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…l flow InferReactivePlaces marks a phi reactive when one of its predecessors is controlled by a reactive `if`, `branch` or `switch` test. A `MaybeThrow` terminal with a handler is a branch too, but it has no test operand, so it never counted. A local that a `catch` handler assigns, a `return` from a `try` / `catch` inside `useMemo`, and the caught value itself were all treated as non-reactive. Values built from them were memoized with no dependency, and a second render with different props returned the element of the first render. A `MaybeThrow` block now throws reactively when one of its instructions has a reactive input, or when the block itself is reactive-controlled. Control dependence on such a block is transitive: in a `try` block every instruction ends its block, so the frontier of a block is mostly the block before it, and one level loses an earlier throw or a reactive `if` around the blocks. The handler binding of a `try` is declared before the statement but only bound in the `catch` clause. It is not marked reactive, or a scope around the statement would depend on it out of scope. Reading it is a reactive input, which makes the `catch` parameter reactive. Upstream (babel-plugin-react-compiler 1.0.0) has the same behaviour, so this is a deliberate difference from the ported ControlDominators.ts.
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Status: fix pushed, waiting for CI and review. Reproduction (1.4.3-canary.1+6a92015fc and main b993710). Use a // stale.jsx
function App(p) {
let ok = "valid";
try { JSON.parse(p.t); } catch { ok = "invalid"; }
return <div>{ok}</div>;
}
console.log(App({ t: "{bad" }).props.children);
console.log(App({ t: "{}" }).props.children);With this change the second command prints Self-reviewed: 4 concerns raised, 3 addressed. Details are in the Notes block of the description. |
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Included review availability: Your plan provides up to 10 included reviews per hour; 0 remain after this review. WalkthroughReactive-place inference now tracks reactive exception control flow through ChangesReactive throw inference
Suggested reviewers: Priority: ➖ Normal Merge Risk: ⚪ Minimal · up to The reactive try/catch inference change has no unresolved, concrete merge-blocking issue in the supplied review context. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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In `@test/bundler/transpiler/react-compiler.test.ts`:
- Line 1060: Strengthen the NothingReactiveInTry assertion by adding a positive
check that the generated bundle contains the staticStatus dependency, alongside
the existing negative check for the !== staticStatus expression.
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I reviewed this PR and didn't find any bugs. Because it deliberately diverges from upstream React Compiler in a dataflow fixpoint pass — a design decision that changes codegen for user components — a human sign-off would still be worthwhile.
What was reviewed:
- Fixpoint termination: both
snapshot()calls run unconditionally before the&&, so neither change flag is left un-reset by short-circuiting. - The
has_handlergate keepsfrontiersempty for functions with no try/catch, so the added per-block work is a no-op on the common path. - Catch bindings are treated as reactive at read sites rather than marked reactive directly, and
ScopeAroundTryin the test covers the out-of-scope-dependency shape that motivated this. - Test covers both directions: nine components must follow props across renders on one persisted memo cache, and
onAfterBundleasserts the static-try case did not gain a spurious dependency.
Extended reasoning...
Overview
The PR teaches infer_reactive_places to treat MaybeThrow { handler: Some(_) } as reactive control flow, fixing wrong memoization of values assigned in catch clauses. It adds a ReactiveThrows struct that records post-dominator frontiers per block (only when the function has any handler), a set of blocks that throw reactively (or are transitively controlled by one), and the set of catch-binding identifiers whose handler is reached by a reactive throw. The fixpoint loop is extended to seed and propagate these sets, is_reactive_controlled_block consults them, and the replay pass is threaded the same is_caught_value check. A large itBundled test with ten component shapes renders each several times against one persisted memo cache and asserts exact stdout plus a negative onAfterBundle check.
Security risks
None. This is a pure compiler-analysis change over internal HIR — no untrusted input parsing, no I/O, no allocation-size arithmetic. The only user-observable effect is which memo dependencies the React Compiler emits.
Level of scrutiny
High. This is a deliberate divergence from upstream ControlDominators.ts in a fixpoint dataflow pass, and the resulting codegen ships to end users' React components. Per REVIEW.md, ported code treats the reference implementation as the spec and deliberate divergence must be justified — the PR does justify it well (doc-comment on ReactiveThrows, extensive differential testing in the description, upstream-fixtures unchanged), but a maintainer should confirm the divergence is wanted and that the transitive-control-dependence rule (stronger than upstream's one-level read) is the right precision trade-off.
Other factors
The implementation looks careful: the double-snapshot termination check avoids the short-circuit trap by binding reactive_map.snapshot() first; .unwrap() on control_tests.get(block_id) is a pre-populated invariant; FxHashSet/FxHashMap come from crate::collections as src/CLAUDE.md requires; the has_handler gate keeps the new work off functions without try/catch. The test is placed in the existing react-compiler.test.ts, uses itBundled, asserts exact stdout, includes a negative contract, and the PR states 9 of 10 lines are stale on main (satisfying the fails-on-USE_SYSTEM_BUN requirement). The known gap (inlined IIFE bodies without MaybeThrow) is disclosed and scoped to a different pass. No issues found by the bug hunt.
The negative check on `!== staticStatus` passes with no effect if the local loses its name in the output. Assert first that the element still reads it by that name.
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Updated 6:07 PM PT - Sep 14th, 2026
✅ @robobun, your commit 73f3c59afd8882cd922fabed9074815a36d18471 passed in 🧪 To try this PR locally: bunx bun-pr 42481That installs a local version of the PR into your bun-42481 --bun |
One line at each place it applies. The long form is in the pull request.
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Review follow-up, two small commits on top of the fix:
The automated review found no bugs. It asks a maintainer to confirm the deliberate difference from upstream |
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Inline comments:
In `@src/react_compiler/inference/infer_reactive_places.rs`:
- Line 484: Update ReactiveThrows::mark_caught_value and the nested-function
propagation and replay paths so caught-binding reactivity is carried alongside
ReactivityMap and reactive_ids. Ensure closures created within catch (error)
mark direct reads of error as reactive and preserve the thrown-value dependency;
add a regression fixture covering this closure scenario.
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…caught error A closure that captures the catch parameter itself is not compiled. One that captures a handler local taken from it is, and has to follow the caught value.
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On the second CodeRabbit finding (a closure in
No source change. All review threads are resolved. |
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I worked on the same report from another side and stop here, because this PR covers it. I did not open a PR. Some results that can help this one: Checked on a build of this branch (dbef50d)
The test has no value block inside That is group (a) of the report. Two components for function TernaryInTry(props) {
let side;
try {
side = props.left ? "left" : "right";
} catch (error) {
side = "caught";
}
return <div>{side}</div>;
}
function TernaryInReturnedJsx(props) {
try {
return <div>{props.left ? "left" : "right"}</div>;
} catch {
return null;
}
}Upstream version 1.0.0 does not compile a value block inside DESIGN.md
My branch robobun/2707f86a/react-compiler-try-catch-reactivity. It has the same rule for a |
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do we really need to allocate more memory
…rows The first version kept, for every block of a function that has a handler, a copy of the block's post-dominator frontier, plus a hash set of blocks. None of that is needed. The blocks that are control-dependent on a block are the post-dominator tree path from each of its branch targets up to its own post-dominator, and that tree is already computed. A block that throws reactively, or that a reactive throw controls, now marks its dependents with that walk. What is left is one flag for each block, allocated only when the function has a handler, and the set of reactive handler bindings. A function with no try / catch allocates nothing new. The precompute loop is the ported code again.
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No, it did not need to. 6dedfda removes it. Before: for each function with a Now: one The stored frontiers were only there to ask "is a block on my frontier a reactive throw". The other direction needs no storage: the blocks that are control-dependent on a block are the post-dominator tree path from each of its branch targets up to its own post-dominator, and that tree is already computed. A block that throws reactively marks its dependents with that walk. Checks:
Time is unchanged. On a function with two nested 150-statement |
`Environment::next_block_id` hands out a new id. Read the first id past the real blocks from the post-dominator tree instead.
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Can we use an AutoBitSet instead of a Vec? |
Up to 127 blocks fit inline, so a typical function with a try / catch allocates nothing for the flags. A larger one takes one bit for each block.
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Yes, done in 2fd1a4d.
I ran a component small enough for the inline arm and one with about 420 blocks for the heap arm. Both follow their props across four renders on one memo cache. |
…t-compiler-catch-reactive
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Problem
bun build --react-compiler --target=browser, a value that depends on whether atryblock throws is memoized with no dependency.try { JSON.parse(p.t) } catch { ok = "invalid" } return <div>{ok}</div>compiles toif ($[0] === __MEMO_CACHE_SENTINEL) t0 = jsxDEV("div", { children: ok }). A second render returns the first element. AtryinuseMemo, JSX built frome.messagein a handler, and a reactiveifinside atrybreak the same way.src/react_compiler/inference/infer_reactive_places.rs:70reads onlyif,branchandswitchtests. AMaybeThrowedge to a handler has no test operand.Fix
MaybeThrowblock with a handler throws reactively when one of its instructions has a reactive input, or when the block is reactive-controlled. It then controls the blocks behind it like a reactiveif, transitively.test/bundler/transpiler/react-compiler.test.ts(TryCatchIsReactiveControlFlow, 9 of 10 lines are stale on main),react-compiler-fixtures.test.ts,bundler_jsx.test.ts.Background
tryblock each instruction ends its block withMaybeThrow { continuation, handler }, so the frontier of a block is mostly the block before it.inference/ports upstreamControlDominators.ts. Upstream has the same bug (babel-plugin-react-compiler 1.0.0). This is a deliberate difference, kept in one struct,ReactiveThrows.Notes
Differential check. 134 components with
try/catch, each rendered 3 to 5 times against one memo cache, compiled output compared with the uncompiled program. With this change 128 match. The 6 that do not are one known gap, below. Main (b993710) is stale on 70 of the 134. Shapes: loops,break/continue/returnin the handler, value blocks,switch, three levels of nesting, atryin acatch,useMemo,useCallback,useState, effect dependency arrays, a mutated, stored, reassigned or returned catch parameter, scopes that span the statement.Real code. 12,864 source files that contain
try, from 22 open-source React codebases (PostHog, Grafana, Supabase, Bluesky, Dub, Metabase, lobe-chat and others), compiled with main and with this change. 1,910 have a compiled function. 11 outputs change. Each gains a dependency on the caught value (e.message) or on a local that thetryor the handler decides (isVerified,isArgumentsStreaming,eligibilityError,error). Example: GrafanaHeatmapPanelbuilds its{ warning }object from the caught exception in the handler. That object was memoized on the sentinel alone, so the first warning stayed. A scanner that resolves each memo guard dependency lexically finds 0 out-of-scope dependencies in all outputs.Memory. A function with no
try/catchallocates nothing new. One that has a handler gets oneAutoBitSetwith a bit for each block (inline up to 127 blocks, so usually no allocation), plus a set with one entry for each reactive catch binding. No frontier is stored: a block that throws reactively marks the blocks that are control-dependent on it by a walk up the post-dominator tree, which the pass already computes. The first version of this PR stored a copy of every frontier. The walk gives the same relation aspost_dominator_frontieron 5,425 compiled functions, and the output is byte-identical to that first version on 1,739 fixture files and 1,145 local files compiled withcompilationMode: "all", so the numbers in these notes carry over.Upstream fixtures. No output in
test/bundler/transpiler/react-compiler-fixtures/changes (1,806 inputs), also not when the 42 fixtures that containtryare compiled with@compilationMode:"all". Upstream has no fixture for this case.Why the handler binding is not marked reactive. An earlier version of this change marked it. The binding temporary is declared before the
trystatement (DeclareLocal Catch), so a scope that spans the statement took it as a dependency and emittedif ($[0] !== props.x || $[1] !== t0) { try {...} catch (t0) {...} }, aReferenceErroron every render.ScopeAroundTryin the test covers this shape.Precision. A
tryblock that reads nothing reactive and always runs adds no dependency (NothingReactiveInTry). Atrystatement inside a reactive-controlled region (after a reactive early return, in a reactive loop) now always has a reactive handler, also when its body reads nothing reactive. That is needed for shapes likefor (const x of props.items) { try { f() } catch { n++; continue } }, and it changes no output in the 12,864 files.Each rule is covered by a line of the test. Old binding rule:
ScopeAroundTrythrowsReferenceError: t0 is not defined. No caught-value rule:CaughtValueandScopeAroundTryare stale on the third render. No transitivity through blocks that do not throw:IfInTryandSplitOnBothSidesare stale. No transitivity between throwing blocks:NestedTryis stale. No seed from a reactive-controlled block:StaticTryInLoopis stale.Known gap, not in this change. An IIFE (or a
useMemocall) that sits inside atryblock and has a branch in its body.InlineImmediatelyInvokedFunctionExpressionsinlines the body withoutMaybeThrowterminals, so the graph has no edge from those blocks to the handler. That is a different pass, and upstream has the same behaviour.Self-review. 4 concerns raised, 3 addressed: the out-of-scope binding (fixed at the source, with a test), coverage of loops /
if/switch/&&insidetrywith a named catch parameter (added), and control dependence read one level deep (made transitive instead of documented). Not taken: a lexical guard incodegen.rsthat skips a function with an out-of-scope dependency. With the binding fixed nothing reaches it, so it would ship untested.[human-review] gate passed · iteration 0 · 2 files touched
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passes on PR (with fix)
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