Do not allow variants to start with - (#18867)

This PR fixes an issue where custom variants with just `-` in the name
were allowed but weren't actually picked up by Oxide so you couldn't use
them anyway.

The reason we allow `-` is for `kebab-style-variants`, which is very
common, but you shouldn't use `-`, `--` or more in a variant name.

It doesn't really solve the issue (#18863), but it fixes the
inconsistencies in that exist today.

Inconsistencies:
|   | `-:flex` | `--:flex` |
| --: | :--: | :--: |
| Oxide | ❌ | ❌ |
| Tailwind Play | ✅ | ❌ |
| Intellisense | ✅ | ✅ |

- Oxide already had the correct rules setup, so this is expected
- Tailwind Play uses Tailwind's core compile step, but it considers
candidates that start with `--` as a CSS variable instead of a utility.
This means that the `--:flex` was considered a CSS variable and skipped
during compilation.
- Intellisense uses the same APIs than Tailwind's core, but it didn't
have the CSS variable check which resulted in the `--:flex` being
"correct".

With this PR, the matrix looks like this now:
|   | `-:flex` | `--:flex` |
| --: | :--: | :--: |
| Oxide | ❌ | ❌ |
| Tailwind Play | ❌ | ❌ |
| Intellisense | ❌ | ❌ |


This should not be considered a breaking change because Oxide didn't
pick up candidates with variants that start with a `-`. CSS for these
candidates was never generated before.

Closes: #18863

---------

Co-authored-by: Jordan Pittman <thecrypticace@gmail.com>

CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Hide internal fields from completions in `matchUtilities` ([#18820](https://github.com/tailwindlabs/tailwindcss/pull/18820))
 - Ignore `.vercel` folders by default (can be overridden by `@source …` rules) ([#18855](https://github.com/tailwindlabs/tailwindcss/pull/18855))
 - Consider variants starting with `@-` to be invalid (e.g. `@-2xl:flex`) ([#18869](https://github.com/tailwindlabs/tailwindcss/pull/18869))
+- Do not allow custom variants to start with a `-` ([#18867](https://github.com/tailwindlabs/tailwindcss/pull/18867))
 - Upgrade: Migrate `aria` theme keys to `@custom-variant` ([#18815](https://github.com/tailwindlabs/tailwindcss/pull/18815))
 - Upgrade: Migrate `data` theme keys to `@custom-variant` ([#18816](https://github.com/tailwindlabs/tailwindcss/pull/18816))
 - Upgrade: Migrate `supports` theme keys to `@custom-variant` ([#18817](https://github.com/tailwindlabs/tailwindcss/pull/18817))

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

@@ -119,7 +119,7 @@ export function buildPluginApi({
     addVariant(name, variant) {
       if (!IS_VALID_VARIANT_NAME.test(name)) {
         throw new Error(
-          `\`addVariant('${name}')\` defines an invalid variant name. Variants should only contain alphanumeric, dashes or underscore characters.`,
+          `\`addVariant('${name}')\` defines an invalid variant name. Variants should only contain alphanumeric, dashes, or underscore characters and start with a lowercase letter or number.`,
         )
       }
 

packages/tailwindcss/src/index.test.ts

@@ -3768,7 +3768,27 @@ describe('@custom-variant', () => {
         @custom-variant foo:bar (&:hover, &:focus);
       `),
     ).rejects.toThrowErrorMatchingInlineSnapshot(
-      `[Error: \`@custom-variant foo:bar\` defines an invalid variant name. Variants should only contain alphanumeric, dashes or underscore characters.]`,
+      `[Error: \`@custom-variant foo:bar\` defines an invalid variant name. Variants should only contain alphanumeric, dashes, or underscore characters and start with a lowercase letter or number.]`,
+    )
+  })
+
+  test('@custom-variant cannot contain dashes on its own', () => {
+    return expect(
+      compileCss(css`
+        @custom-variant - (&.dash);
+      `),
+    ).rejects.toThrowErrorMatchingInlineSnapshot(
+      `[Error: \`@custom-variant -\` defines an invalid variant name. Variants should only contain alphanumeric, dashes, or underscore characters and start with a lowercase letter or number.]`,
+    )
+  })
+
+  test('@custom-variant cannot contain multiple dashes on their own', () => {
+    return expect(
+      compileCss(css`
+        @custom-variant --- (&.dashed);
+      `),
+    ).rejects.toThrowErrorMatchingInlineSnapshot(
+      `[Error: \`@custom-variant ---\` defines an invalid variant name. Variants should only contain alphanumeric, dashes, or underscore characters and start with a lowercase letter or number.]`,
     )
   })
 

packages/tailwindcss/src/index.ts

@@ -356,7 +356,7 @@ async function parseCss(
 
       if (!IS_VALID_VARIANT_NAME.test(name)) {
         throw new Error(
-          `\`@custom-variant ${name}\` defines an invalid variant name. Variants should only contain alphanumeric, dashes or underscore characters.`,
+          `\`@custom-variant ${name}\` defines an invalid variant name. Variants should only contain alphanumeric, dashes, or underscore characters and start with a lowercase letter or number.`,
         )
       }
 

packages/tailwindcss/src/variants.ts

@@ -18,7 +18,7 @@ import { DefaultMap } from './utils/default-map'
 import { isPositiveInteger } from './utils/infer-data-type'
 import { segment } from './utils/segment'
 
-export const IS_VALID_VARIANT_NAME = /^@?[a-zA-Z0-9_-]*$/
+export const IS_VALID_VARIANT_NAME = /^@?[a-z0-9][a-zA-Z0-9_-]*$/
 
 type VariantFn<T extends Variant['kind']> = (
   rule: Rule,