From 4e0b4321c8311bb0bf3dea530405960dbca97a9d Mon Sep 17 00:00:00 2001
From: Richard Smith <richard-llvm@metafoo.co.uk>
Date: Wed, 12 Nov 2014 23:38:38 +0000
Subject: [PATCH] PR19372: Keep checking template arguments after we see an
 argument pack expansion into a parameter pack; we know that we're still
 filling in that parameter's arguments. Previously, if we hit this case for an
 alias template, we'd try to substitute using non-canonical template
 arguments.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221832 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Sema/SemaTemplate.cpp       | 39 ++++++++++-----------------------
 test/SemaTemplate/deduction.cpp | 14 ++++++++++++
 2 files changed, 25 insertions(+), 28 deletions(-)

diff --git a/lib/Sema/SemaTemplate.cpp b/lib/Sema/SemaTemplate.cpp
index 50a4298333..3fff8b1c86 100644
--- a/lib/Sema/SemaTemplate.cpp
+++ b/lib/Sema/SemaTemplate.cpp
@@ -3695,12 +3695,12 @@ bool Sema::CheckTemplateArgumentList(TemplateDecl *Template,
                                 ArgumentPack.size(), Converted))
         return true;
 
-      if (TemplateArgs[ArgIdx].getArgument().isPackExpansion() &&
-          isa<TypeAliasTemplateDecl>(Template) &&
-          !(Param + 1 == ParamEnd && (*Param)->isTemplateParameterPack() &&
-            !getExpandedPackSize(*Param))) {
+      bool PackExpansionIntoNonPack =
+          TemplateArgs[ArgIdx].getArgument().isPackExpansion() &&
+          (!(*Param)->isTemplateParameterPack() || getExpandedPackSize(*Param));
+      if (PackExpansionIntoNonPack && isa<TypeAliasTemplateDecl>(Template)) {
         // Core issue 1430: we have a pack expansion as an argument to an
-        // alias template, and it's not part of a final parameter pack. This
+        // alias template, and it's not part of a parameter pack. This
         // can't be canonicalized, so reject it now.
         Diag(TemplateArgs[ArgIdx].getLocation(),
              diag::err_alias_template_expansion_into_fixed_list)
@@ -3723,16 +3723,11 @@ bool Sema::CheckTemplateArgumentList(TemplateDecl *Template,
         ++Param;
       }
 
-      // If we just saw a pack expansion, then directly convert the remaining
-      // arguments, because we don't know what parameters they'll match up
-      // with.
-      if (TemplateArgs[ArgIdx-1].getArgument().isPackExpansion()) {
-        bool InFinalParameterPack = Param != ParamEnd &&
-                                    Param + 1 == ParamEnd &&
-                                    (*Param)->isTemplateParameterPack() &&
-                                    !getExpandedPackSize(*Param);
-
-        if (!InFinalParameterPack && !ArgumentPack.empty()) {
+      // If we just saw a pack expansion into a non-pack, then directly convert
+      // the remaining arguments, because we don't know what parameters they'll
+      // match up with.
+      if (PackExpansionIntoNonPack) {
+        if (!ArgumentPack.empty()) {
           // If we were part way through filling in an expanded parameter pack,
           // fall back to just producing individual arguments.
           Converted.insert(Converted.end(),
@@ -3741,22 +3736,10 @@ bool Sema::CheckTemplateArgumentList(TemplateDecl *Template,
         }
 
         while (ArgIdx < NumArgs) {
-          if (InFinalParameterPack)
-            ArgumentPack.push_back(TemplateArgs[ArgIdx].getArgument());
-          else
-            Converted.push_back(TemplateArgs[ArgIdx].getArgument());
+          Converted.push_back(TemplateArgs[ArgIdx].getArgument());
           ++ArgIdx;
         }
 
-        // Push the argument pack onto the list of converted arguments.
-        if (InFinalParameterPack && !ArgumentPack.empty()) {
-          Converted.push_back(
-            TemplateArgument::CreatePackCopy(Context,
-                                             ArgumentPack.data(),
-                                             ArgumentPack.size()));
-          ArgumentPack.clear();
-        }
-
         return false;
       }
 
diff --git a/test/SemaTemplate/deduction.cpp b/test/SemaTemplate/deduction.cpp
index 797f7f8261..c2abf262d6 100644
--- a/test/SemaTemplate/deduction.cpp
+++ b/test/SemaTemplate/deduction.cpp
@@ -178,3 +178,17 @@ namespace PR21536 {
   template<typename ...T> void f(S<T...>);
   void g() { f(S<int, int>()); }
 }
+
+namespace PR19372 {
+  template <template<typename...> class C, typename ...Us> struct BindBack {
+    template <typename ...Ts> using apply = C<Ts..., Us...>;
+  };
+  template <typename, typename...> struct Y;
+  template <typename ...Ts> using Z = Y<Ts...>;
+
+  using T = BindBack<Z, int>::apply<>;
+  using T = Z<int>;
+
+  using U = BindBack<Z, int, int>::apply<char>;
+  using U = Z<char, int, int>;
+}
-- 
2.40.0