https://bugs.llvm.org/show_bug.cgi?id=38044
Bug ID: 38044
Summary: clang fails to deduce the most specialized template
definition when it's partialy specialized with
multiple template template parameters
Product: clang
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: C++
Assignee: unassignedclangb...@nondot.org
Reporter: michaelroyn...@gmail.com
CC: dgre...@apple.com, llvm-bugs@lists.llvm.org
I was doing simple metaprogramming recently and wanted to code the "front"
helper: get the first type of a type list.
So I experimented on wandbox this code :
#include <type_traits>
template <typename... Ts>
struct type_list {
static constexpr std::size_t size = sizeof...(Ts);
};
template <typename List, typename = void>
struct front {};
template <template <typename, typename...> class List, typename T, typename...
Ts>
struct front<List<T, Ts...>, std::enable_if_t<(List<T, Ts...>::size > 0)>> {
using type = T;
};
template <typename List>
using front_t = typename front<List>::type;
There's nothing fancy (Please let me know if the code above is not legal C++).
Usage is :
static_assert(std::is_same_v<int, front_t<type_list<int, double>>>);
The problem is when I'm specializing with a template template like this :
template <typename, typename...> class List
Then clang throws it away and doesn't find anything.
However, if I specialize only with :
template <typename...> class List
Then it's fine and clang doesn't throw it away (but it's not ok because I can't
do what I want to do this way).
I think clang is wrong here and should consider this specialization.
It consistently fails since clang 4.0 haven't tested the version before) up
until trunk.
FWIW, both gcc and msvc compile it just fine.
Here's a godbolt repro : https://godbolt.org/g/M5fgbj
Please let me know if what I'm writing is not legal C++.
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