fix(postcss-fix-rules): prevent merging of :host(tag) and tag

Fixes: #730

CONTRIBUTORS.md

@@ -13,7 +13,7 @@ Thanks goes to these wonderful people ([emoji key](https://github.com/kentcdodds
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+| [<img src="https://avatars0.githubusercontent.com/u/4847190?v=4" width="100px;"/><br /><sub><b>Ben McKernan</b></sub>](https://github.com/ben-mckernan)<br />[💻](https://github.com/cssnano/cssnano/commits?author=ben-mckernan "Code") [⚠️](https://github.com/cssnano/cssnano/commits?author=ben-mckernan "Tests") | [<img src="https://avatars0.githubusercontent.com/u/122028?v=4" width="100px;"/><br /><sub><b>Ian Walter</b></sub>](https://iankwalter.com)<br />[💻](https://github.com/cssnano/cssnano/commits?author=ianwalter "Code") | [<img src="https://avatars1.githubusercontent.com/u/5821279?s=460&v=4" width="100px;"/><br /><sub><b>MapTo0</b></sub>](https://github.com/MapTo0)<br />[💻](https://github.com/cssnano/cssnano/commits?author=MapTo0 "Code") [⚠️](https://github.com/cssnano/cssnano/commits?author=MapTo0 "Tests") 
 <!-- ALL-CONTRIBUTORS-LIST:END -->
 
 This project follows the [all-contributors](https://github.com/kentcdodds/all-contributors) specification. Contributions of any kind welcome!

packages/postcss-merge-rules/src/__tests__/index.js

@@ -597,6 +597,13 @@ test(
     '.a,.b{display: flex;ALL: initial;border-radius: 2px;color: blue;}.a{margin: 10px 20px;font-size: 10px;}.b{margin: 10px 30px;font-size: 20px;}'
 );
 
+test(
+    'should not merge :host(tagname) with tagname',
+    processCSS,
+    ':host(tag){display:block}tag{display:block}',
+    ':host(tag){display:block}tag{display:block}',
+);
+
 test(
     'should merge multiple media queries',
     processCSS,

packages/postcss-merge-rules/src/index.js

@@ -89,6 +89,11 @@ function canMerge (ruleA, ruleB, browsers, compatibilityCache) {
     const b = ruleB.selectors;
 
     const selectors = a.concat(b);
+    const hasHost = selectors.some(selector => selector.includes(":host"));
+
+    if (hasHost) {
+        return false;
+    }
 
     if (!ensureCompatibility(selectors, browsers, compatibilityCache)) {
         return false;