为什么 C++ 20 中使用 require 表达式的“Oneable”概念的某些实现无法在某些编译器上编译？
// `Oneable` implemented using `requires` with a simple assignment expression
// clang, gcc, and msvc all compile
template <class T>
concept Oneable = requires(T n) { n = 1; };
static_assert(Oneable<int>);
static_assert(!Oneable<int*>);
// `Oneable` implemented using `requires` with an assignment statement inside
// the body of a lambda expression
// clang and msvc compile
// gcc gives a compiler error on instantiating Oneable<T*>
template <class T>
concept Oneable = requires { []() { T …