[llvm-bugs] [Bug 45782] New: Unused arguments in alias template specialisation are not treated as substitution failure

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45782

            Bug ID: 45782
           Summary: Unused arguments in alias template specialisation are
                    not treated as substitution failure
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++17
          Assignee: unassignedclangbugs at nondot.org
          Reporter: v at vsamko.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

template<typename... T>
  using void_t = void;
  template<typename T, typename=void>
  struct has_reflect_members {};
  template<typename T>
  struct has_reflect_members<T, void_t<typename T::mytype>> {};
  template<typename T>
  struct has_reflect_members<T, void_t<typename T::anothertype>> {};

results in 
  <source>:9:8: error: redefinition of 'has_reflect_members<T, void_t<typename 
  T::anothertype> >'
  struct has_reflect_members<T, void_t<typename T::anothertype>> {};
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  <source>:7:8: note: previous definition is here
  struct has_reflect_members<T, void_t<typename T::mytype>> {};

But according to http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1558
these specialisations can rely on substitution failure and they wouldn't be
redefinitions.

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