函数是一种callable type吗? c++标准里面说道:
20.8.1 Definitions [func.def]
1 The following definitions apply to this Clause:
2 A call signature is the name of a return type ...
declval里面,什么是"unevaluated context"? cppreference的网站上,关于declval的介绍里面提到en.cppreference.com/w/cpp/utility/declval)
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Note that because no definition exist ...