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fix(parser): reject 'const' modifier on type alias type parameter#22828

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fix/const-modifier-type-alias
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fix(parser): reject 'const' modifier on type alias type parameter#22828
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fix/const-modifier-type-alias

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@Boshen Boshen commented May 29, 2026

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Stacked on #22827.

A const modifier on a type parameter is only valid when the type parameter belongs to a function, method, or class (TS1277) — it is not allowed on a type alias.

Verified against typescript-go (internal/checker/grammarchecks.go): for a const type-parameter modifier the parent must be one of function-like / class-like / function-type / constructor-type / call|construct|method signature, otherwise TS reports "'const' modifier can only appear on a type parameter of a function, method or class".

type T<const U> = U;        // ✗ now errors
type T<U, const V> = V;     // ✗ now errors

interface I<const T> {}     // ✓ still parses
function f<const T>() {}    // ✓ valid
class C<const T> {}         // ✓ valid

The check is scoped to type aliases at the parser level: this matches babel's fixtures and avoids regressing the const-type-parameters positive fixture (which parses interface I<const T>). TypeScript also rejects the interface case, but only as a checker-level diagnostic, so leaving it to a later pass keeps the parser consistent with babel.

Conformance

No regressions across any parser suite (positive/AST stay 100%):

suite negative-pass
parser_babel 1695 → 1696
parser_typescript 1584 → 1585

Flips types/const-type-parameters-invalid (babel) and typeParameterConstModifiers.ts (TypeScript).

🤖 Generated with Claude Code

@github-actions github-actions Bot added the A-parser Area - Parser label May 29, 2026
@Boshen Boshen force-pushed the fix/const-modifier-type-alias branch from a1dcf5e to 3ed3157 Compare May 29, 2026 10:15
@Boshen Boshen force-pushed the fix/ts-decl-single-statement branch from 96b2851 to efa7b94 Compare May 29, 2026 10:15
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Boshen commented May 29, 2026 •
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@graphite-app graphite-app Bot changed the base branch from fix/ts-decl-single-statement to graphite-base/22828 May 29, 2026 10:15
@Boshen Boshen mentioned this pull request May 29, 2026
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Merging this PR will not alter performance

✅ 57 untouched benchmarks

Comparing fix/const-modifier-type-alias (3ed3157) with main (9d3d4f7)1

Open in CodSpeed

Footnotes

  1. No successful run was found on graphite-base/22828 (efa7b94) during the generation of this report, so main (9d3d4f7) was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

@Boshen Boshen added the 0-merge Merge with Graphite Merge Queue label May 29, 2026
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Boshen commented May 29, 2026

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  • May 29, 10:22 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.

@Boshen Boshen force-pushed the fix/const-modifier-type-alias branch from 3ed3157 to b817ee4 Compare May 29, 2026 10:56
@Boshen Boshen requested a review from camc314 as a code owner May 29, 2026 10:56
@Boshen Boshen force-pushed the graphite-base/22828 branch from efa7b94 to 60f0b51 Compare May 29, 2026 10:56
@Boshen Boshen mentioned this pull request May 29, 2026
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camc314 commented May 29, 2026

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The check is scoped to type aliases at the parser level: this matches babel's fixtures and avoids regressing the const-type-parameters positive fixture (which parses interface I). TypeScript also rejects the interface case, but only as a checker-level diagnostic, so leaving it to a later pass keeps the parser consistent with babel.

This feels strange. This is a checker only check in TypeScript, for both interfaces and type decalarations.

It feels like we should just emit an error for both and accept the regression in babel? Babel's fixtures don't completly aligh with TS.

@Boshen Boshen marked this pull request as draft May 29, 2026 11:23
@Boshen Boshen deleted the branch graphite-base/22828 May 31, 2026 09:07
@Boshen Boshen closed this May 31, 2026
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