[Clang] Resolved type of expression indexing into pack of values of a non-dependent type

[TilakChad]

For use of non-dependent type inside template, clang verifies eagerly that the type of variable being declared matches to that type.
With this PR, we assign the type of the PackIndexingExpr to the type of the pack it refers to.

This fixes #121242.

[llvmbot]

@llvm/pr-subscribers-clang

Author: TilakChad (TilakChad)

Changes

For use of non-dependent type inside template, clang verifies eagerly that the type of variable being declared matches to that type.
With this PR, we assign the type of the PackIndexingExpr to the type of the pack it refers to.

This fixes #121242.

---

Full diff: https://github.com/llvm/llvm-project/pull/121405.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/lib/AST/ExprCXX.cpp (+1-1)
• (modified) clang/test/SemaCXX/cxx2c-pack-indexing.cpp (+16)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 983c1da20ed4c8..612541b0ddf84c 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -886,6 +886,7 @@ Bug Fixes to C++ Support
   out of a module (which is the case e.g. in MSVC's implementation of ``std`` module). (#GH118218)
 - Fixed a pack expansion issue in checking unexpanded parameter sizes. (#GH17042)
 - Fixed a bug where captured structured bindings were modifiable inside non-mutable lambda (#GH95081)
+- Fixed an issue while resolving type of expression indexing into a pack of values of non-dependent type (#GH121242)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/AST/ExprCXX.cpp b/clang/lib/AST/ExprCXX.cpp
index fc09d24fc30cb4..5bf5d6adf525a8 100644
--- a/clang/lib/AST/ExprCXX.cpp
+++ b/clang/lib/AST/ExprCXX.cpp
@@ -1722,7 +1722,7 @@ PackIndexingExpr *PackIndexingExpr::Create(
   if (Index && FullySubstituted && !SubstitutedExprs.empty())
     Type = SubstitutedExprs[*Index]->getType();
   else
-    Type = Context.DependentTy;
+    Type = PackIdExpr->getType();
 
   void *Storage =
       Context.Allocate(totalSizeToAlloc<Expr *>(SubstitutedExprs.size()));
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index cb679a6c3ad879..58b642d2735b6e 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -305,3 +305,19 @@ template <class... Args> struct mdispatch_ {
 mdispatch_<int, int> d;
 
 } // namespace GH116105
+
+namespace GH121242 {
+    // Non-dependent type pack access
+    template <int...x>
+    int y = x...[0];
+
+    struct X {};
+
+    template <X...x>
+    X z = x...[0];
+
+    void foo() {
+        (void)y<0>;
+        (void)z<X{}>;
+    }
+} // namespace GH121242

[TilakChad]

Alternatively, we could've made the PackIndexingExpr type-dependent and verify type compatibility lazily during instantiation.
It is how MSVC and GCC seems to behave.

Considering that the clang seems to be eager about such type compatibility, this changes is along the lines of how clang currently behaves.

https://godbolt.org/z/v5v3vqY8W

[cor3ntin]

LGTM