Ensure --alpha(…) is seen as a color, and --spacing(…) is seen as a length

[RobinMalfait]

This PR fixes an issue where intellisense recommends this canonicalization:

- text-[calc(var(--spacing)*4)]
+ text-[--spacing(4)]

Which is correct, but the issue is that the result is different due to ambiguity of the text-* utilities.

.text-\[calc\(var\(--spacing\)\*4\)\] {
  font-size: calc(var(--spacing) * 4);
}
.text-\[--spacing\(4\)\] {
  color: calc(var(--spacing, 0.25rem) * 4);
}

Notice that we're using color all of a sudden? This is because that's the default and we infer the data type based on the arbitrary value. The calc(…) infers that the type is length, but we don't know what the type of --spacing(…) is so we fallback to the default type which would be color.

This PR makes sure that built in functions like --alpha(…) and --spacing(…) are resolved as color and length respectively.

With this fix in place, this is the result:

.text-\[calc\(var\(--spacing\)\*4\)\] {
  font-size: calc(var(--spacing) * 4);
}
.text-\[--spacing\(4\)\] {
  font-size: calc(var(--spacing, 0.25rem) * 4);
}

Fixes: #20256
Fixes: #20258

Test plan

1. Added a regression test to make sure this doesn't happen anymore

[greptile-apps]

Confidence Score: 5/5

The change is minimal and surgical — two small regex additions, each directly tested with snapshot assertions covering the previously-broken cases.

Both --alpha and --spacing are the only built-in Tailwind CSS functions with a well-defined return type (color and length respectively); the third built-in, --theme, is intentionally left out because it is type-agnostic. The fix is consistent with the existing patterns in both files and the regression tests confirm the corrected CSS output.

No files require special attention.

_{Reviews (2): Last reviewed commit: "update changelog" | Re-trigger Greptile}

[coderabbitai]

Walkthrough

Two data-type recognizer functions are updated to correctly classify custom CSS function values. In infer-data-type.ts, a new IS_LENGTH_FN regex is added so that isLength returns true for var(--spacing)(...)-style values. In is-color.ts, the IS_COLOR_FN regex is extended to include the --alpha( prefix, making isColor return true for those values. Test snapshots are extended with text-[--spacing(2)] (generating font-size) and text-[--alpha(red/20%)] (generating color), and a changelog entry is added.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	Title clearly summarizes the main changes: recognizing --alpha(...) as a color function and --spacing(...) as a length function.
Linked Issues check	✅ Passed	The PR fully addresses both linked issues (#20256, #20258) by making --spacing(...) resolve as a length type, ensuring text-[--spacing(n)] generates font-size instead of color.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the data type inference issue: test updates, is-color.ts for --alpha(...), infer-data-type.ts for --spacing(...), and changelog entry.
Description check	✅ Passed	The description clearly explains the issue, the root cause of type inference ambiguity, the solution, and provides concrete before/after examples showing how the fix resolves the problem.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@packages/tailwindcss/src/utils/math-operators.ts`:
- Line 39: The issue is that in the addWhitespaceAroundMathOperators function,
the function-name scanner at lines 95-99 strips hyphens from function names,
causing --spacing( to be parsed as spacing instead of --spacing. When the code
checks MATH_FUNCTIONS.includes(fn) at line 107, it fails to match because it's
checking for spacing instead of --spacing. To fix this, modify the function-name
checking logic in addWhitespaceAroundMathOperators to also check for the
hyphenated version of custom properties (like --spacing) against MATH_FUNCTIONS,
either by preserving the original function name with hyphens before the check or
by adding both the stripped and hyphenated versions to the includes check.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: e9c54fb2-ff61-41ef-a75e-48dc69bbb662

📥 Commits

Reviewing files that changed from the base of the PR and between 34c2b04 and 9ae165b.

📒 Files selected for processing (3)

• packages/tailwindcss/src/utilities.test.ts
• packages/tailwindcss/src/utils/is-color.ts
• packages/tailwindcss/src/utils/math-operators.ts