[Clang] Diagnose unexpanded packs for NTTP type constraints

[zyn0217]

Otherwise, the invalid unexpanded type would be passed to getAutoType and lead to a crash.

Fixes #88866

[llvmbot]

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

Changes

Otherwise, the invalid unexpanded type would be passed to getAutoType and lead to a crash.

Fixes #88866

---

Full diff: https://github.com/llvm/llvm-project/pull/121296.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/lib/Sema/SemaTemplate.cpp (+10-2)
• (modified) clang/test/SemaCXX/cxx2c-fold-exprs.cpp (+42)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 983c1da20ed4c8..db40a722ce1ed8 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)
+- Clang now identifies unexpanded parameter packs within the type constraint on a non-type template parameter. (#GH88866)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 5e7a3c8484c88f..eac184d468ce9a 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -1530,9 +1530,17 @@ NamedDecl *Sema::ActOnNonTypeTemplateParameter(Scope *S, Declarator &D,
   Param->setAccess(AS_public);
 
   if (AutoTypeLoc TL = TInfo->getTypeLoc().getContainedAutoTypeLoc())
-    if (TL.isConstrained())
-      if (AttachTypeConstraint(TL, Param, Param, D.getEllipsisLoc()))
+    if (TL.isConstrained()) {
+      if (const ASTTemplateArgumentListInfo *ArgumentList =
+              TL.getConceptReference()->getTemplateArgsAsWritten())
+        for (const TemplateArgumentLoc &Loc : ArgumentList->arguments()) {
+          Invalid |= DiagnoseUnexpandedParameterPack(
+              Loc, UnexpandedParameterPackContext::UPPC_TypeConstraint);
+        }
+      if (!Invalid &&
+          AttachTypeConstraint(TL, Param, Param, D.getEllipsisLoc()))
         Invalid = true;
+    }
 
   if (Invalid)
     Param->setInvalidDecl();
diff --git a/clang/test/SemaCXX/cxx2c-fold-exprs.cpp b/clang/test/SemaCXX/cxx2c-fold-exprs.cpp
index 0674135aac483f..509922c0a8f4fe 100644
--- a/clang/test/SemaCXX/cxx2c-fold-exprs.cpp
+++ b/clang/test/SemaCXX/cxx2c-fold-exprs.cpp
@@ -305,3 +305,45 @@ static_assert(__is_same_as(_Three_way_comparison_result_with_tuple_like<tuple<in
 static_assert(__is_same_as(_Three_way_comparison_result_with_tuple_like<tuple<int>, 0>::type, long));
 
 }
+
+namespace GH88866 {
+
+template <typename...Ts> struct index_by;
+
+template <typename T, typename Indices>
+concept InitFunc = true;
+
+namespace Invalid {
+
+template <typename Indices, InitFunc<Indices> auto... init>
+struct LazyLitMatrix;
+
+template <
+    typename...Indices,
+    InitFunc<index_by<Indices>> auto... init
+    // expected-error@-1 {{type constraint contains unexpanded parameter pack 'Indices'}}
+>
+struct LazyLitMatrix<index_by<Indices...>, init...> {
+};
+
+using T = LazyLitMatrix<index_by<int, char>, 42, 43>;
+
+}
+
+namespace Valid {
+
+template <typename Indices, InitFunc<Indices> auto... init>
+struct LazyLitMatrix;
+
+template <
+    typename...Indices,
+    InitFunc<index_by<Indices...>> auto... init
+>
+struct LazyLitMatrix<index_by<Indices...>, init...> {
+};
+
+using T = LazyLitMatrix<index_by<int, char>, 42, 43>;
+
+}
+
+}

[zyn0217]

@cor3ntin I assume we are unable to deduce template arguments in the type constraint, right? This logic would otherwise end up matching the NTTP argument against the NTTP constraint

[cor3ntin]

Unclear. @zygoloid asked that exact question last year (https://lists.isocpp.org/core/2023/03/14099.php) but there are no replies yet

[zyn0217]

Sigh. Looks like having run into another dark corner of C++ :/