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`parse_paren_expr` tries the parentheses after `async` as arrow function parameters when a ":" follows them in TypeScript. Three parts of that try changed what a call means: - The conversion loop unwrapped spread items in place, so `a ? async(b, ...c) : d` called `async(b, c)`. The loop now works on a copy of each item. - The error for a rest argument with an initializer went to the log at once, so `a ? async(b, ...c = d) : e` failed. It now goes to the list of binding errors, which is reported only when the parentheses are parameters. - Above assignment level a ":" cannot be a return type, but the code reported `Unexpected :` for `a ? 1 + async(b) : d`. The ":" is now left alone there.
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WalkthroughThe TypeScript parser now distinguishes calls to ChangesTypeScript async-call parsing
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| Check name | Status | Explanation |
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| Title check |  |
The title clearly and concisely describes the main parser change: preserving async(...) as a call before a TypeScript conditional-expression colon. |
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Check skipped because no linked issues were found for this pull request. |
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Check skipped because no linked issues were found for this pull request. |
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The description is complete and relevant. It explains the problem, the fix, scope, downsides, and verification results. It does not use the template headings exactly, but it provides the required info… |
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The description is complete and relevant. It explains the problem, the fix, scope, downsides, and verification results. It does not use the template headings exactly, but it provides the required information under equivalent sections.
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Updated 12:24 AM PT - Oct 27th, 2026
✅ @robobun, your commit 4f6bfcf9cd7d757315faf17cfcf0082af98fe240 passed in 🧪 To try this PR locally: bunx bun-pr 45743That installs a local version of the PR into your bun-45743 --bun |
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Status Reproduced on main (5866b5f) with a release build. Save one line as
With this PR (#45743) the three lines print as written. |
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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🟣
src/js_parser/parse/mod.rs— TypeScript users whosea ? async(b, ...c) : dnow keeps its spread get a source-map segment for...cthat points at theasynckeyword, not at the.... src/js_parser/parse/mod.rs:470 wraps each spread item in E::Spread withloc, the paren/async loc passed into parse_paren_expr, and the printer maps that node at src/js_printer/lib.rs:3272. The array-literal path uses the dots loc (src/js_parser/parse/parse_prefix.rs:744) and esbuild uses itemLoc. Fix: capture the lexer loc before consuming...at mod.rs:455-461 and use it at mod.rs:470 so every spread kept in the E::Call fallback (mod.rs:638) maps to its own column.Why this was flagged
Trigger: any TS/TSX file with a spread argument to a call named
asyncthat is followed by:(the exact input this PR starts preserving), reaching parse_paren_expr via parse_async_prefix_expr (src/js_parser/parse/mod.rs:1717). At mod.rs:470 the ESpread node is created withloc, the location of the opening paren orasynckeyword, not the...token. The fallback at mod.rs:638 now places that node in the call args; the printer calls add_source_mapping(expr.loc) for ESpread at src/js_printer/lib.rs:3272, so the generated...maps back to theasynccolumn. On the base branch the TS path dropped the ESpread entirely (wrong output, but no such mapping); the JS path already had the same mapping, which is why it was dismissed as pre-existing. The sibling array-literal path uses dots_loc (parse_prefix.rs:744) and esbuild's parseParenExpr uses itemLoc, so this PR is where the divergence becomes reachable for the TS population it fixes. Remedy: record the item loc before the...token and use it at mod.rs:470.Verification: Triggering condition: a call to a function named
asyncwith a spread argument. src/js_parser/parse/mod.rs:470 builds E::Spread withloc, theasynckeyword's location, not the...token. The printer maps ESpread fromexpr.locat src/js_printer/lib.rs:3272. Line 470 is untouched by this diff; the base already produces the same mislocated ESpread forasync(b, ...c)in.jsfiles.
…ap a spread to its own column A rest argument with an initializer is no reason to skip the check for a return type: `async (...a = 1): T => 0` is an arrow function with a bad parameter, and it gets the same error as before. Only other conversion errors mean that the parentheses are not parameters. `E::Spread` took the location of the parentheses. It now takes the location of its "...", so the spread of a call of `async` maps to its own column.
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The spread location is fixed in 21dc213. |
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Review comments at @test/bundler/transpiler/transpiler.test.js:
- Around line 537-539: Remove the exact-empty stderr assertion from this
subprocess test while keeping stderr drained through proc.stderr.text().
Preserve the stdout and exitCode assertions, with exitCode last.
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📒 Files selected for processing: 5
src/js_parser/p.rssrc/js_parser/parse/mod.rssrc/js_parser/parser.rssrc/jsc/RuntimeTranspilerCache.rstest/bundler/transpiler/transpiler.test.js
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
The follow-up commit (21dc213) addresses the async (...a = 1): T => 0 gap raised in the earlier review: the return-type probe now runs when the only conversion error is a rest initializer, and the new err(...) cases cover it. Beyond the inline findings (both pre-existing), I also checked that the new RestInitializer entry cannot go unreported: parse_paren_expr is the only external caller of convert_expr_to_binding_and_initializer and replays invalid_log once it commits to an arrow, and InvalidLoc::add_error is the only match on InvalidLocTag, so the new variant is handled everywhere.
Extended reasoning...
The PR changes TypeScript arrow-vs-call disambiguation in src/js_parser/parse/mod.rs (immutable items slice, level gate on the : probe, RestInitializer deferred into invalid_log), adds the enum variant in parser.rs, bumps EXPECTED_VERSION to 36, and adds transpiler and sourcemap tests; it touches no security-sensitive surface. The earlier-flagged lost-error case is fixed in the second commit and tested. The two remaining findings are pre-existing and do not stem from this diff, but since this alters core parser control flow a maintainer look is still warranted rather than an automated approve.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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src/js_parser/parse/mod.rs— pre-existing nit: TypeScript users writinga ? async<T>(b) : c => dora ? <T>(b) : c => dstill getExpected ":" but found ";", while tsc accepts both as a call or cast before:. The sibling callers at mod.rs:1746 and parse_prefix.rs:969 pass default opts, so parse_paren_expr never sets is_after_question_and_before_colon and the cheap probe at mod.rs:593 accepts: c =>as a return type. Fix: every caller of parse_paren_expr that can sit between?and:should forwardflags == EFlags::AfterQuestionAndBeforeColon, as mod.rs:1727 and parse_prefix.rs:68 already do, which covers both sites. Theasync<T>(...b) : ctest at transpiler.test.js:492 only covers the case without a following=>.Why this was flagged
Input
x = a ? async<T>(b) : c => din a .ts file. The conditional parses its yes branch with EFlags::AfterQuestionAndBeforeColon (parse_suffix.rs:456-460). mod.rs:1746-1755 calls parse_paren_expr with only is_async and force_arrow_fn set; is_after_question_and_before_colon stays false. In parse_paren_expr the token after)is:at Level::Comma, the binding conversion succeeds, and mod.rs:580 is false, so mod.rs:593 runs try_skip_type_script_arrow_return_type_with_backtracking, which sees: c =>and returns true. The arrow bodydis consumed, parse_suffix then expects the conditional's:and finds;. tsc instead rejects the arrow (no:after the body with allowReturnTypeInArrowFunction false) and parsesasync<T>(b)as a call withc => das the else branch. The same path exists fora ? <T>(b) : c => dvia parse_prefix.rs:965-977. The base branch behaves the same way; this PR's test matrix includesasync<T>(...b) : cbut not the=> dcontinuation that trips this path.Verification: pre-existing (nit): the base fails the same way by the same route. Trigger: a .ts/.tsx file containing
a ? async<T>(b) : c => dora ? <T>(b) : c => d. At src/js_parser/parse/mod.rs:1746-1755is_after_question_and_before_colonstays false. Same for the non-async<T>(path at src/js_parser/parse/parse_prefix.rs:969-977.
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About |
…d arrow function The check for "only rest initializers" put a loop in front of the return type check, and LLVM then left that check out of line in `parse_paren_expr`. The loop is now in a cold function that runs only when the conversion has errors. test: assert the result of the spawned process as one object
…c-call-before-colon
…c-call-before-colon
…c-call-before-colon # Conflicts: # src/jsc/RuntimeTranspilerCache.rs
Problem
.tsfile,a ? async(b, ...c) : dcallsasync(b, c). The spread is lost with no error.:followsasync(...),parse_paren_exprfirst tries the parentheses as arrow function parameters. Its loop unwraps each spread in place (src/js_parser/parse/mod.rs:540), and the call reuses the items.a ? async(b, ...c = d) : e(A rest argument cannot have a default initializer,src/js_parser/p.rs:4036) anda ? 1 + async(b) : d(Unexpected :,mod.rs:522).Fix
invalid_log, which is reported only for parameters (from js_parser: reject a parenthesized assignment as a destructuring target #41697). Above assignment level, a:after)is not tried as a return type.E::Spreadtakes the location of its..., so a spread argument ofasync(...)maps to its own column in a source map, also in.jsfiles.test/bundler/transpiler/transpiler.test.js(2 new blocks fail on main). Of 166,340 generated conditionals, 6,851 change, each only by the restored....Background
async (a) => bandasync(a)start alike. The parser reads the parentheses as expressions and converts them to bindings when=>follows, and in TypeScript also when:follows (: T =>can be a return type).invalid_logholds the errors of that conversion. They are reported only for an arrow function.Downsides
a ? 1 + <T>(b) : cand theasyncforms above were errors and parse now. tsc accepts them..text+256 B. No new call, allocation or token on a path that parses today.EXPECTED_VERSIONis 44, so each cached file is transpiled once more.Notes
Reproduction
main prints
a+b,c. This PR printsa+b+c. The same text in a.jsfile is correct on main..ts)x = a ? async(b, ...c) : dx = a ? async(b, c) : d...cx = a ? async(b, ...c = d) : eA rest argument cannot have a default initializerx = a ? ([...b = c]) : dx = a ? !async(b, ...c) : dUnexpected :x = a ? 1 + async(b) : dUnexpected :x = a ? 1 + <T>(b) : cUnexpected :x = (...a = 1) => 0A rest argument cannot have a default initializerx = async (...a = 1): T => 0A rest argument cannot have a default initializerx = 1 + async(b) => cUnexpected =>Why each part is here
E::Spreaditems were replaced by their value while the parser only tried the conversion.convert_expr_to_binding_and_initializerwrote it to the log at once. Its other errors go toinvalid_log. With the loop fixed alone,a ? async(b, ...c = d) : ewould still fail. js_parser: reject a parenthesized assignment as a destructuring target #41697 has the same hunk for another reason and stays open for its own fix.async (...a = 1): T => 0is an arrow function with a bad parameter. The check for a return type still runs when the only conversion errors are rest initializers, so this input keeps its error. That test is in a cold function: with a loop in the condition, LLVM left the return type check out of line inparse_paren_expr, and every arrow function with a return type paid a call.E::Spread. It was the location of the parentheses, which is theasynckeyword for a call. The printer maps that node, so...cpointed atasyncin a source map. It did so on main forasync(b, ...c)in a.jsfile. It is now the location of the....parse_async_prefix_exprcallsparse_paren_exprat every precedence level. The block that handles=>and:starts withif level.gt(Level::Assign) { unexpected }, which is right for=>and wrong for a:that cannot be a return type there. tsc tries an arrow function only at assignment level.Checks
Bun.Transpiler.transformSyncagainstts.transpileModule(tsc 6.0.2), outputs compared as acorn trees:.ts: tsc accepts 75,336. Same tree as tsc for 70,844 on main and 70,844 here. 6,851 outputs differ from main, each only by.... tsc rejects those inputs (it readsasync(...b) :as the start of an arrow function, Parser misinterprets async() calls in conditional expressions as async arrow functions microsoft/TypeScript#64231), and 1,403 of them are legal JavaScript that now get the JavaScript tree..tsx: the same 6,851.test/bundler/transpiler/transpiler.test.js: 240 pass with the debug build and with a release build. With a release build of main, 238 pass and the 2 new blocks fail.size -A,nm -S):.text81,071,045 to 81,071,301 B.parse_paren_expr+102 B and -15 B (TypeScript copies), +46 B and +73 B (JavaScript copies).convert_expr_to_binding-65 B per copy. The check for a return type and the arrow function attempt are inlined inparse_paren_expr, as on main.invalid_log, and stop the:check above assignment level. All three are in this PR.perfandvalgrindare not available there.Not in this PR
a ? b => (c) : d => ein a.tsfile still fails withExpected ":" but found ";". That is the mark of the conditional, which stops at arrow function bodies. It is a separate change.