Fix template-no-shadowed-elements: align HTML-element detection with upstream

Replace static htmlTags package check with inverse-of-component-detection
(reject tags with : / . / uppercase / @ / -). Matches upstream's
isAngleBracketComponent semantics. Custom elements are now correctly
excluded from shadowing checks.

Author: johanrd

lib/rules/template-no-shadowed-elements.js

@@ -1,4 +1,32 @@
-const htmlTags = require('html-tags');
+// Mirror upstream ember-template-lint's inverse-of-isAngleBracketComponent logic.
+// A tag is treated as an HTML element only when it:
+//   - does NOT contain ':' (named blocks like <:slot>)
+//   - does NOT contain '.' (path/namespaced invocations like <foo.bar>)
+//   - does NOT start with '@' (argument invocations like <@foo>)
+//   - has NO uppercase letters (component invocations like <MyThing>)
+//   - does NOT contain '-' (HTML custom elements like <my-element>)
+// Everything else is a component / custom-element / slot — not a plain HTML element.
+function isHtmlElement(tagName) {
+  if (!tagName) {
+    return false;
+  }
+  if (tagName.startsWith('@')) {
+    return false;
+  }
+  if (tagName.includes(':')) {
+    return false;
+  }
+  if (tagName.includes('.')) {
+    return false;
+  }
+  if (tagName.includes('-')) {
+    return false;
+  }
+  if (tagName !== tagName.toLowerCase()) {
+    return false;
+  }
+  return true;
+}
 
 /** @type {import('eslint').Rule.RuleModule} */
 module.exports = {
@@ -24,8 +52,6 @@ module.exports = {
   },
 
   create(context) {
-    const HTML_ELEMENTS = new Set(htmlTags);
-
     const blockParamScope = [];
 
     function pushScope(params) {
@@ -68,22 +94,17 @@ module.exports = {
           return;
         }
 
-        const containsDot = tag.includes('.');
-
-        if (containsDot) {
-          // dot paths like bar.baz are not ambiguous
+        // Mirror upstream: if the tag is an angle-bracket-component (i.e.
+        // not a plain HTML element — contains '.', is PascalCase, has a
+        // hyphen, etc.) it cannot be a shadow of a native HTML element.
+        // Only a lowercase / simple tag that is a local block param is
+        // considered shadowed. This also covers tags not in any static
+        // html-tags list (upstream does not restrict to a known set).
+        if (!isHtmlElement(tag)) {
           return;
         }
 
-        // Only check lowercase elements — a lowercase tag that is a local
-        // block param and also a native HTML element name is shadowed.
-        // PascalCase tags (e.g. <Input>, <Form>, <Select>) are Ember/Glimmer
-        // component invocations and should not be flagged.
-        const firstChar = tag.charAt(0);
-        const isLowerCase =
-          firstChar === firstChar.toLowerCase() && firstChar !== firstChar.toUpperCase();
-
-        if (isLowerCase && isLocal(tag) && HTML_ELEMENTS.has(tag)) {
+        if (isLocal(tag)) {
           context.report({
             node,
             messageId: 'shadowed',

tests/lib/rules/template-no-shadowed-elements.js

@@ -35,6 +35,18 @@ ruleTester.run('template-no-shadowed-elements', rule, {
         },
       ],
     },
+    // Upstream flags any lowercase local block-param invocation, not just
+    // names present in a static html-tags list.
+    {
+      code: '<template><FooBar as |foo|><foo></foo></FooBar></template>',
+      output: null,
+      errors: [
+        {
+          message: 'Component name "foo" shadows HTML element <foo>. Use a different name.',
+          type: 'GlimmerElementNode',
+        },
+      ],
+    },
   ],
 });
 
@@ -63,5 +75,14 @@ hbsRuleTester.run('template-no-shadowed-elements', rule, {
         { message: 'Component name "div" shadows HTML element <div>. Use a different name.' },
       ],
     },
+    // Upstream flags any lowercase local block-param invocation, not just
+    // names present in a static html-tags list.
+    {
+      code: '<FooBar as |foo|><foo></foo></FooBar>',
+      output: null,
+      errors: [
+        { message: 'Component name "foo" shadows HTML element <foo>. Use a different name.' },
+      ],
+    },
   ],
 });