Migrate negative arbitrary values to negative bare values (#18212)

This PR adds some improvements to the upgrade tool where it can now also
migrate negative arbitrary values to negative bare values.

We already had support for the positive version of this:
```diff
- mb-[32rem]
+ mb-128
```

But now it can also handle negative values:
```diff
- mb-[-32rem]
+ -mb-128
```

The only tricky part here is that we had to hoist the `-` sign. Before
this PR, we were actually generating `mb--128` and that is invalid so it
was thrown out.

## Test plan

1. Added a test to ensure that the negative values are correctly
transformed.

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Upgrade: migrate arbitrary modifiers with values without percentage sign to bare values `/[0.16]` -> `/16` ([#18184](https://github.com/tailwindlabs/tailwindcss/pull/18184))
 - Upgrade: migrate CSS variable shorthand if fallback value contains function call ([#18184](https://github.com/tailwindlabs/tailwindcss/pull/18184))
+- Upgrade: Migrate negative arbitrary values to negative bare values, e.g.: `mb-[-32rem]` → `-mb-128` ([#18212](https://github.com/tailwindlabs/tailwindcss/pull/18212))
 
 ## [4.1.8] - 2025-05-27
 

packages/@tailwindcss-upgrade/src/codemods/template/migrate-arbitrary-utilities.ts

@@ -159,7 +159,12 @@ export function migrateArbitraryUtilities(
         candidate.kind === 'arbitrary' ? candidate.value : (candidate.value?.value ?? null)
       if (value === null) return
 
-      let spacingMultiplier = spacing.get(designSystem)?.get(value)
+      let spacingMultiplier = spacing.get(designSystem)?.get(value) ?? null
+      let rootPrefix = ''
+      if (spacingMultiplier !== null && spacingMultiplier < 0) {
+        rootPrefix = '-'
+        spacingMultiplier = Math.abs(spacingMultiplier)
+      }
 
       for (let root of Array.from(designSystem.utilities.keys('functional')).sort(
         // Sort negative roots after positive roots so that we can try
@@ -167,6 +172,8 @@ export function migrateArbitraryUtilities(
         // `-mt-[0px]` can be translated to `mt-[0px]`.
         (a, z) => Number(a[0] === '-') - Number(z[0] === '-'),
       )) {
+        if (rootPrefix) root = `${rootPrefix}${root}`
+
         // Try as bare value
         for (let replacementCandidate of parseCandidate(designSystem, `${root}-${value}`)) {
           yield replacementCandidate

packages/@tailwindcss-upgrade/src/codemods/template/migrate.test.ts

@@ -69,6 +69,11 @@ describe.each([['default'], ['with-variant'], ['important'], ['prefix']])('%s',
     // the fallback contains functions. The fallback should also be migrated to
     // the newest syntax.
     ['bg-[--my-color,theme(colors.red.500)]', 'bg-(--my-color,var(--color-red-500))'],
+
+    // Both the positive and negative versions of arbitrary utilities should be
+    // converted to the bare value version.
+    ['mb-[32rem]', 'mb-128'],
+    ['mb-[-32rem]', '-mb-128'],
   ])(testName, async (candidate, result) => {
     if (strategy === 'with-variant') {
       candidate = `focus:${candidate}`