics are one site at the tsconfig that causes them (#45). Rules, expectations, ground truth and companions are unchanged from #45's last state.

uptide pack test output

verification.json on this branch records the run on main after #51 (commit 8573cc0); pack test react was not rerun here, since the merge touched neither packages/core/src/check nor the pack. That run:

uptide pack test react · verified
fixtures      3 cases, 6 sites · pass
ground truth
  excalidraw/excalidraw@9ee0b8ffcbd3           react 18.2.0 → 19.0.0  predicted 60, expected 73, false positives 1, false negatives 14
  tldraw/tldraw@5957867a9c5c                   react 18.3.1 → 19.2.1  predicted 39, expected 60, false positives 6, false negatives 26

  rule                         precision  recall    tp   fp   fn
  element-props-unknown             100%     60%     6    0    4
  generic                             4%     33%     1   26    2
  global-jsx-namespace              100%     51%    20    0   19
  mutable-ref-object                100%    100%     2    0    0
  props-with-ref                    100%    100%     1    0    0
  ref-callback-return               100%    100%     1    0    0
  ref-object-nullable               100%     37%    20    0   34
  removed-react-dom                 100%    100%    18    0    0
  use-ref-argument                  100%     80%     4    0    1
  overall (sites)                    93%     70%    92    7   40
  breaking (sites)                  100%     69%    89    0   40

Breaking findings: precision 100% (89 sites, none false), recall 69%, on 2 public repositories. The 7 non-breaking false positives are deprecated or unverified findings and do not count toward the verdict.

What is not covered

• Recall on breaking sites is 69%: 40 expected sites are missed, most of them ref-object-nullable (34) and global-jsx-namespace (19 in the per-rule table, counted at the tsconfig anchor) sites the compiler does not reach, plus 4 element-props-unknown and 1 use-ref-argument. They are listed by pack test react as false negatives.
• Behavior changes the compiler cannot see are reported as notes for review, never rewritten.
• The engine links one copy of the target types; a nested second @types/react copy, as a real install can keep, is not modeled.

This PR on top of #45

• .changeset/react-pack.md is minor (a new verified pack) and opens with the two repositories; numbers unchanged.
• README "Verified packs" regenerated with pnpm docs:packs; pnpm docs:packs --check and pnpm docs:links pass.
• packages/core/src/fix/isolate.test.ts used react as its example of a package without a pack; it is vitest now, like the two cli.test.ts cases. No other test did.
• pnpm lint, pnpm typecheck, core (919 tests) and CLI (350 tests) pass.

Checklist

• Every rule and note comes from a source listed in meta.sources
• Every rule that rewrites or detects has a fixture case, with a site it must leave alone (keep)
• Ground truth: public repositories at the commit before their upgrade; expected findings read from their own upgrade commit and the compiler, never copied from Uptide's output
• At least two public repositories for verified; otherwise the pack ships as a candidate
• pnpm uptide pack test <package> passes and its output is pasted above; verification.json written with --write
• pnpm lint && pnpm typecheck && pnpm test pass
• Changeset added (pnpm changeset): a new pack is something users notice
• No private code, names or paths anywhere: fixtures are synthetic, ground truth is public

Supersedes #45.

🤖 Generated with Claude Code

https://claude.ai/code/session_018u9b3Pp5H68DdYZGp5Qg1U

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