Re-scan '<<' when parsing type arguments in heritage clauses, typeof queries, and JSX by ahmad65saad · Pull Request #64716 · microsoft/TypeScript

ahmad65saad · 2026-10-10T02:47:32Z

The scanner produces << as a single token, and the parser is expected to call reScanLessThanToken() wherever a < may open a type argument list. parseTypeArgumentsOfTypeReference and tryParseTypeArgumentsInExpression already do this, but parseTypeArguments only checked for <. So when the first type argument itself starts with < (e.g. a generic function type), the list was never parsed and the rest was misread as a shift/relational expression:

declare const f: <T>(x: T) => T;
declare class B<F> {}
interface I<F> {}
interface J<F> {}

class C1 implements I<<T>(x: T) => T> {}   // error before, OK now
class C2 extends B<<T>(x: T) => T> {}      // error before, OK now
interface I2 extends J<<T>(x: T) => T> {}  // error before, OK now
type A1 = typeof f<<T>(x: T) => T>;        // error before, OK now
<Comp<<T>(x: T) => T> value={f} />;        // error before, OK now

const e = f<<T>(x: T) => T>;               // already worked (expression path re-scans)

Fix

parseTypeArguments now re-scans << before checking for <. This covers heritage clauses (parseExpressionWithTypeArguments) and JSX opening/self-closing elements. In those positions << can't be a left shift, so no valid code changes meaning. JSDoc @augments/@implements also go through parseTypeArguments, but the JSDoc scanner never produces << there, so they already worked; jsdocAugments_genericFunctionTypeArgument.ts pins that (its baselines are identical with and without this change).
typeof type queries are the exception: there << can legitimately be a left shift after a type in an expression, e.g. a as typeof x << 1, which compiles today. So in parseTypeQuery, when the token is <<, the type arguments are parsed speculatively (mark/rewind, same diagnostics-count check as skipParameterStart) and dropped if they don't parse cleanly.

Tests

New test parseGenericArrowRatherThanLeftShift2.tsx covers each position above, plus the instantiation/call expression forms, and ordinary << shift expressions (including inside JSX attributes and after as typeof / satisfies typeof) to make sure they still parse as shifts. The first commit adds the test with the old (erroring) baselines so the change is visible in the second commit's diff. No other baselines changed.

Note: the JSDoc test's baseline includes TS8023 (JSDoc '@augments A' does not match the 'extends A' clause). That also happens on main without this change: checkJSDocAugmentsTagMatchesExtends compares the two A<<T>(x: T) => T> types with isTypeIdenticalTo, and the separately written generic function types aren't identical. It's a checker issue unrelated to parsing, so I left it out of scope here.

npx hereby test ✅
npx hereby lint ✅
npx hereby check:format ✅
npx hereby validate ✅

This PR was written in part with the assistance of generative AI (Claude).

parseTypeArguments only checked for a '<' token, so a type argument list whose first type starts with '<' (e.g. a generic function type) was scanned as '<<' and never parsed. This affected heritage clauses, typeof type queries, and JSX opening elements. Re-scan the token first, as the type reference and expression paths already do. In a typeof type query, '<<' can also be a left shift following a type in an expression, as in `x as typeof y << 1`, which already compiles. There the type arguments are parsed speculatively and only kept if they parse without errors; otherwise the '<<' is left alone. Fixes microsoft#47410

Copilot

🟡 Changes recommended

The newly affected JSDoc parsing path lacks regression coverage.

1 open finding

Add JSDoc regression coverage for generic function type arguments · New

What changed in this PR

Fixes parsing of generic function types beginning with << in type arguments while preserving left-shift expressions.

Changes:

Re-scans << when parsing type arguments.
Speculatively handles ambiguous typeof queries.
Adds regression coverage and baselines.

File	Description
`tsc/internal/parser/parser.go`	Updates type-argument parsing and ambiguity handling.
`tsc/testdata/tests/cases/compiler/parseGenericArrowRatherThanLeftShift2.tsx`	Adds parser regression cases.
`tsc/testdata/baselines/reference/compiler/parseGenericArrowRatherThanLeftShift2.types`	Records inferred types.
`tsc/testdata/baselines/reference/compiler/parseGenericArrowRatherThanLeftShift2.symbols`	Records resolved symbols.
`tsc/testdata/baselines/reference/compiler/parseGenericArrowRatherThanLeftShift2.js`	Records emitted JavaScript.

🧠 Review effort: Balanced


 func (p *Parser) parseTypeArguments() *ast.NodeList {
-	if p.token == ast.KindLessThanToken {
+	if p.reScanLessThanToken() == ast.KindLessThanToken {


ahmad65saad · 2026-10-10T03:47:28Z

@microsoft-github-policy-service agree

…ument

Copilot

🟢 Approval recommended

The parser ambiguity is handled conservatively and the affected contexts have focused regression coverage.

1 open finding

Add JSDoc regression coverage for generic function type arguments

🧠 Review effort: Balanced

Jake Bailey (jakebailey) · 2026-10-13T20:54:31Z

I think this is right and esbuild also accepts this syntax, which is good.

TypeScript Bot (@typescript-bot) test it

typescript-automation · 2026-10-13T20:54:38Z

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
`test top400`	✅ Started	✅ Results
`user test this`	✅ Started	✅ Results
`run dt`	✅ Started	✅ Results
`perf test this faster`	✅ Started	👀 Results

typescript-automation · 2026-10-13T21:17:57Z

Hey Jake Bailey (@jakebailey), the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

typescript-automation · 2026-10-13T21:24:07Z

Jake Bailey (@jakebailey) Here are the results of running the user tests with tsc comparing baseline and pr:

Everything looks good!

typescript-automation · 2026-10-13T21:24:12Z

Jake Bailey (@jakebailey)
The results of the perf run you requested are in!

Here they are:

tsc

Comparison Report - baseline..pr

_Metric	_baseline	_pr	_Delta	_Best	_Worst	_p-value
_{Compiler-Unions - native}
_Errors	₄₁	₄₁	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_115,276	_115,276	_~	_~	_~	_{p=1.000 n=12}
_Types	_79,613	_79,613	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{133,224k (± 0.07%)}	_{133,213k (± 0.06%)}	_~	_133,044k	_133,473k	_{p=0.932 n=12}
_{Memory Allocs}	_{2,190,272 (± 0.01%)}	_{2,190,377 (± 0.01%)}	_~	_2,190,185	_2,190,911	_{p=0.219 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.041s (± 3.18%)}	_{0.041s (± 2.74%)}	_~	_0.039s	_0.044s	_{p=0.459 n=12}
_{Bind Time}	_{0.012s (± 7.61%)}	_{0.013s (±10.84%)}	_~	_0.010s	_0.016s	_{p=0.450 n=12}
_{Check Time}	_{0.506s (± 0.73%)}	_{0.499s (± 0.81%)}	_{-0.007s (- 1.40%)}	_0.492s	_0.511s	_{p=0.016 n=12}
_{Emit Time}	_{0.268s (± 2.47%)}	_{0.272s (± 1.91%)}	_~	_0.253s	_0.281s	_{p=0.297 n=12}
_{Total Time}	_{0.834s (± 0.86%)}	_{0.831s (± 0.73%)}	_~	_0.813s	_0.852s	_{p=0.523 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{845,733 (± 0.04%)}	_{846,106 (± 0.06%)}	_~	_845,146	_848,383	_{p=0.347 n=12}
_Types	_{248,125 (± 0.00%)}	_{248,125 (± 0.00%)}	_~	_248,122	_248,134	_{p=0.918 n=12}
_{Memory Used}	_{746,089k (± 0.02%)}	_{746,138k (± 0.01%)}	_~	_745,885k	_746,283k	_{p=0.579 n=12}
_{Memory Allocs}	_{12,913,075 (± 0.02%)}	_{12,914,938 (± 0.04%)}	_~	_12,908,663	_12,932,026	_{p=0.671 n=12}
_{Config Time}	_0.017s	_0.017s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.249s (± 3.04%)}	_{0.258s (± 2.34%)}	_{+0.009s (+ 3.61%)}	_0.239s	_0.267s	_{p=0.046 n=12}
_{Bind Time}	_{0.068s (±13.25%)}	_{0.061s (± 7.63%)}	_{🟩-0.007s (-10.43%)}	_0.056s	_0.078s	_{p=0.042 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.562s (± 1.07%)}	_{1.559s (± 0.91%)}	_~	_1.516s	_1.604s	_{p=0.943 n=12}
_{Total Time}	_{1.911s (± 1.21%)}	_{1.907s (± 0.88%)}	_~	_1.863s	_1.974s	_{p=0.477 n=12}
_{mui-docs - native}
_Errors	_11,405	_11,405	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_4,339,337	_4,339,337	_~	_~	_~	_{p=1.000 n=12}
_Types	_1,379,387	_1,379,387	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{2,774,842k (± 0.06%)}	_{2,775,102k (± 0.06%)}	_~	_2,770,268k	_2,779,406k	_{p=0.671 n=12}
_{Memory Allocs}	_{32,721,445 (± 0.05%)}	_{32,725,483 (± 0.04%)}	_~	_32,692,199	_32,762,903	_{p=0.630 n=12}
_{Config Time}	_0.017s	_0.017s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.497s (± 2.85%)}	_{0.498s (± 2.21%)}	_~	_0.467s	_0.520s	_{p=0.810 n=12}
_{Bind Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{8.874s (± 0.59%)}	_{8.849s (± 0.61%)}	_~	_8.764s	_9.027s	_{p=0.551 n=12}
_{Emit Time}	_{0.473s (±10.89%)}	_{0.460s (±11.84%)}	_~	_0.419s	_0.654s	_{p=0.468 n=12}
_{Total Time}	_{10.636s (± 0.55%)}	_{10.602s (± 0.56%)}	_~	_10.483s	_10.749s	_{p=0.660 n=12}
_{strada-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,388,669	_1,388,669	_~	_~	_~	_{p=1.000 n=12}
_Types	_441,862	_441,862	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{1,598,704k (± 0.45%)}	_{1,594,161k (± 0.44%)}	_~	_1,577,681k	_1,614,957k	_{p=0.242 n=12}
_{Memory Allocs}	_{82,453,841 (± 0.08%)}	_{82,408,345 (± 0.14%)}	_~	_82,023,659	_82,537,415	_{p=0.630 n=12}
_{Config Time}	_{0.003s (±16.58%)}	_{0.003s (±14.42%)}	_~	_0.002s	_0.004s	_{p=0.384 n=12}
_{Parse Time}	_{0.255s (± 2.49%)}	_{0.261s (± 1.95%)}	_~	_0.251s	_0.273s	_{p=0.272 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.793s (± 0.61%)}	_{1.799s (± 0.53%)}	_~	_1.765s	_1.820s	_{p=0.266 n=12}
_{Emit Time}	_{0.278s (± 4.79%)}	_{0.275s (± 1.88%)}	_~	_0.261s	_0.291s	_{p=0.723 n=12}
_{Total Time}	_{20.568s (± 0.52%)}	_{20.787s (± 0.53%)}	_{+0.219s (+ 1.07%)}	_20.489s	_21.138s	_{p=0.011 n=12}
_{strada-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_334,953	_334,953	_~	_~	_~	_{p=1.000 n=12}
_Types	_197,616	_197,616	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{296,038k (± 0.03%)}	_{296,092k (± 0.03%)}	_~	_295,886k	_296,373k	_{p=0.682 n=12}
_{Memory Allocs}	_{4,622,032 (± 0.01%)}	_{4,621,491 (± 0.01%)}	_~	_4,620,893	_4,622,820	_{p=0.104 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.112s (± 2.96%)}	_{0.116s (± 3.30%)}	_{+0.004s (+ 3.72%)}	_0.103s	_0.125s	_{p=0.046 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.041s (± 0.66%)}	_{1.045s (± 0.52%)}	_~	_1.031s	_1.060s	_{p=0.163 n=12}
_{Emit Time}	_{0.144s (± 9.19%)}	_{0.148s (± 6.29%)}	_~	_0.125s	_0.176s	_{p=0.876 n=12}
_{Total Time}	_{1.354s (± 0.96%)}	_{1.361s (± 1.02%)}	_~	_1.317s	_1.396s	_{p=0.451 n=12}
_{ts-pre-modules - native}
_Errors	₈₇	₈₇	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_303,834	_303,834	_~	_~	_~	_{p=1.000 n=12}
_Types	_181,663	_181,663	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{256,642k (± 0.02%)}	_{256,666k (± 0.02%)}	_~	_256,588k	_256,862k	_{p=0.434 n=12}
_{Memory Allocs}	_{1,614,062 (± 0.01%)}	_{1,614,167 (± 0.01%)}	_~	_1,613,450	_1,614,804	_{p=0.478 n=12}
_{Config Time}	_0.000s	_{0.000s (±217.90%)}	_~	_0.000s	_0.001s	_{p=1.000 n=12}
_{Parse Time}	_{0.101s (± 4.68%)}	_{0.099s (± 4.96%)}	_~	_0.090s	_0.111s	_{p=0.599 n=12}
_{Bind Time}	_{0.037s (± 9.89%)}	_{0.034s (± 7.88%)}	_~	_0.028s	_0.041s	_{p=0.114 n=12}
_{Check Time}	_{0.818s (± 0.99%)}	_{0.818s (± 0.59%)}	_~	_0.812s	_0.831s	_{p=1.000 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.970s (± 0.92%)}	_{0.964s (± 0.80%)}	_~	_0.945s	_0.983s	_{p=0.434 n=12}
_{vscode - native}
_Errors	₃₉₆	₃₉₆	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_11,386,701	_11,386,701	_~	_~	_~	_{p=1.000 n=12}
_Types	_3,759,032	_3,759,032	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{6,490,472k (± 0.01%)}	_{6,490,166k (± 0.01%)}	_~	_6,488,769k	_6,493,425k	_{p=0.291 n=12}
_{Memory Allocs}	_{55,749,504 (± 0.01%)}	_{55,754,889 (± 0.02%)}	_~	_55,741,722	_55,783,485	_{p=0.551 n=12}
_{Config Time}	_{0.075s (± 0.56%)}	_{0.075s (± 0.43%)}	_~	_0.075s	_0.076s	_{p=1.000 n=12}
_{Parse Time}	_{1.923s (± 3.84%)}	_{1.816s (± 3.04%)}	_{🟩-0.107s (- 5.56%)}	_1.712s	_1.961s	_{p=0.010 n=12}
_{Bind Time}	_{0.565s (±23.04%)}	_{0.645s (±17.24%)}	_~	_0.451s	_0.950s	_{p=0.103 n=12}
_{Check Time}	_{13.586s (± 1.26%)}	_{13.430s (± 1.54%)}	_~	_13.079s	_13.823s	_{p=0.340 n=12}
_{Emit Time}	_{4.697s (±11.42%)}	_{5.150s (±10.43%)}	_~	_4.109s	_6.147s	_{p=0.291 n=12}
_{Total Time}	_{20.927s (± 2.07%)}	_{21.210s (± 1.93%)}	_~	_20.453s	_22.116s	_{p=0.242 n=12}
_{webpack - native}
_Errors	₅₈₅	₅₈₅	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,325,855	_1,325,855	_~	_~	_~	_{p=1.000 n=12}
_Types	_627,988	_627,988	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{951,170k (± 0.01%)}	_{951,293k (± 0.01%)}	_~	_950,927k	_951,584k	_{p=0.139 n=12}
_{Memory Allocs}	_{6,740,654 (± 0.04%)}	_{6,739,402 (± 0.04%)}	_~	_6,735,724	_6,747,408	_{p=0.242 n=12}
_{Config Time}	_0.009s	_{0.009s (± 3.96%)}	_~	_0.009s	_0.011s	_{p=1.000 n=12}
_{Parse Time}	_{0.296s (± 4.76%)}	_{0.320s (± 2.97%)}	_{🔻+0.024s (+ 7.97%)}	_0.294s	_0.345s	_{p=0.008 n=12}
_{Bind Time}	_{0.103s (±16.96%)}	_{0.084s (±20.65%)}	_{🟩-0.019s (-18.24%)}	_0.066s	_0.151s	_{p=0.023 n=12}
_{Check Time}	_{1.944s (± 0.68%)}	_{1.989s (± 2.87%)}	_~	_1.921s	_2.245s	_{p=0.193 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{2.389s (± 0.46%)}	_{2.438s (± 2.39%)}	_~	_2.361s	_2.681s	_{p=0.117 n=12}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,062,854	_1,062,854	_~	_~	_~	_{p=1.000 n=12}
_Types	_390,845	_390,845	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{548,158k (± 0.02%)}	_{548,057k (± 0.03%)}	_~	_547,816k	_548,625k	_{p=0.114 n=12}
_{Memory Allocs}	_{4,889,883 (± 0.07%)}	_{4,891,081 (± 0.07%)}	_~	_4,878,762	_4,899,820	_{p=0.443 n=12}
_{Config Time}	_0.003s	_0.003s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.145s (± 2.90%)}	_{0.141s (± 3.77%)}	_~	_0.129s	_0.155s	_{p=0.234 n=12}
_{Bind Time}	_{0.042s (±14.84%)}	_{0.055s (±19.86%)}	_{🔻+0.014s (+33.00%)}	_0.033s	_0.075s	_{p=0.046 n=12}
_{Check Time}	_{1.020s (± 0.58%)}	_{1.022s (± 0.36%)}	_~	_1.014s	_1.036s	_{p=0.944 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.219s (± 0.93%)}	_{1.229s (± 0.46%)}	_~	_1.211s	_1.241s	_{p=0.164 n=12}

System info unknown

Hosts

native

Scenarios

Compiler-Unions - native
angular-1 - native
mui-docs - native
strada-build-src - native
strada-compiler - native
ts-pre-modules - native
vscode - native
webpack - native
xstate-main - native

_Benchmark	_Name	_Iterations
_Current	_pr	₁₂
_Baseline	_baseline	₁₂

Developer Information:

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typescript-automation · 2026-10-13T22:08:38Z

Jake Bailey (@jakebailey) Here are the results of running the top 400 repos with tsc comparing baseline and pr:

Everything looks good!

ahmad65saad added 2 commits October 10, 2026 05:41

Add test for type arguments starting with '<<'

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github-project-automation Bot added this to PR Backlog Oct 10, 2026

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Comment thread tsc/internal/parser/parser.go

func (p *Parser) parseTypeArguments() *ast.NodeList {

if p.token == ast.KindLessThanToken {

if p.reScanLessThanToken() == ast.KindLessThanToken {

Add JSDoc @augments/@implements case with a generic function type arg…

65e2afc

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Jake Bailey (jakebailey) requested a balanced review from Copilot October 13, 2026 20:43

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Re-scan '<<' when parsing type arguments in heritage clauses, typeof queries, and JSX - #64716

Re-scan '<<' when parsing type arguments in heritage clauses, typeof queries, and JSX#64716
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ahmad65saad:fix-shift-token-in-type-arguments

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