Improve upgrade migrations (#18184)

This PR fixes 2 issues with the migration tool where certain classes
weren't migrated. This PR fixes those 2 scenarios:

### Scenario 1

When you have an arbitrary opacity modifier that doesn't use `%`, but is
just a number typically between `0` and `1` then this was not converted
to the bare value equivalent before.

E.g.:

```html
<div class="bg-[#f00]/[0.16]"></dv>
```

Will now be converted to:

```html
<div class="bg-[#f00]/16"></dv>
```

### Scenario 2

Fixes a bug when a CSS function was used in a fallback value in the CSS
variable shorthand syntax. In that case we didn't migrate the class to
the new syntax.

This was because we assumed that a `(` was found, that we are dealing
with a CSS function.

E.g.: 
```html
<div class="w-[--spacing(1)]"></div>
                        ^  This indicates a CSS function, we should not be 
                           converting this to `w-(--spacing(1))`
```

But if a function was used as a fallback value, for example:

```html
<div class="bg-[--my-color,theme(colors.red.500)]"></dv>
```

Then we also didn't migrate it, but since the function call is in the
fallback, we can still migrate it.

Will now properly be converted to:

```html
<div class="bg-(--my-color,var(--color-red-500))"></dv>
```


## Test plan

1. Added a test for the first case
2. Added a test for the second case
3. Also added an integration-like test that runs all the migration steps
to make sure that the `theme(…)` in the fallback also gets updated to
`var(…)`. This one caught an issue because the `var(…)` wasn't handling
prefixes correctly.

Author: RobinMalfait

CHANGELOG.md

@@ -7,7 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-- Nothing yet!
+### Fixed
+
+- 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))
 
 ## [4.1.8] - 2025-05-27
 

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

@@ -40,6 +40,11 @@ test.each([
   ['w-[--spacing(5)]', 'w-[--spacing(5)]'],
   ['bg-[--theme(--color-red-500)]', 'bg-[--theme(--color-red-500)]'],
 
+  // Fallback values should be included inside the `var(…)` function
+  ['bg-[--my-color,red]', 'bg-(--my-color,red)'],
+  // Fallback values can contain CSS functions
+  ['bg-[--my-color,theme(spacing.1)]', 'bg-(--my-color,theme(spacing.1))'],
+
   // Some properties never had var() injection in v3.
   ['[scroll-timeline-name:--myTimeline]', '[scroll-timeline-name:--myTimeline]'],
   ['[timeline-scope:--myScope]', '[timeline-scope:--myScope]'],

packages/@tailwindcss-upgrade/src/codemods/template/migrate-automatic-var-injection.ts

@@ -2,6 +2,7 @@ import { walk, WalkAction } from '../../../../tailwindcss/src/ast'
 import { type Candidate, type Variant } from '../../../../tailwindcss/src/candidate'
 import type { Config } from '../../../../tailwindcss/src/compat/plugin-api'
 import type { DesignSystem } from '../../../../tailwindcss/src/design-system'
+import * as ValueParser from '../../../../tailwindcss/src/value-parser'
 
 export function migrateAutomaticVarInjection(
   designSystem: DesignSystem,
@@ -73,9 +74,23 @@ export function migrateAutomaticVarInjection(
 
 function injectVar(value: string): { value: string; didChange: boolean } {
   let didChange = false
-  if (value.startsWith('--') && !value.includes('(')) {
-    value = `var(${value})`
-    didChange = true
+  if (value.startsWith('--')) {
+    // E.g.:
+    //
+    // - `--my-color` → `var(--my-color)`             Convert variable
+    // - `--my-color,red` → `var(--my-color,red)`     Convert variable with fallback
+    // - `--theme(color.red)` → `--theme(color.red)`  Do not convert functions
+    //
+    if (
+      // No `(` definitely means there is no function
+      !value.includes('(') ||
+      // There could be a function call in the fallback value, but it cannot be
+      // top-level, so we can safely check the first part
+      ValueParser.parse(value)[0]?.kind !== 'function'
+    ) {
+      value = `var(${value})`
+      didChange = true
+    }
   } else if (value.startsWith(' --')) {
     value = value.slice(1)
     didChange = true

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

@@ -37,6 +37,9 @@ describe.each([['default'], ['with-variant'], ['important'], ['prefix']])('%s',
     // Use a bare value modifier
     ['bg-red-500/[25%]', 'bg-red-500/25'],
 
+    // Convert 0-1 values to bare values
+    ['bg-[#f00]/[0.16]', 'bg-[#f00]/16'],
+
     // Drop unnecessary modifiers
     ['bg-red-500/[100%]', 'bg-red-500'],
     ['bg-red-500/100', 'bg-red-500'],

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

@@ -55,6 +55,23 @@ export function migrateOptimizeModifier(
         }
       }
 
+      // 3. Try to remove the square brackets, but multiply by 100. E.g.: `[0.16]` -> `16`
+      if (!changed) {
+        let newModifier: NamedUtilityValue = {
+          kind: 'named',
+          value: `${parseFloat(modifier.value) * 100}`,
+          fraction: null,
+        }
+
+        if (
+          targetSignature ===
+          signatures.get(designSystem.printCandidate({ ...candidate, modifier: newModifier }))
+        ) {
+          changed = true
+          candidate.modifier = newModifier
+        }
+      }
+
       return changed ? designSystem.printCandidate(candidate) : rawCandidate
     }
   }

packages/@tailwindcss-upgrade/src/codemods/template/migrate-theme-to-var.ts

@@ -193,6 +193,10 @@ export function createConverter(designSystem: DesignSystem, { prettyPrint = fals
     let variable = `--${keyPathToCssProperty(toKeyPath(path))}` as const
     if (!designSystem.theme.get([variable])) return null
 
+    if (designSystem.theme.prefix) {
+      return `--${designSystem.theme.prefix}-${variable.slice(2)}`
+    }
+
     return variable
   }
 

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

@@ -64,6 +64,11 @@ describe.each([['default'], ['with-variant'], ['important'], ['prefix']])('%s',
     // non-negative version first so we can replace `-mt-[0px]` with `mt-[0px]`.
     ['mt-[0px]', 'mt-[0px]'],
     ['-mt-[0px]', 'mt-[0px]'],
+
+    // Shorthand CSS Variables should be converted to the new syntax, even if
+    // 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))'],
   ])(testName, async (candidate, result) => {
     if (strategy === 'with-variant') {
       candidate = `focus:${candidate}`