[llvm-bugs] [Bug 38268] New: class template partial specialization contains a template parameter that cannot be deduced

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38268

            Bug ID: 38268
           Summary: class template partial specialization contains a
                    template parameter that cannot be deduced
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zhonghao at pku.org.cn
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

The code is as follow:

template <typename T>
 struct outer
 {
 template <typename U>
 struct inner
 {

 };
 };


 template <typename T>
 struct is_inner_for
 {
 template <typename Whatever>
 struct predicate
 {
 static constexpr bool value = false;
 };

 template <typename U>
 struct predicate<typename outer<T>::template inner<U>>
 {
 static constexpr bool value = true;
 };
 };

clang++ rejects the code:

code0.cpp:22:9: error: class template partial specialization contains a
      template parameter that cannot be deduced; this partial specialization
      will never be used [-Wunusable-partial-specialization]
 struct predicate<typename outer<T>::template inner<U>>
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
code0.cpp:21:21: note: non-deducible template parameter 'U'
 template <typename U>
                    ^
1 error generated.

g++ 4.8, 5.2 accept the above code. It seems to be difficult to determine
whether the code is legal or not. I found that g++ 6 rejects the code with a
similar message:

        code0.cpp:22:9: error: template parameters not deducible in partial
specialization:
          struct predicate<typename outer<T>::template inner<U>> :
std::true_type
                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        code0.cpp:22:9: note:         'U'
        code0.cpp:26:1: error: static assertion failed: Yay!
         static_assert(
         ^~~~~~~~~~~~~

However, g++ 9.0 again accepts the code.

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