only migrate @layer in v3 projects

Author: RobinMalfait

packages/@tailwindcss-upgrade/src/codemods/css/migrate-at-layer-utilities.test.ts

@@ -1,10 +1,12 @@
 import dedent from 'dedent'
 import postcss from 'postcss'
-import { describe, expect, it } from 'vitest'
+import { describe, expect, it, vi } from 'vitest'
 import { Stylesheet } from '../../stylesheet'
+import * as versions from '../../utils/version'
 import { formatNodes } from './format-nodes'
 import { migrateAtLayerUtilities } from './migrate-at-layer-utilities'
 import { sortBuckets } from './sort-buckets'
+vi.spyOn(versions, 'isMajor').mockReturnValue(true)
 
 const css = dedent
 

packages/@tailwindcss-upgrade/src/codemods/css/migrate-at-layer-utilities.ts

@@ -2,10 +2,16 @@ import { type AtRule, type Comment, type Plugin, type Rule } from 'postcss'
 import SelectorParser from 'postcss-selector-parser'
 import { segment } from '../../../../tailwindcss/src/utils/segment'
 import { Stylesheet } from '../../stylesheet'
+import * as version from '../../utils/version'
 import { walk, WalkAction, walkDepth } from '../../utils/walk'
 
 export function migrateAtLayerUtilities(stylesheet: Stylesheet): Plugin {
   function migrate(atRule: AtRule) {
+    // Migrating `@layer utilities` to `@utility` is only supported in Tailwind
+    // CSS v3 projects. Tailwind CSS v4 projects could also have `@layer
+    // utilities` but those aren't actual utilities.
+    if (!version.isMajor(3)) return
+
     // Only migrate `@layer utilities` and `@layer components`.
     if (atRule.params !== 'utilities' && atRule.params !== 'components') return