ensure DEFAULT is taken into account for matchVariant (#9603)

This means that if you define your `matchVariant` as:

```js
matchVariant('foo', (value) => '.foo-${value} &')
```

Then you can't use `foo:underline`, if you want to be able to use
`foo:underline` then you have to define a `DEFAULT` value:

```js
matchVariant('foo', (value) => '.foo-${value} &', {
  values: {
    DEFAULT: 'bar'
  }
})
```

Now `foo:underline` will generate `.foo-bar &` as a selector!

src/lib/generateRules.js

@@ -145,7 +145,7 @@ function applyVariant(variant, matches, context) {
   }
 
   /** @type {{modifier: string | null, value: string | null}} */
-  let args = { modifier: null, value: null }
+  let args = { modifier: null, value: sharedState.NONE }
 
   // Retrieve "modifier"
   {

src/lib/setupContextUtils.js

@@ -577,6 +577,8 @@ function buildPluginApi(tailwindConfig, context, { variantList, variantMap, offs
       let modifiersEnabled = flagEnabled(tailwindConfig, 'generalizedModifiers')
 
       for (let [key, value] of Object.entries(options?.values ?? {})) {
+        if (key === 'DEFAULT') continue
+
         api.addVariant(
           isSpecial ? `${variant}${key}` : `${variant}-${key}`,
           ({ args, container }) =>
@@ -594,13 +596,20 @@ function buildPluginApi(tailwindConfig, context, { variantList, variantMap, offs
         )
       }
 
+      let hasDefault = 'DEFAULT' in (options?.values ?? {})
+
       api.addVariant(
         variant,
-        ({ args, container }) =>
-          variantFn(
-            args.value,
+        ({ args, container }) => {
+          if (args.value === sharedState.NONE && !hasDefault) {
+            return null
+          }
+
+          return variantFn(
+            args.value === sharedState.NONE ? options.values.DEFAULT : args.value,
             modifiersEnabled ? { modifier: args.modifier, container } : { container }
-          ),
+          )
+        },
         {
           ...options,
           id,

src/lib/sharedState.js

@@ -8,6 +8,8 @@ export const contextSourcesMap = new Map()
 export const sourceHashMap = new Map()
 export const NOT_ON_DEMAND = new String('*')
 
+export const NONE = Symbol('__NONE__')
+
 export function resolveDebug(debug) {
   if (debug === undefined) {
     return false

tests/match-variants.test.js

@@ -656,3 +656,135 @@ it('should guarantee that we are not passing values from other variants to the w
     `)
   })
 })
+
+it('should default to the DEFAULT value for variants', () => {
+  let config = {
+    content: [
+      {
+        raw: html`
+          <div>
+            <div class="foo:underline"></div>
+          </div>
+        `,
+      },
+    ],
+    corePlugins: { preflight: false },
+    plugins: [
+      ({ matchVariant }) => {
+        matchVariant('foo', (value) => `.foo${value} &`, {
+          values: {
+            DEFAULT: '.bar',
+          },
+        })
+      },
+    ],
+  }
+
+  let input = css`
+    @tailwind utilities;
+  `
+
+  return run(input, config).then((result) => {
+    expect(result.css).toMatchFormattedCss(css`
+      .foo.bar .foo\:underline {
+        text-decoration-line: underline;
+      }
+    `)
+  })
+})
+
+it('should not generate anything if the matchVariant does not have a DEFAULT value configured', () => {
+  let config = {
+    content: [
+      {
+        raw: html`
+          <div>
+            <div class="foo:underline"></div>
+          </div>
+        `,
+      },
+    ],
+    corePlugins: { preflight: false },
+    plugins: [
+      ({ matchVariant }) => {
+        matchVariant('foo', (value) => `.foo${value} &`)
+      },
+    ],
+  }
+
+  let input = css`
+    @tailwind utilities;
+  `
+
+  return run(input, config).then((result) => {
+    expect(result.css).toMatchFormattedCss(css``)
+  })
+})
+
+it('should be possible to use `null` as a DEFAULT value', () => {
+  let config = {
+    content: [
+      {
+        raw: html`
+          <div>
+            <div class="foo:underline"></div>
+          </div>
+        `,
+      },
+    ],
+    corePlugins: { preflight: false },
+    plugins: [
+      ({ matchVariant }) => {
+        matchVariant('foo', (value) => `.foo${value === null ? '-good' : '-bad'} &`, {
+          values: { DEFAULT: null },
+        })
+      },
+    ],
+  }
+
+  let input = css`
+    @tailwind utilities;
+  `
+
+  return run(input, config).then((result) => {
+    expect(result.css).toMatchFormattedCss(css`
+      .foo-good .foo\:underline {
+        text-decoration-line: underline;
+      }
+    `)
+  })
+})
+
+it('should be possible to use `undefined` as a DEFAULT value', () => {
+  let config = {
+    content: [
+      {
+        raw: html`
+          <div>
+            <div class="foo:underline"></div>
+          </div>
+        `,
+      },
+    ],
+    corePlugins: { preflight: false },
+    plugins: [
+      ({ matchVariant }) => {
+        matchVariant('foo', (value) => `.foo${value === undefined ? '-good' : '-bad'} &`, {
+          values: { DEFAULT: undefined },
+        })
+      },
+    ],
+  }
+
+  let input = css`
+    @tailwind utilities;
+  `
+
+  return run(input, config).then((result) => {
+    expect(result.css).toMatchFormattedCss(css`
+      .foo-good .foo\:underline {
+        text-decoration-line: underline;
+      }
+    `)
+  })
+})