Allow variants to be overridden (#14008)

* Allow variants to be overridden

* Update changelog

* Update changelog

* Update tests

CHANGELOG.md

@@ -17,8 +17,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-- Add support for basic `addVariant` plugins with new `@plugin` directive ([#13982](https://github.com/tailwindlabs/tailwindcss/pull/13982))
-- Add `@variant` at-rule for defining custom variants in CSS ([#13992](https://github.com/tailwindlabs/tailwindcss/pull/13992))
+- Add support for basic `addVariant` plugins with new `@plugin` directive ([#13982](https://github.com/tailwindlabs/tailwindcss/pull/13982), [#14008](https://github.com/tailwindlabs/tailwindcss/pull/14008))
+- Add `@variant` at-rule for defining custom variants in CSS ([#13992](https://github.com/tailwindlabs/tailwindcss/pull/13992), [#14008](https://github.com/tailwindlabs/tailwindcss/pull/14008))
 
 ## [4.0.0-alpha.17] - 2024-07-04
 

packages/tailwindcss/src/index.test.ts

@@ -1301,6 +1301,47 @@ describe('plugins', () => {
       }"
     `)
   })
+
+  test('built-in variants can be overridden while keeping their order', () => {
+    let compiled = compile(
+      css`
+        @plugin "my-plugin";
+        @layer utilities {
+          @tailwind utilities;
+        }
+      `,
+
+      {
+        loadPlugin: () => {
+          return ({ addVariant }) => {
+            addVariant('dark', '&:is([data-theme=dark] *)')
+          }
+        },
+      },
+    ).build(
+      // Make sure the order does not change by including the variants
+      // immediately before and after `dark`
+      ['rtl:flex', 'dark:flex', 'starting:flex'],
+    )
+
+    expect(optimizeCss(compiled).trim()).toMatchInlineSnapshot(`
+      "@layer utilities {
+        .rtl\\:flex:where([dir="rtl"], [dir="rtl"] *) {
+          display: flex;
+        }
+
+        .dark\\:flex:is([data-theme="dark"] *) {
+          display: flex;
+        }
+
+        @starting-style {
+          .starting\\:flex {
+            display: flex;
+          }
+        }
+      }"
+    `)
+  })
 })
 
 describe('@variant', () => {
@@ -1737,4 +1778,37 @@ describe('@variant', () => {
       `)
     })
   })
+
+  test('built-in variants can be overridden while keeping their order', () => {
+    expect(
+      compileCss(
+        css`
+          @variant dark (&:is([data-theme='dark'] *));
+          @layer utilities {
+            @tailwind utilities;
+          }
+        `,
+
+        // Make sure the order does not change by including the variants
+        // immediately before and after `dark`
+        ['rtl:flex', 'dark:flex', 'starting:flex'],
+      ),
+    ).toMatchInlineSnapshot(`
+      "@layer utilities {
+        .rtl\\:flex:where([dir="rtl"], [dir="rtl"] *) {
+          display: flex;
+        }
+
+        .dark\\:flex:is([data-theme="dark"] *) {
+          display: flex;
+        }
+
+        @starting-style {
+          .starting\\:flex {
+            display: flex;
+          }
+        }
+      }"
+    `)
+  })
 })

packages/tailwindcss/src/variants.ts

@@ -158,22 +158,18 @@ export class Variants {
     name: string,
     { kind, applyFn, compounds }: { kind: T; applyFn: VariantFn<T>; compounds: boolean },
   ) {
-    // In test mode, throw an error if we accidentally override another variant
-    // by mistake when implementing a new variant that shares the same root
-    // without realizing the definitions need to be merged.
-    if (process.env.NODE_ENV === 'test') {
-      if (this.variants.has(name)) {
-        throw new Error(`Duplicate variant prefix [${name}]`)
-      }
+    let existing = this.variants.get(name)
+    if (existing) {
+      Object.assign(existing, { kind, applyFn, compounds })
+    } else {
+      this.lastOrder = this.nextOrder()
+      this.variants.set(name, {
+        kind,
+        applyFn,
+        order: this.lastOrder,
+        compounds,
+      })
     }
-
-    this.lastOrder = this.nextOrder()
-    this.variants.set(name, {
-      kind,
-      applyFn,
-      order: this.lastOrder,
-      compounds,
-    })
   }
 
   private nextOrder() {