std::common_type 定義変遷

• C++11 https://timsong-cpp.github.io/cppwp/n3337/meta.trans.other#3
• C++14 https://timsong-cpp.github.io/cppwp/n4140/meta.trans.other#3
• C++17 https://timsong-cpp.github.io/cppwp/n4659/meta.trans.other#3 + https://timsong-cpp.github.io/cppwp/n4659/meta.trans.other#4
• C++20 https://timsong-cpp.github.io/cppwp/n4861/meta.trans.other#4 + https://timsong-cpp.github.io/cppwp/n4861/meta.trans.other#5

LWG

• 2408. SFINAE-friendly common_type/iterator_traits is missing in C++14
• 2141. common_type trait produces reference types
• 2465. SFINAE-friendly common_type is nearly impossible to specialize correctly and regresses key functionality