[clang] [clang][Sema] fix crash on __type_pack_element with dependent packs (GH180307) (PR #180407)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Serosh (Serosh-commits)

<details>
<summary>Changes</summary>

dependent pack expansions in __type_pack_element can result in single-element template argument lists. When performing semantic analysis for these builtins, the compiler needs to account for the dependent expansions and handle them without triggering strict size assertions. The patch adds this analysis and ensures we either defer evaluation for dependent cases or report clear out-of-bounds diagnostics instead of crashing

fixes #<!-- -->180307

---
Full diff: https://github.com/llvm/llvm-project/pull/180407.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaTemplate.cpp (+16-4) 
- (modified) clang/test/SemaCXX/type_pack_element.cpp (+5) 


``````````diff
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 3497ff7856eed..5496cd766adb8 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -3462,16 +3462,28 @@ static QualType checkBuiltinTemplateIdType(
     // Specializations of
     //    __type_pack_element<Index, T_1, ..., T_N>
     // are treated like T_Index.
-    assert(Converted.size() == 2 &&
-           "__type_pack_element should be given an index and a parameter pack");
+    assert((Converted.size() == 1 || Converted.size() == 2) &&
+      "__type_pack_element should be given an index and a parameter pack");
 
-    TemplateArgument IndexArg = Converted[0], Ts = Converted[1];
-    if (IndexArg.isDependent() || Ts.isDependent())
+    TemplateArgument IndexArg = Converted[0];
+    if (IndexArg.isDependent())
       return QualType();
 
     llvm::APSInt Index = IndexArg.getAsIntegral();
     assert(Index >= 0 && "the index used with __type_pack_element should be of "
                          "type std::size_t, and hence be non-negative");
+
+    // If the pack is empty, we only have the index argument.
+    if (Converted.size() == 1) {
+      SemaRef.Diag(TemplateArgs[0].getLocation(),
+                   diag::err_type_pack_element_out_of_bounds);
+      return QualType();
+    }
+
+    TemplateArgument Ts = Converted[1];
+    if (Ts.isDependent())
+      return QualType();
+
     // If the Index is out of bounds, the program is ill-formed.
     if (Index >= Ts.pack_size()) {
       SemaRef.Diag(TemplateArgs[0].getLocation(),
diff --git a/clang/test/SemaCXX/type_pack_element.cpp b/clang/test/SemaCXX/type_pack_element.cpp
index d22d5fa2ba67c..1ac5c861c18e8 100644
--- a/clang/test/SemaCXX/type_pack_element.cpp
+++ b/clang/test/SemaCXX/type_pack_element.cpp
@@ -43,3 +43,8 @@ static_assert(__is_same(__type_pack_element<5, X<0>, X<1>, X<2>, X<3>, X<4>, X<5
 template <SizeT Index, typename ...T>
 using ErrorTypePackElement1 = __type_pack_element<Index, T...>; // expected-error{{may not be accessed at an out of bounds index}}
 using illformed1 = ErrorTypePackElement1<3, X<0>, X<1>>;  // expected-note{{in instantiation}}
+
+template <SizeT... Seq>
+void gh180307(__type_pack_element<Seq...>) {}
+
+using gh180307_empty = __type_pack_element<0>; // expected-error {{a parameter pack may not be accessed at an out of bounds index}}

``````````

</details>


https://github.com/llvm/llvm-project/pull/180407