[Clang] Fix unexpanded packs in NTTP type constraints (#121296)

In the case where a type-constraint on an NTTP contains a pack, we form
a PackExpansionType to model it. However, there are a few places
expecting it to be a non-pack expansion, and luckily only small changes
could make them work.

Fixes #88866

Author: zyn0217

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)
 - Fixed an issue while resolving type of expression indexing into a pack of values of non-dependent type (#GH121242)
 
 Bug Fixes to AST Handling

clang/lib/AST/ASTContext.cpp

@@ -6376,7 +6376,7 @@ ASTContext::getAutoType(QualType DeducedType, AutoTypeKeyword Keyword,
 }
 
 QualType ASTContext::getUnconstrainedType(QualType T) const {
-  QualType CanonT = T.getCanonicalType();
+  QualType CanonT = T.getNonPackExpansionType().getCanonicalType();
 
   // Remove a type-constraint from a top-level auto or decltype(auto).
   if (auto *AT = CanonT->getAs<AutoType>()) {

clang/lib/Sema/SemaTemplate.cpp

@@ -1228,7 +1228,7 @@ bool Sema::AttachTypeConstraint(AutoTypeLoc TL,
                                 NonTypeTemplateParmDecl *NewConstrainedParm,
                                 NonTypeTemplateParmDecl *OrigConstrainedParm,
                                 SourceLocation EllipsisLoc) {
-  if (NewConstrainedParm->getType() != TL.getType() ||
+  if (NewConstrainedParm->getType().getNonPackExpansionType() != TL.getType() ||
       TL.getAutoKeyword() != AutoTypeKeyword::Auto) {
     Diag(NewConstrainedParm->getTypeSourceInfo()->getTypeLoc().getBeginLoc(),
          diag::err_unsupported_placeholder_constraint)
@@ -1530,9 +1530,19 @@ 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 (D.getEllipsisLoc().isInvalid() &&
+          T->containsUnexpandedParameterPack()) {
+        assert(TL.getConceptReference()->getTemplateArgsAsWritten());
+        for (auto &Loc :
+             TL.getConceptReference()->getTemplateArgsAsWritten()->arguments())
+          Invalid |= DiagnoseUnexpandedParameterPack(
+              Loc, UnexpandedParameterPackContext::UPPC_TypeConstraint);
+      }
+      if (!Invalid &&
+          AttachTypeConstraint(TL, Param, Param, D.getEllipsisLoc()))
         Invalid = true;
+    }
 
   if (Invalid)
     Param->setInvalidDecl();

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -857,7 +857,10 @@ class PackDeductionScope {
       if (auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(
               TemplateParams->getParam(Index))) {
         if (!NTTP->isExpandedParameterPack())
-          if (auto *Expansion = dyn_cast<PackExpansionType>(NTTP->getType()))
+          // FIXME: CWG2982 suggests a type-constraint forms a non-deduced
+          // context, however it is not yet resolved.
+          if (auto *Expansion = dyn_cast<PackExpansionType>(
+                  S.Context.getUnconstrainedType(NTTP->getType())))
             ExtraDeductions.push_back(Expansion->getPattern());
       }
       // FIXME: Also collect the unexpanded packs in any type and template

clang/test/SemaCXX/cxx2c-fold-exprs.cpp

@@ -305,3 +305,82 @@ 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 ExpandsBoth {
+
+template <typename Indices, InitFunc<Indices> auto... init>
+struct LazyLitMatrix; // expected-note {{here}}
+
+template <
+    typename...Indices,
+    InitFunc<index_by<Indices>> auto... init
+>
+struct LazyLitMatrix<index_by<Indices...>, init...> {
+};
+
+// FIXME: Explain why we didn't pick up the partial specialization - pack sizes don't match.
+template struct LazyLitMatrix<index_by<int, char>, 42>;
+// expected-error@-1 {{instantiation of undefined template}}
+template struct LazyLitMatrix<index_by<int, char>, 42, 43>;
+
+}
+
+namespace ExpandsRespectively {
+
+template <typename Indices, InitFunc<Indices> auto... init>
+struct LazyLitMatrix;
+
+template <
+    typename...Indices,
+    InitFunc<index_by<Indices...>> auto... init
+>
+struct LazyLitMatrix<index_by<Indices...>, init...> {
+};
+
+template struct LazyLitMatrix<index_by<int, char>, 42>;
+template struct LazyLitMatrix<index_by<int, char>, 42, 43>;
+
+}
+
+namespace TypeParameter {
+
+template <typename Indices, InitFunc<Indices>... init>
+struct LazyLitMatrix; // expected-note {{here}}
+
+template <
+    typename...Indices,
+    InitFunc<index_by<Indices>>... init
+>
+struct LazyLitMatrix<index_by<Indices...>, init...> {
+};
+
+// FIXME: Explain why we didn't pick up the partial specialization - pack sizes don't match.
+template struct LazyLitMatrix<index_by<int, char>, float>;
+// expected-error@-1 {{instantiation of undefined template}}
+template struct LazyLitMatrix<index_by<int, char>, unsigned, float>;
+
+}
+
+namespace Invalid {
+
+template <typename Indices, InitFunc<Indices>... init>
+struct LazyLitMatrix;
+
+template <
+    typename...Indices,
+    InitFunc<index_by<Indices>> init
+    // expected-error@-1 {{unexpanded parameter pack 'Indices'}}
+>
+struct LazyLitMatrix<index_by<Indices...>, init> {
+};
+
+}
+
+}