Fix crash and rejects-valid when a later template parameter or default

template argument contains a backreference to a dependently-typed
earlier parameter.

In a case like:
  template<typename T, T A, decltype(A) = A> struct X {};
  template<typename U> auto Y = X<U, 0>();
we previously treated both references to `A` in the third parameter as
being of type `int` when checking the template-id in `Y`. That`s wrong;
the type of `A` in these contexts is the dependent type `U`.

When we encounter a non-type template argument that we can't convert to
the parameter type because of type-dependence, we now insert a dependent
conversion node so that the SubstNonTypeTemplateParmExpr for the
template argument will have the parameter's type rather than whatever
type the argument had.

llvm-svn: 363972

1 files changed, 35 insertions, 10 deletions

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 74a891bcb70..7b0a3a3e094 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -4888,10 +4888,22 @@ bool Sema::CheckTemplateArgument(NamedDecl *Param,
 
       TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack,
                                         Converted);
-      NTTPType = SubstType(NTTPType,
-                           MultiLevelTemplateArgumentList(TemplateArgs),
-                           NTTP->getLocation(),
-                           NTTP->getDeclName());
+
+      // If the parameter is a pack expansion, expand this slice of the pack.
+      if (auto *PET = NTTPType->getAs<PackExpansionType>()) {
+        Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(*this,
+                                                           ArgumentPackIndex);
+        NTTPType = SubstType(PET->getPattern(),
+                             MultiLevelTemplateArgumentList(TemplateArgs),
+                             NTTP->getLocation(),
+                             NTTP->getDeclName());
+      } else {
+        NTTPType = SubstType(NTTPType,
+                             MultiLevelTemplateArgumentList(TemplateArgs),
+                             NTTP->getLocation(),
+                             NTTP->getDeclName());
+      }
+
       // If that worked, check the non-type template parameter type
       // for validity.
       if (!NTTPType.isNull())
@@ -6310,9 +6322,7 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
     // When checking a deduced template argument, deduce from its type even if
     // the type is dependent, in order to check the types of non-type template
     // arguments line up properly in partial ordering.
-    Optional<unsigned> Depth;
-    if (CTAK != CTAK_Specified)
-      Depth = Param->getDepth() + 1;
+    Optional<unsigned> Depth = Param->getDepth() + 1;
     if (DeduceAutoType(
             Context.getTrivialTypeSourceInfo(ParamType, Param->getLocation()),
             Arg, ParamType, Depth) == DAR_Failed) {
@@ -6367,9 +6377,24 @@ 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.
   if (ParamType->isDependentType() || Arg->isTypeDependent()) {
-    // FIXME: Produce a cloned, canonical expression?
-    Converted = TemplateArgument(Arg);
-    return Arg;
+    // Force the argument to the type of the parameter to maintain invariants.
+    auto *PE = dyn_cast<PackExpansionExpr>(Arg);
+    if (PE)
+      Arg = PE->getPattern();
+    ExprResult E = ImpCastExprToType(
+        Arg, ParamType.getNonLValueExprType(Context), CK_Dependent,
+        ParamType->isLValueReferenceType() ? VK_LValue :
+        ParamType->isRValueReferenceType() ? VK_XValue : VK_RValue);
+    if (E.isInvalid())
+      return ExprError();
+    if (PE) {
+      // Recreate a pack expansion if we unwrapped one.
+      E = new (Context)
+          PackExpansionExpr(E.get()->getType(), E.get(), PE->getEllipsisLoc(),
+                            PE->getNumExpansions());
+    }
+    Converted = TemplateArgument(E.get());
+    return E;
   }
 
   // The initialization of the parameter from the argument is