Ensure --value(…) is required in functional @utility definitions (#20005)

While working on #19989, I noticed that `--value(…)` inside functional
`@utility` definitions is not required right now.

That means that the following CSS is valid:
```css
@utility foo-* {
  color: red;
}
```

But this doesn't really makes sense, because this now accepts a value
and `foo-a`, `foo-b` and `foo-c` would generate the following CSS:
```css
.foo-a {
  color: red;
}
.foo-b {
  color: red;
}
.foo-c {
  color: red;
}
```
The `a`, `b`, and `c` are not doing anything here apart from making your
CSS bigger. So this is very likely an actual bug that you forgot to use
`--value(…)`.

Additionally, if a `--value(…)` was used, but it didn't resolve
anything, then we already properly discared the candidate.

## Test plan

1. Add test to ensure `--value(…)` is required in functional `@utility`
definitions
2. Existing tests pass

CHANGELOG.md

@@ -27,6 +27,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Allow multiple `@utility` definitions with the same name but different value types ([#19777](https://github.com/tailwindlabs/tailwindcss/pull/19777))
 - Export missing `PluginWithConfig` type from `tailwindcss/plugin` to fix errors when inferring plugin config types ([#19707](https://github.com/tailwindlabs/tailwindcss/pull/19707))
 - Ensure `start` and `end` legacy utilities without values do not generate CSS ([#20003](https://github.com/tailwindlabs/tailwindcss/pull/20003))
+- Ensure `--value(…)` is required in functional `@utility` definitions ([#20005](https://github.com/tailwindlabs/tailwindcss/pull/20005))
 
 ## [4.2.4] - 2026-04-21
 

packages/tailwindcss/src/utilities.test.ts

@@ -28870,6 +28870,40 @@ describe('custom utilities', () => {
   })
 
   describe('functional utilities', () => {
+    test('functional utilities require a `--value(…)`', async () => {
+      let input = css`
+        @utility tab-* {
+          tab-size: 4;
+        }
+
+        @tailwind utilities;
+      `
+
+      expect(await compileCss(input, ['tab', 'tab-foo'])).toEqual('')
+    })
+
+    test('functional utilities must resolve at least one `--value(…)`', async () => {
+      let input = css`
+        @utility tab-* {
+          tab-size: --value(integer);
+        }
+
+        @tailwind utilities;
+      `
+
+      expect(await compileCss(input, ['tab-1', 'tab-2'])).toMatchInlineSnapshot(`
+        ".tab-1 {
+          tab-size: 1;
+        }
+
+        .tab-2 {
+          tab-size: 2;
+        }"
+      `)
+
+      expect(await compileCss(input, ['tab', 'tab-foo', 'tab-2.5'])).toEqual('')
+    })
+
     test('resolving values from `@theme`', async () => {
       let input = css`
         @theme reference {

packages/tailwindcss/src/utilities.ts

@@ -6178,14 +6178,14 @@ export function createCssUtility(node: AtRule) {
           let shouldRemoveDeclaration = false
 
           let valueAst = ValueParser.parse(node.value)
-          walk(valueAst, (valueNode) => {
-            if (valueNode.kind !== 'function') return
+          walk(valueAst, (fnNode) => {
+            if (fnNode.kind !== 'function') return
 
             // Value function, e.g.: `--value(integer)`
-            if (valueNode.value === '--value') {
+            if (fnNode.value === '--value') {
               usedValueFn = true
 
-              let resolved = resolveValueFunction(value, valueNode, designSystem)
+              let resolved = resolveValueFunction(value, fnNode, designSystem)
               if (resolved) {
                 resolvedValueFn = true
                 if (resolved.ratio) {
@@ -6203,7 +6203,7 @@ export function createCssUtility(node: AtRule) {
             }
 
             // Modifier function, e.g.: `--modifier(integer)`
-            else if (valueNode.value === '--modifier') {
+            else if (fnNode.value === '--modifier') {
               // If there is no modifier present in the candidate, then the
               // declaration can be removed.
               if (modifier === null) {
@@ -6213,7 +6213,7 @@ export function createCssUtility(node: AtRule) {
 
               usedModifierFn = true
 
-              let replacement = resolveValueFunction(modifier, valueNode, designSystem)
+              let replacement = resolveValueFunction(modifier, fnNode, designSystem)
               if (replacement) {
                 resolvedModifierFn = true
                 return WalkAction.ReplaceSkip(replacement.nodes)
@@ -6233,8 +6233,9 @@ export function createCssUtility(node: AtRule) {
           node.value = ValueParser.toCss(valueAst)
         })
 
-        // Used `--value(…)` but nothing resolved
-        if (usedValueFn && !resolvedValueFn) return null
+        // Functional CSS utilities require `--value(…)`, and one of those
+        // branches must resolve for the candidate to be valid.
+        if (!usedValueFn || !resolvedValueFn) return null
 
         // Used `--modifier(…)` but nothing resolved
         if (usedModifierFn && !resolvedModifierFn) return null