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

[via cfe-commits]

https://github.com/cor3ntin updated https://github.com/llvm/llvm-project/pull/106054

>From 2d3b50688e7b0faea4452860cacc24a596cd685c Mon Sep 17 00:00:00 2001
From: Corentin Jabot <corentinjabot at gmail.com>
Date: Mon, 26 Aug 2024 10:48:34 +0200
Subject: [PATCH 1/2] [Clang] Evaluate dependent indexes of pack indexing in a
 constant context

Fixes #105900
---
 clang/docs/ReleaseNotes.rst                |  1 +
 clang/lib/Sema/TreeTransform.h             | 23 +++++++++++++-----
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 27 ++++++++++++++++++++++
 3 files changed, 45 insertions(+), 6 deletions(-)

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
+}
+}

>From 767f13ece43f78f02e8e250922f8d6e5a867d6ac Mon Sep 17 00:00:00 2001
From: Corentin Jabot <corentinjabot at gmail.com>
Date: Mon, 26 Aug 2024 15:36:50 +0200
Subject: [PATCH 2/2] remove copy pasta comments

---
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 37dceaee64fac2..7d7e808746217f 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -254,7 +254,8 @@ struct vars  {
 };
 
 void f() {
-    types<void>::x<0>(); // Error
-    vars<0>::x<0>(); // Error
+    types<void>::x<0>();
+    vars<0>::x<0>();
 }
+
 }