MSVC里弄了个条件定义，set时确实也是iterator  = _Tree_const_iterator<_Scary_val>

template <class _Traits>
class _Tree { // ordered red-black tree for map/multimap/set/multiset
public:
    static constexpr bool _Is_set = is_same_v<key_type, value_type>;

public:

    using iterator       = conditional_t<_Is_set, _Tree_const_iterator<_Scary_val>, _Tree_iterator<_Scary_val>>;


【 在 gameplayer 的大作中提到: 】
: 差不多，glibc中set的iterator和const_iterator是相同的，在stl_set.h中有:
:       // _GLIBCXX_RESOLVE_LIB_DEFECTS
:       // DR 103. set::iterator is required to be modifiable,
: ...................
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