[clang] Fix handling of unexpanded packs in template argument expressions.

Closes #58679.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Differential Revision: https://reviews.llvm.org/D136977

clang/docs/ReleaseNotes.rst

@@ -266,6 +266,8 @@ Bug Fixes
   `Issue 45736 <https://github.com/llvm/llvm-project/issues/45736>`_
 - Fix an issue when performing constraints partial ordering on non-template
   functions. `Issue 56154 <https://github.com/llvm/llvm-project/issues/56154>`_
+- Fix handling of unexpanded packs in template argument expressions.
+  `Issue 58679 <https://github.com/llvm/llvm-project/issues/58679>`_
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/Sema.h

@@ -6818,7 +6818,8 @@ public:
         Expr, Expr ? Expr->getExprLoc() : SourceLocation(), DiscardedValue);
   }
   ExprResult ActOnFinishFullExpr(Expr *Expr, SourceLocation CC,
-                                 bool DiscardedValue, bool IsConstexpr = false);
+                                 bool DiscardedValue, bool IsConstexpr = false,
+                                 bool IsTemplateArgument = false);
   StmtResult ActOnFinishFullStmt(Stmt *Stmt);
 
   // Marks SS invalid if it represents an incomplete type.

clang/lib/Sema/SemaExprCXX.cpp

@@ -8713,14 +8713,14 @@ Sema::CorrectDelayedTyposInExpr(Expr *E, VarDecl *InitDecl,
 }
 
 ExprResult Sema::ActOnFinishFullExpr(Expr *FE, SourceLocation CC,
-                                     bool DiscardedValue,
-                                     bool IsConstexpr) {
+                                     bool DiscardedValue, bool IsConstexpr,
+                                     bool IsTemplateArgument) {
   ExprResult FullExpr = FE;
 
   if (!FullExpr.get())
     return ExprError();
 
-  if (DiagnoseUnexpandedParameterPack(FullExpr.get()))
+  if (!IsTemplateArgument && DiagnoseUnexpandedParameterPack(FullExpr.get()))
     return ExprError();
 
   if (DiscardedValue) {

clang/lib/Sema/SemaOverload.cpp

@@ -5862,9 +5862,9 @@ static ExprResult CheckConvertedConstantExpression(Sema &S, Expr *From,
 
   // C++2a [intro.execution]p5:
   //   A full-expression is [...] a constant-expression [...]
-  Result =
-      S.ActOnFinishFullExpr(Result.get(), From->getExprLoc(),
-                            /*DiscardedValue=*/false, /*IsConstexpr=*/true);
+  Result = S.ActOnFinishFullExpr(Result.get(), From->getExprLoc(),
+                                 /*DiscardedValue=*/false, /*IsConstexpr=*/true,
+                                 CCE == Sema::CCEKind::CCEK_TemplateArg);
   if (Result.isInvalid())
     return Result;
 

clang/lib/Sema/SemaTemplate.cpp

@@ -7156,11 +7156,8 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
   }
 
   // If either the parameter has a dependent type or the argument is
-  // type-dependent, there's nothing we can check now. The argument only
-  // contains an unexpanded pack during partial ordering, and there's
-  // nothing more we can check in that case.
-  if (ParamType->isDependentType() || Arg->isTypeDependent() ||
-      Arg->containsUnexpandedParameterPack()) {
+  // type-dependent, there's nothing we can check now.
+  if (ParamType->isDependentType() || Arg->isTypeDependent()) {
     // Force the argument to the type of the parameter to maintain invariants.
     auto *PE = dyn_cast<PackExpansionExpr>(Arg);
     if (PE)

clang/test/CXX/temp/temp.decls/temp.variadic/p5.cpp

@@ -502,3 +502,22 @@ template <class> using B = char;
 template <class ...Cs> int C{ A<B<Cs>>{}... }; // expected-error {{implicit instantiation of undefined template}}
 #endif
 } // namespace GH56094
+
+namespace GH58679 {
+#if __cplusplus >= 201402L
+template <class> constexpr int A = 1;
+
+template <int> struct B;
+template <> struct B<1> { using b1 = void; };
+
+template <class> using C = char;
+
+template <class... Ds> int D{ B<A<C<Ds>>>{}... };
+
+struct E {
+  template <class E1, class = typename B<A<E1>>::b1> E(E1);
+};
+
+template <typename... Es> int F{ E(C<Es>{})... };
+#endif
+} // namespace GH58679