[llvm-bugs] [Bug 43496] New: A bug for template specialization

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43496

            Bug ID: 43496
           Summary: A bug for template specialization
           Product: clang
           Version: 8.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++17
          Assignee: unassignedclangbugs at nondot.org
          Reporter: liwei.cpp at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

The code is as follows:

#include <type_traits>

template <typename> class AddLayer;

template <template <typename> class TLayer>
struct LayerInputPortSet
{
    using type = int;
};

template <>
struct LayerInputPortSet<AddLayer>
{
    using type = float;
};

using type1 = typename LayerInputPortSet<AddLayer>::type;
static_assert(std::is_same_v<type1, float>);

template <template<typename> class TLayer>
struct Sublayer
{
    template <typename TInputs>
    using LayerType = TLayer<TInputs>;
};

using type = typename LayerInputPortSet<Sublayer<AddLayer>::template
LayerType>::type;
static_assert(std::is_same_v<type, float>);

When compiling (with -std=c++17), type1 == float, but with the introduction of
a wrapper (Sublayer), type != float. In fact, the compiler thought that type is
int. 

A similar issue is as follows:

#include <type_traits>

template <typename> class AddLayer;

template <template <typename> class TLayer>
struct LayerInputPortSet;

template <>
struct LayerInputPortSet<AddLayer>
{
    using type = float;
};

using type1 = typename LayerInputPortSet<AddLayer>::type;
static_assert(std::is_same_v<type1, float>);

template <template<typename> class TLayer>
struct Sublayer
{
    template <typename TInputs>
    using LayerType = TLayer<TInputs>;
};

using type = typename LayerInputPortSet<Sublayer<AddLayer>::template
LayerType>::type;
static_assert(std::is_same_v<type, float>);

When compiling, the compiler reports that 
implicit instantiation of undefined template
'LayerInputPortSet<Sublayer<AddLayer>::template LayerType>'

This bug happened on clang++ 6, 7, 8.

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