Don’t replace all instances of the same class

This isn’t 100% correct either so we’re backing out this change

CHANGELOG.md

@@ -17,7 +17,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Use absolute paths when resolving changed files for resilience against working directory changes ([#9032](https://github.com/tailwindlabs/tailwindcss/pull/9032))
 - Fix ring color utility generation when using `respectDefaultRingColorOpacity` ([#9070](https://github.com/tailwindlabs/tailwindcss/pull/9070))
-- Replace all occurrences of a class in a selector when using `@apply` ([#9107](https://github.com/tailwindlabs/tailwindcss/pull/9107))
 - Sort tags before classes when `@applying` a selector with joined classes ([#9107](https://github.com/tailwindlabs/tailwindcss/pull/9107))
 
 ## [3.1.8] - 2022-08-05

src/lib/expandApplyAtRules.js

@@ -315,6 +315,7 @@ function processApply(root, context, localCache) {
       let replaced = new Set()
 
       utilitySelectorsList.each((utilitySelector) => {
+        let hasReplaced = false
         utilitySelector = utilitySelector.clone()
 
         utilitySelector.walkClasses((node) => {
@@ -322,6 +323,19 @@ function processApply(root, context, localCache) {
             return
           }
 
+          // Don't replace multiple instances of the same class
+          // This is theoretically correct but only partially
+          // We'd need to generate every possible permutation of the replacement
+          // For example with `.foo + .foo { … }` and `section { @apply foo; }`
+          // We'd need to generate all of these:
+          // - `.foo + .foo`
+          // - `.foo + section`
+          // - `section + .foo`
+          // - `section + section`
+          if (hasReplaced) {
+            return
+          }
+
           // Since you can only `@apply` class names this is sufficient
           // We want to replace the matched class name with the selector the user is using
           // Ex: Replace `.text-blue-500` with `.foo.bar:is(.something-cool)`
@@ -329,6 +343,8 @@ function processApply(root, context, localCache) {
 
           // Record that we did something and we want to use this new selector
           replaced.add(utilitySelector)
+
+          hasReplaced = true
         })
       })
 

tests/apply.test.js

@@ -1641,7 +1641,10 @@ it('can apply joined classes when using elements', async () => {
   `)
 })
 
-it('can produce selectors that replace multiple instances of the same class', async () => {
+it('should not replace multiple instances of the same class in a single selector', async () => {
+  // NOTE: This test is non-normative and is not part of the spec of how `@apply` works per-se
+  // It describes how it currently works because the "correct" way produces a combinatorial explosion
+  // of selectors that is not easily doable
   let config = {
     content: [{ raw: html`<div class="foo-1 -foo-1 new-class"></div>` }],
     plugins: [],
@@ -1674,14 +1677,16 @@ it('can produce selectors that replace multiple instances of the same class', as
     .bar + .bar {
       color: fuchsia;
     }
-    header + header {
+    header + .foo {
       color: blue;
     }
-    main + main {
+    main + .foo {
       color: blue;
+    }
+    main + .bar {
       color: fuchsia;
     }
-    footer + footer {
+    footer + .bar {
       color: fuchsia;
     }
   `)