[clang] [Clang] Evaluate dependent indexes of pack indexing in a constant context (PR #106054)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: cor3ntin (cor3ntin)

<details>
<summary>Changes</summary>

Fixes #<!-- -->105900

---
Full diff: https://github.com/llvm/llvm-project/pull/106054.diff


3 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+1) 
- (modified) clang/lib/Sema/TreeTransform.h (+17-6) 
- (modified) clang/test/SemaCXX/cxx2c-pack-indexing.cpp (+27) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8f98167dff31ef..384471bf860c5b 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -292,6 +292,7 @@ Bug Fixes to C++ Support
 - Correctly check constraints of explicit instantiations of member functions. (#GH46029)
 - Fixed an assertion failure about a constraint of a friend function template references to a value with greater
   template depth than the friend function template. (#GH98258)
+- Fix evaluation of the index of dependent pack indexing expressions/types specifiers (#GH105900)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 62287c2d26375c..b3854cd8f82220 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6669,9 +6669,15 @@ QualType
 TreeTransform<Derived>::TransformPackIndexingType(TypeLocBuilder &TLB,
                                                   PackIndexingTypeLoc TL) {
   // Transform the index
-  ExprResult IndexExpr = getDerived().TransformExpr(TL.getIndexExpr());
-  if (IndexExpr.isInvalid())
-    return QualType();
+  ExprResult IndexExpr;
+  {
+    EnterExpressionEvaluationContext ConstantContext(
+        SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+
+    IndexExpr = getDerived().TransformExpr(TL.getIndexExpr());
+    if (IndexExpr.isInvalid())
+      return QualType();
+  }
   QualType Pattern = TL.getPattern();
 
   const PackIndexingType *PIT = TL.getTypePtr();
@@ -15299,9 +15305,14 @@ TreeTransform<Derived>::TransformPackIndexingExpr(PackIndexingExpr *E) {
     return E;
 
   // Transform the index
-  ExprResult IndexExpr = getDerived().TransformExpr(E->getIndexExpr());
-  if (IndexExpr.isInvalid())
-    return ExprError();
+  ExprResult IndexExpr;
+  {
+    EnterExpressionEvaluationContext ConstantContext(
+        SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+    IndexExpr = getDerived().TransformExpr(E->getIndexExpr());
+    if (IndexExpr.isInvalid())
+      return ExprError();
+  }
 
   SmallVector<Expr *, 5> ExpandedExprs;
   if (!E->expandsToEmptyPack() && E->getExpressions().empty()) {
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 9ea90a4c3e30fd..37dceaee64fac2 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -231,3 +231,30 @@ struct type_info {
 namespace GH93650 {
 auto func(auto... inputArgs) { return typeid(inputArgs...[0]); }
 } // namespace GH93650
+
+
+namespace GH105900 {
+
+template <typename... opts>
+struct types  {
+    template <unsigned idx>
+    static constexpr __SIZE_TYPE__ get_index() { return idx; }
+
+    template <unsigned s>
+    static auto x() -> opts...[get_index<s>()] {}
+};
+
+template <auto... opts>
+struct vars  {
+    template <unsigned idx>
+    static constexpr __SIZE_TYPE__ get_index() { return idx; }
+
+    template <unsigned s>
+    static auto x() -> decltype(opts...[get_index<s>()]) {return 0;}
+};
+
+void f() {
+    types<void>::x<0>(); // Error
+    vars<0>::x<0>(); // Error
+}
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/106054