Show suggestions for known matchVariant values (#18798)

Given this variant:
```js
matchVariant(
  "foo",
  (value) => `&:is([data-foo='${value}'])`,
  {
    values: {
      DEFAULT: "",
      bar: "bar",
      baz: "bar",
    },
  }
)
```

We weren't listing `foo-bar` and `foo-baz` in IntelliSense. This PR
fixes that.

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Don't transition `visibility` when using `transition` ([#18795](https://github.com/tailwindlabs/tailwindcss/pull/18795))
 - Discard matched variants with unknown named values ([#18799](https://github.com/tailwindlabs/tailwindcss/pull/18799))
 - Discard matched variants with non-string values ([#18799](https://github.com/tailwindlabs/tailwindcss/pull/18799))
+- Show suggestions for known `matchVariant` values ([#18798](https://github.com/tailwindlabs/tailwindcss/pull/18798))
 
 ## [4.1.12] - 2025-08-13
 

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

@@ -247,6 +247,10 @@ export function buildPluginApi({
           return aValue < zValue ? -1 : 1
         },
       )
+
+      designSystem.variants.suggest(name, () =>
+        Object.keys(options?.values ?? {}).filter((v) => v !== 'DEFAULT'),
+      )
     },
 
     addUtilities(utilities) {

packages/tailwindcss/src/intellisense.test.ts

@@ -675,3 +675,40 @@ test('Custom @utility and existing utility with names matching theme keys dont g
   expect(matches).toHaveLength(1)
   expect(classMap.get('text-header')?.modifiers).toEqual(['sm'])
 })
+
+test('matchVariant', async () => {
+  let input = css`
+    @import 'tailwindcss/utilities';
+    @plugin "./plugin.js";
+  `
+
+  let design = await __unstable__loadDesignSystem(input, {
+    loadStylesheet: async (_, base) => ({
+      path: '',
+      base,
+      content: '@tailwind utilities;',
+    }),
+    loadModule: async () => ({
+      path: '',
+      base: '',
+      module: plugin(({ matchVariant }) => {
+        matchVariant('foo', (val) => `&:is(${val})`, {
+          values: {
+            DEFAULT: '1',
+            a: 'a',
+            b: 'b',
+          },
+        })
+      }),
+    }),
+  })
+
+  let variants = design.getVariants()
+  let v1 = variants.find((v) => v.name === 'foo')!
+  expect(v1).not.toBeUndefined()
+
+  expect(v1.hasDash).toEqual(true)
+  expect(v1.isArbitrary).toEqual(true)
+  expect(v1.name).toEqual('foo')
+  expect(v1.values).toEqual(['a', 'b'])
+})