Allow @variant to be used inside addBase (#19480)

This seems like an unintentional omission

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

CHANGELOG.md

@@ -26,6 +26,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Ensure standalone `@tailwindcss/cli` binaries are ignored when scanning for class candidates ([#20139](https://github.com/tailwindlabs/tailwindcss/pull/20139))
 - Ensure class candidates are extracted from Twig `addClass(…)` and `removeClass(…)` calls ([#20198](https://github.com/tailwindlabs/tailwindcss/pull/20198))
 - Don't crash in the Ruby or Vue preprocessors when scanning files containing invalid UTF-8 bytes ([#19588](https://github.com/tailwindlabs/tailwindcss/pull/19588))
+- Allow `@variant` to be used inside `addBase` ([#19480](https://github.com/tailwindlabs/tailwindcss/pull/19480))
 
 ### Changed
 

packages/tailwindcss/src/compat/plugin-api.test.ts

@@ -1478,6 +1478,55 @@ describe('addBase', () => {
       "
     `)
   })
+
+  test('@variant is replaced inside rules using addBase', async () => {
+    expect(
+      await compileCss(
+        css`
+          @plugin "my-plugin";
+          @theme {
+            --breakpoint-supertiny: 128px;
+          }
+        `,
+        {
+          loadModule: async () => ({
+            path: '',
+            base: '/root',
+            module: plugin(function ({ addBase }) {
+              addBase({
+                ':root': {
+                  '@variant supertiny': {
+                    '--PascalCase': '1',
+                    '--camelCase': '1',
+                    '--UPPERCASE': '1',
+                  },
+                  '@variant data-enabled:focus, disabled': {
+                    '--x': '1',
+                  },
+                },
+              })
+            }),
+          }),
+        },
+      ),
+    ).toMatchInlineSnapshot(`
+      "
+      @layer base {
+        @media (min-width: 128px) {
+          :root {
+            --PascalCase: 1;
+            --camelCase: 1;
+            --UPPERCASE: 1;
+          }
+        }
+
+        :root[data-enabled]:focus, :root:disabled {
+          --x: 1;
+        }
+      }
+      "
+    `)
+  })
 })
 
 describe('addVariant', () => {

packages/tailwindcss/src/compat/plugin-api.ts

@@ -13,7 +13,12 @@ import { escape } from '../utils/escape'
 import { inferDataType } from '../utils/infer-data-type'
 import { segment } from '../utils/segment'
 import { toKeyPath } from '../utils/to-key-path'
-import { compoundsForSelectors, IS_VALID_VARIANT_NAME, substituteAtSlot } from '../variants'
+import {
+  compoundsForSelectors,
+  IS_VALID_VARIANT_NAME,
+  substituteAtSlot,
+  substituteAtVariant,
+} from '../variants'
 import { walk, WalkAction } from '../walk'
 import type { ResolvedConfig, UserConfig } from './config/types'
 import { createThemeFn } from './plugin-functions'
@@ -110,6 +115,7 @@ export function buildPluginApi({
       if (referenceMode) return
       let baseNodes = objectToAst(css)
       featuresRef.current |= substituteFunctions(baseNodes, designSystem)
+      featuresRef.current |= substituteAtVariant(baseNodes, designSystem)
       let rule = atRule('@layer', 'base', baseNodes)
       walk([rule], (node) => {
         node.src = src