[clang] [clang] Fix assertion failure on direct initialization of an alias with an unexpanded pack from a parenthesized braced-init-list (PR #236838)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Harlen Batagelo (hbatagelo)

<details>
<summary>Changes</summary>

Fixes #<!-- -->113762.

Given
```cpp
template&lt;typename T&gt; using Int = int;
template&lt;typename ...Ts&gt; void Pack() {
  Int&lt;Ts&gt;({});
}
```
Sema wraps the ill-formed `Int&lt;Ts&gt;({})` in a `RecoveryExpr` whose type is 
the sugared `Int&lt;Ts&gt;`, so `containsUnexpandedParameterPack()` is `true`. 
But the collector invoked by `Sema::DiagnoseUnexpandedParameterPack` only walks 
the subexpression (`InitListExpr`) which doesn't mention the pack. The 
traversal finds nothing, triggering the assertion 
https://github.com/llvm/llvm-project/blob/3bf0a2073bd4ef0a744601305d62ab058233a9ed/clang/lib/Sema/SemaTemplateVariadic.cpp#L566-L567

Fix by overriding `TraverseRecoveryExpr` in 
`CollectUnexpandedParameterPacksVisitor` to also traverse the `RecoveryExpr`'s 
type.

---
Full diff: https://github.com/llvm/llvm-project/pull/236838.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaTemplateVariadic.cpp (+7) 
- (modified) clang/test/SemaTemplate/find-unexpanded-packs.cpp (+1) 


``````````diff
diff --git a/clang/lib/Sema/SemaTemplateVariadic.cpp 
b/clang/lib/Sema/SemaTemplateVariadic.cpp
index 2d6e75bd77b402..bda26f5d6aeca6 100644
--- a/clang/lib/Sema/SemaTemplateVariadic.cpp
+++ b/clang/lib/Sema/SemaTemplateVariadic.cpp
@@ -384,6 +384,13 @@ class CollectUnexpandedParameterPacksVisitor
       return DynamicRecursiveASTVisitor::TraverseUnresolvedLookupExpr(E);
     }
 
+    bool TraverseRecoveryExpr(RecoveryExpr *E) override {
+      if (!DynamicRecursiveASTVisitor::TraverseRecoveryExpr(E))
+        return false;
+      TraverseType(E->getType());
+      return true;
+    }
+
     bool TraverseSubstBuiltinTemplatePackType(SubstBuiltinTemplatePackType *T,
                                               bool TraverseQualifier) override 
{
       addUnexpanded(T);
diff --git a/clang/test/SemaTemplate/find-unexpanded-packs.cpp 
b/clang/test/SemaTemplate/find-unexpanded-packs.cpp
index ae989c68b50e22..5bb9fb1d6cde09 100644
--- a/clang/test/SemaTemplate/find-unexpanded-packs.cpp
+++ b/clang/test/SemaTemplate/find-unexpanded-packs.cpp
@@ -162,6 +162,7 @@ template void HiddenPack<int, float>();
 template<typename ...Ts> void HiddenPackInvalid() {
   Int<Ts>(); // expected-error {{unexpanded}}
   Int<Ts>{}; // expected-error {{unexpanded}}
+  Int<Ts>({}); // expected-error {{unexpanded}} expected-error {{cannot 
initialize non-class type}}
   Class<Ts>(); // expected-error {{unexpanded}}
   Class<Ts>{}; // expected-error {{unexpanded}}
 }

``````````

</details>


https://github.com/llvm/llvm-project/pull/236838
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