Fix: skip scope-shadowed fn in template-no-redundant-fn for gjs/gts

When `fn` is imported from a local module in gjs/gts, it shadows
Ember's built-in helper. Use scope analysis to detect this and
avoid false positives.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: NullVoxPopuli

lib/rules/template-no-redundant-fn.js

@@ -23,6 +23,23 @@ module.exports = {
   },
 
   create(context) {
+    const sourceCode = context.sourceCode;
+
+    /**
+     * Check whether `fn` in the template resolves to a local/imported binding.
+     * If it does, the user has shadowed Ember's built-in `fn` helper and we
+     * should not flag the usage.
+     */
+    function isFnShadowed(node) {
+      const head = node.path?.head;
+      if (!head) {
+        return false;
+      }
+      const scope = sourceCode.getScope(node);
+      const ref = scope.references.find((r) => r.identifier === head);
+      return ref?.resolved != null;
+    }
+
     function checkFnUsage(node) {
       // Check if this is an (fn) call with only one argument (the function itself)
       if (
@@ -33,6 +50,10 @@ module.exports = {
         node.params.length === 1 &&
         !node.hash?.pairs?.length
       ) {
+        // In gjs/gts, `fn` may be shadowed by a local import — skip if so.
+        if (isFnShadowed(node)) {
+          return;
+        }
         const param = node.params[0];
         const paramText =
           param.type === 'GlimmerPathExpression'

tests/lib/rules/template-no-redundant-fn.js

@@ -35,6 +35,15 @@ ruleTester.run('template-no-redundant-fn', rule, {
     '<template><SomeComponent @onClick={{fn this.handleClick "foo"}} /></template>',
     '<template>{{foo bar=this.handleClick}}></template>',
     '<template>{{foo bar=(fn this.handleClick "foo")}}></template>',
+
+    // `fn` is imported from a local module — not Ember's built-in helper.
+    // Should NOT be flagged even with a single argument.
+    `
+      import { fn } from './my-utils';
+      <template>
+        <button {{on "click" (fn this.handleClick)}}>Click</button>
+      </template>
+    `,
   ],
 
   invalid: [