comparing iterator with const_iterator breaks the C++ standard

In containers like QMap, comparing iterator with const_iterator does not always return the expected output.

A quick search through the bug reports shows that this issue has been dismissed in the past as "it's wrong to mix iterator and const_iterator": QTBUG-50314 QTBUG-10025 QTBUG-69182

However it is in the C++ standard, since C++14, that you can compare iterator and const_iterator.

[container.requirements.general] paragraph 7:

In the expressions
  i == j
  i != j
  i < j
  i <= j
  i >= j
  i > j
  i <=> j
  i - j
where i and j denote objects of a container’s iterator type, either or both may be replaced by an object of the container’s const_iterator type referring to the same element with no change in semantics