From 5c76de72baa408d6cd32cd7ba97660617da6ff34 Mon Sep 17 00:00:00 2001
From: Jordan Pittman <jordan@cryptica.me>
Date: Fri, 15 Apr 2022 11:57:06 -0400
Subject: [PATCH] Require matching prefix when detecting negatives (#8121)

* Require matching prefix when detecting negatives

* Update changelog
---
 CHANGELOG.md             |  1 +
 src/lib/generateRules.js |  6 +++++-
 tests/prefix.test.js     | 16 ++++++++++++++++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ae4fc61fa214..1e94446bec2e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Types: allow for arbitrary theme values (for 3rd party plugins) ([#7926](https://github.com/tailwindlabs/tailwindcss/pull/7926))
 - Don’t split vars with numbers in them inside arbitrary values ([#8091](https://github.com/tailwindlabs/tailwindcss/pull/8091))
+- Require matching prefix when detecting negatives ([#8121](https://github.com/tailwindlabs/tailwindcss/pull/8121))
 
 ### Added
 
diff --git a/src/lib/generateRules.js b/src/lib/generateRules.js
index fc9a6bbd6ec1..bbed6812eb48 100644
--- a/src/lib/generateRules.js
+++ b/src/lib/generateRules.js
@@ -382,7 +382,11 @@ function* resolveMatchedPlugins(classCandidate, context) {
   const twConfigPrefix = context.tailwindConfig.prefix
 
   const twConfigPrefixLen = twConfigPrefix.length
-  if (candidatePrefix[twConfigPrefixLen] === '-') {
+
+  const hasMatchingPrefix =
+    candidatePrefix.startsWith(twConfigPrefix) || candidatePrefix.startsWith(`-${twConfigPrefix}`)
+
+  if (candidatePrefix[twConfigPrefixLen] === '-' && hasMatchingPrefix) {
     negative = true
     candidatePrefix = twConfigPrefix + candidatePrefix.slice(twConfigPrefixLen + 1)
   }
diff --git a/tests/prefix.test.js b/tests/prefix.test.js
index 9124fd5b64c0..a82b98624ace 100644
--- a/tests/prefix.test.js
+++ b/tests/prefix.test.js
@@ -358,3 +358,19 @@ it('prefix with negative values and variants in the safelist', async () => {
     }
   `)
 })
+
+it('prefix does not detect and generate unnecessary classes', async () => {
+  let config = {
+    prefix: 'tw-_',
+    content: [{ raw: html`-aaa-filter aaaa-table aaaa-hidden` }],
+    corePlugins: { preflight: false },
+  }
+
+  let input = css`
+    @tailwind utilities;
+  `
+
+  const result = await run(input, config)
+
+  expect(result.css).toMatchFormattedCss(css``)
+})