fix(template-require-iframe-title): flag title={{null|undefined|number}}

Before: the rule only rejected title={{false}} among mustache literals.
title={{null}}, title={{undefined}}, and title={{42}} (any non-string
literal) were silently accepted even though none produces a useful
accessible name for an <iframe>.

Fix: extract isInvalidTitleLiteral(); treat boolean, null, undefined,
and numeric literals as invalid title values in both GlimmerMustacheStatement
and single-part GlimmerConcatStatement positions.

Six new invalid tests (3 literal types × 2 syntax forms).

Note: whitespace-only title values (e.g. title="   ") remain flagged —
the audit suggested upstream jsx-a11y accepts these, but an all-whitespace
title provides no accessible name, so our stricter behavior is kept.

Author: johanrd

lib/rules/template-require-iframe-title.js

@@ -1,3 +1,22 @@
+// Mustache path nodes that produce no accessible name. Booleans, null, undefined
+// all coerce to empty-ish strings; numeric literals ("42") are accepted by HTML
+// but provide no useful title for assistive tech.
+function isInvalidTitleLiteral(path) {
+  if (!path) {
+    return false;
+  }
+  if (path.type === 'GlimmerBooleanLiteral') {
+    return true;
+  }
+  if (path.type === 'GlimmerNullLiteral' || path.type === 'GlimmerUndefinedLiteral') {
+    return true;
+  }
+  if (path.type === 'GlimmerNumberLiteral') {
+    return true;
+  }
+  return false;
+}
+
 /** @type {import('eslint').Rule.RuleModule} */
 module.exports = {
   meta: {
@@ -93,19 +112,21 @@ module.exports = {
               break;
             }
             case 'GlimmerMustacheStatement': {
-              // title={{false}} → BooleanLiteral false is invalid
-              if (titleAttr.value.path?.type === 'GlimmerBooleanLiteral') {
+              // title={{false}} / title={{null}} / title={{undefined}} / title={{42}}
+              // — any literal that doesn't produce a meaningful accessible name.
+              if (isInvalidTitleLiteral(titleAttr.value.path)) {
                 context.report({ node, messageId: 'dynamicFalseTitle' });
               }
               break;
             }
             case 'GlimmerConcatStatement': {
-              // title="{{false}}" → ConcatStatement with single BooleanLiteral part
+              // title="{{false}}" / "{{undefined}}" / etc. — ConcatStatement
+              // with a single literal part that doesn't produce a name.
               const parts = titleAttr.value.parts || [];
               if (
                 parts.length === 1 &&
                 parts[0].type === 'GlimmerMustacheStatement' &&
-                parts[0].path?.type === 'GlimmerBooleanLiteral'
+                isInvalidTitleLiteral(parts[0].path)
               ) {
                 context.report({ node, messageId: 'dynamicFalseTitle' });
               }

tests/lib/rules/template-require-iframe-title.js

@@ -104,6 +104,38 @@ ruleTester.run('template-require-iframe-title', rule, {
       output: null,
       errors: [{ messageId: 'emptyTitle' }],
     },
+
+    // Mustache literals that don't coerce to a useful accessible name.
+    {
+      code: '<template><iframe title={{null}} /></template>',
+      output: null,
+      errors: [{ messageId: 'dynamicFalseTitle' }],
+    },
+    {
+      code: '<template><iframe title={{undefined}} /></template>',
+      output: null,
+      errors: [{ messageId: 'dynamicFalseTitle' }],
+    },
+    {
+      code: '<template><iframe title={{42}} /></template>',
+      output: null,
+      errors: [{ messageId: 'dynamicFalseTitle' }],
+    },
+    {
+      code: '<template><iframe title="{{null}}" /></template>',
+      output: null,
+      errors: [{ messageId: 'dynamicFalseTitle' }],
+    },
+    {
+      code: '<template><iframe title="{{undefined}}" /></template>',
+      output: null,
+      errors: [{ messageId: 'dynamicFalseTitle' }],
+    },
+    {
+      code: '<template><iframe title="{{42}}" /></template>',
+      output: null,
+      errors: [{ messageId: 'dynamicFalseTitle' }],
+    },
   ],
 });