Checking rvalue insert result for std::map gives unexpected results

Question

Test code snippet below (CHECK is macro from test framework and works fine):

typedef std::map<int, double> M;
typedef M::value_type VT;

const VT v0;
M mm;

CHECK(mm.begin() == mm.insert(v0).first);

gives false as result:

ERROR: check is NOT correct CHECK( mm.begin() == mm.insert(v0).first )

values: CHECK( {?} == {?} )

Changing last string to

1)

CHECK(bool(mm.begin() == mm.insert(v0).first));

CHECK(bool(mm.insert(v0).first == mm.begin()));

CHECK(mm.insert(v0).first == mm.begin());

const bool result = mm.insert(v0).first == mm.begin();
CHECK(result);

change nothing: the result is still false

However if I save rvalue of insert result into temporary everything just works:

typedef std::map<int, double> M;
typedef M::value_type VT;

const VT v0;
M mm;

M::iterator it = mm.insert(v0).first;
CHECK(mm.begin() == it);

SUCCESS: check is correct CHECK( mm.begin() == it )

values: CHECK( {?} == {?} )

This behaviour happens only in Release build mode in VS2019 (C++17 or higher) and doesn't happen in any other mode or compiler I've tested.

Could anyone please explain why this is normal or a bug? With C++ standard quotes if necessary.

P.S.: Just to be clear. In my understanding of standard and C++ result is strange because:

1. std::map::insert does NOT invalidate any iterators (by standard and MS VS implementation). So call order is not an issue (however I tried all 1-4 possible combinations).
2. bool operator(const std::map::iterator &lhs, const std::map::iterator &rhs) should prolong lifetime of insertion result rvalue iterator(second) and std::map::end() rvalue iterator. So comparison should be well defined.
3. C++17 does not define any other rules in that case than C++11 or even C++03.

Answer 1

As Jarod42 said, mm.begin() has unpredictable value.

For different compilers, the results are different:

For issues with different C++ standard running results, you can report the problem to the Developer Community and post a link in the comment so that we can follow up.