Handle this-fallback for @arg Attr Node values within Element Nodes

README.md

@@ -51,23 +51,29 @@ For each `PathExpression` with a `VarHead` that is NOT in the local template sco
 
   - If there is NO `tail`:
 
-    - If the `MustacheStatement` is the child of an `AttrNode` with a name NOT starting with `@`:
+    - If the `MustacheStatement` is the child of an `AttrNode`
 
-      Wrap the invocation with the `tryLookupHelper` helper to determine if it is a helper at runtime and fall back to the `this` property if not ("ambiguous attribute fallback").
+      - And the `AttrNode` represents a component argument (the name starts with `'@'`):
 
-      ```hbs
-      {{! before }}
-      <Parent id={{property}} />
+        Prefix the `head` with `this`, making it a `ThisHead` ("expression fallback"), as shown above.
 
-      {{! after }}
-      {{#let (hash property=(tryLookupHelper "property")) as |maybeHelpers|}}
-        <Parent
-          id={{(if
-            maybeHelpers.property (maybeHelpers.property) this.property
-          )}}
-        />
-      {{/let}}
-      ```
+      - And the `AttrNode` represents an attribute (the name does not start with `'@'`):
+
+        Wrap the invocation with the `tryLookupHelper` helper to determine if it is a helper at runtime and fall back to the `this` property if not ("ambiguous attribute fallback").
+
+        ```hbs
+        {{! before }}
+        <Parent id={{property}} />
+
+        {{! after }}
+        {{#let (hash property=(tryLookupHelper "property")) as |maybeHelpers|}}
+          <Parent
+            id={{(if
+              maybeHelpers.property (maybeHelpers.property) this.property
+            )}}
+          />
+        {{/let}}
+        ```
 
     - Otherwise:
 

lib/this-fallback-plugin.ts

@@ -92,7 +92,6 @@ class ThisFallbackPlugin implements ASTPlugin {
         for (const attrNode of elementNode.attributes) {
           const value = attrNode.value;
           if (
-            !attrNode.name.startsWith('@') &&
             isNode(value, 'MustacheStatement') &&
             mustacheNeedsFallback(value, this.scopeStack)
           ) {
@@ -101,11 +100,15 @@ class ThisFallbackPlugin implements ASTPlugin {
               'attrNode.value is not a MustacheStatement',
               isNode(attrNode.value, 'MustacheStatement')
             );
-            ambiguousHeads.set(value.path.head.name, value.loc);
-            attrNode.value.path = helperOrExpressionFallback(
-              blockParamName,
-              value
-            );
+            if (attrNode.name.startsWith('@')) {
+              attrNode.value.path = expressionFallback(value.path);
+            } else {
+              ambiguousHeads.set(value.path.head.name, value.loc);
+              attrNode.value.path = helperOrExpressionFallback(
+                blockParamName,
+                value
+              );
+            }
           } else if (isNode(value, 'ConcatStatement')) {
             for (const part of value.parts) {
               const p = part;
@@ -182,8 +185,7 @@ class ThisFallbackPlugin implements ASTPlugin {
         if (mustacheNeedsFallback(n, this.scopeStack)) {
           assert(
             'unexpected AmbiguousMustacheExpression in attribute value',
-            path.parentNode?.type !== 'AttrNode' ||
-              path.parentNode.name.startsWith('@')
+            path.parentNode?.type !== 'AttrNode'
           );
           if (n.path.tail.length > 0) {
             node.path = expressionFallback(n.path);

tests/integration/plugin/mustache-statement-test.ts

@@ -86,6 +86,22 @@ module('Integration | Plugin | MustacheStatement', function (hooks) {
           });
 
           module('for an @arg', function () {
+            test('has this-fallback', async function (assert) {
+              this.set('property', 'property-on-this');
+              await render<{ property: string }>(hbs`
+                {{!--
+                  @glint-expect-error:
+                  Unknown name 'property' (Glint knows better than to let us do this)
+                --}}
+                <GlobalComponent @arg={{property}} as |yielded|>
+                  {{yielded}}
+                </GlobalComponent>
+              `);
+              assert
+                .dom()
+                .hasText('global-component-contents property-on-this');
+            });
+
             test('does nothing to ThisHead PathExpression', async function (assert) {
               this.set('property', 'property-on-this');
               await render<{ property: string }>(hbs`