[llvm-bugs] [Bug 35264] New: Compilation error when pass variadic type arguments to non-variadic template type alias

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35264

            Bug ID: 35264
           Summary: Compilation error when pass variadic type arguments to
                    non-variadic template type alias
           Product: clang
           Version: 5.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: gordon.dmitry at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

The following code fails to compile:

template <template <typename...> class Template>
struct TemplateCheckerOuter
{
    template <typename T>
    struct TemplateCheckerInner
    {
        static constexpr auto Result = false;
    };

    template <typename...Args>
    struct TemplateCheckerInner<Template<Args...>>
    {
        static constexpr auto Result = true;
    };
};

template <typename Val>
class TemplateClass {};

// If change to the following code compiles well:
// template <typename ...Val>
// using TemplateClassAlias = TemplateClass<Val...>;
template <typename Val>
using TemplateClassAlias = TemplateClass<Val>;

// This line succedes to compile
static_assert(TemplateCheckerOuter<TemplateClass>::TemplateCheckerInner<TemplateClassAlias<int>>::Result,"");
// This line fails to compile
static_assert(TemplateCheckerOuter<TemplateClassAlias>::TemplateCheckerInner<TemplateClassAlias<double>>::Result,"");


The error is:
11 : <source>:11:42: error: pack expansion used as argument for non-pack
parameter of alias template
    struct TemplateCheckerInner<Template<Args...>>
                                         ^~~~~~~
29 : <source>:29:15: note: in instantiation of template class
'TemplateCheckerOuter<TemplateClassAlias>' requested here
static_assert(TemplateCheckerOuter<TemplateClassAlias>::TemplateCheckerInner<TemplateClassAlias<double>>::Result,"");
              ^
23 : <source>:23:20: note: template parameter is declared here
template <typename Val>
                   ^
29 : <source>:29:1: error: static_assert failed ""
static_assert(TemplateCheckerOuter<TemplateClassAlias>::TemplateCheckerInner<TemplateClassAlias<double>>::Result,"");
^

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