why are these std::set iterators of different type when compiling with -D_GLIBCXX_DEBUG

[Dennis Luehring]

using latest gcc/STL

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#include <set>

using int_set1 = std::set<int, std::greater<int>>;
using int_set2 = std::set<int>;

static_assert(std::is_same<int_set1::iterator, int_set2::iterator>());
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the two iterators are equal when not using _GLIBCXX_DEBUG but become
different when using the define?