[Clang] Fix a pack expansion bug in template argument deduction

[zyn0217]

I think the intent of df18ee9 was to substitute only those non-packs into a pack expansion type (e.g. T in T::pack...), so let's hold off pack expansions explicitly, in case there are calls coming from a substitution of pack expansion.

Fixes #53609

[llvmbot]

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

Changes

I think the intent of df18ee9 was to substitute only those non-packs into a pack expansion type (e.g. T in T::pack...), so let's hold off pack expansions explicitly, in case there are calls coming from a substitution of pack expansion.

Fixes #53609

---

Full diff: https://github.com/llvm/llvm-project/pull/141547.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/lib/Sema/SemaTemplateDeduction.cpp (+1)
• (modified) clang/test/CXX/temp/temp.fct.spec/temp.deduct/p7.cpp (+17)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index b93fa33acc2a0..2c46a8f8ac86d 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -795,6 +795,7 @@ Bug Fixes to C++ Support
 - Fix instantiation of default-initialized variable template specialization. (#GH140632) (#GH140622)
 - Clang modules now allow a module and its user to differ on TrivialAutoVarInit*
 - Fixed an access checking bug when initializing non-aggregates in default arguments (#GH62444), (#GH83608)
+- Fixed a pack substitution bug in deducing class template partial specializations. (#GH53609)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 217d57d67f067..75ae04b27d06a 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -2946,6 +2946,7 @@ ConvertDeducedTemplateArgument(Sema &S, NamedDecl *Param,
       LocalInstantiationScope Scope(S);
       MultiLevelTemplateArgumentList Args(Template, CTAI.SugaredConverted,
                                           /*Final=*/true);
+      Sema::ArgPackSubstIndexRAII OnlySubstNonPackExpansion(S, std::nullopt);
 
       if (auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(Param)) {
         Sema::InstantiatingTemplate Inst(S, Template->getLocation(), Template,
diff --git a/clang/test/CXX/temp/temp.fct.spec/temp.deduct/p7.cpp b/clang/test/CXX/temp/temp.fct.spec/temp.deduct/p7.cpp
index f0ab7b8ea7612..de6fa0c837e2a 100644
--- a/clang/test/CXX/temp/temp.fct.spec/temp.deduct/p7.cpp
+++ b/clang/test/CXX/temp/temp.fct.spec/temp.deduct/p7.cpp
@@ -54,3 +54,20 @@ namespace reversed_operator_substitution_order {
   float &s = no_adl::f<int>(true);
 }
 #endif
+
+namespace GH53609 {
+
+template <class, int>
+struct a;
+
+template <class, class...>
+struct b;
+
+template <class x, class... y, y... z>
+struct b<x, a<y, z>...> {};
+
+template <class... x> struct c: b<x>...  {};
+
+c<int> d;
+
+}