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

Fixes #105900

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -309,6 +309,8 @@ Bug Fixes to C++ Support
   template depth than the friend function template. (#GH98258)
 - Clang now rebuilds the template parameters of out-of-line declarations and specializations in the context
   of the current instantiation in all cases.
+- Fix evaluation of the index of dependent pack indexing expressions/types specifiers (#GH105900)
+
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

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()) {

clang/test/SemaCXX/cxx2c-pack-indexing.cpp

@@ -231,3 +231,31 @@ 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>();
+    vars<0>::x<0>();
+}
+
+}