fix(formatter): preserve trailing comma in TSX arrow functions with default type params

[jsmecham]

Summary

Fixes #21150

When a single-parameter generic arrow function in a .tsx file has a default type (e.g. <Options = any>), the formatter was incorrectly removing the trailing comma. Prettier preserves it as <Options = any,>.

The existing should_force_trailing_comma_for_arrow_function logic handles the simple case (<T,>) correctly, but it also skipped forcing the comma when the parameter had a default type. Prettier's equivalent only skips the forced comma when a constraint (extends clause) is present — not for default types. This PR aligns with that behavior.

Input

const AppWrapper = <Options = any,>({
  appName,
}: {
  appName: string;
}) => null;

const getProps = <T = Record<string, unknown>,>(
  option: T | undefined,
): Partial<T> => ({});

Before (incorrect)

const AppWrapper = <Options = any>({ appName }: { appName: string }) => null;

const getProps = <T = Record<string, unknown>>(
  option: T | undefined,
): Partial<T> => ({});

After (matches Prettier)

const AppWrapper = <Options = any,>({ appName }: { appName: string }) => null;

const getProps = <T = Record<string, unknown>,>(
  option: T | undefined,
): Partial<T> => ({});

Root cause

In should_force_trailing_comma_for_arrow_function (type_parameters.rs), the early-return condition checked for both a constraint and a default type:

// Before
if params.first().is_some_and(|t| t.constraint().is_some() || t.default().is_some()) {
    return false;
}

Prettier's equivalent only skips forcing the comma when a constraint is present. See Prettier source.

Fix

Removed || t.default().is_some() from the condition:

// After
if params.first().is_some_and(|t| t.constraint().is_some()) {
    return false;
}

Prettier conformance: no regressions (746/753 JS, 591/601 TS — unchanged).

AI Disclosure

This PR was co-authored with Claude Code (AI assistant), as noted in the commit. The fix was reviewed, tested against the full Prettier conformance suite, and verified to produce no regressions.

[codspeed-hq]

Merging this PR will not alter performance

✅ 44 untouched benchmarks
⏩ 7 skipped benchmarks¹

---

_{Comparing jsmecham:fix/tsx-trailing-comma-with-default-type-param (ea5bfe1) with main (eab13b3)²}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (addcd02) during the generation of this report, so eab13b3 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[leaysgur]

Thank you~!

[graphite-app]

Merge activity

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