Comparing two map::iterators: why does it need the copy constructor of std::pair?

Question

The very simple code below compiles and links without a warning in C++98 but gives an incomprehensible compile error in C++11 mode.

#include 

Answer 1

I think I found it after trying to reduce the error. First, the comparison doesn't seem required to make the program ill-formed. Then, the error message contained the dtor, so I tried not to instantiate the dtor. Result:

#include 

But the output message still contains, now for the line where the ctor of m is called:

error: the parameter for this explicitly-defaulted copy constructor is const, but a member or base requires it to be non-const

 constexpr pair(const pair&) = default;

Which hints to [dcl.fct.def.default]/4

A user-provided explicitly-defaulted function (i.e., explicitly defaulted after its first declaration) is defined at the point where it is explicitly defaulted; if such a function is implicitly defined as deleted, the program is ill-formed.

[emphasis mine]

If, as I assume, [class.copy]/11 says that this ctor should be defined as deleted, then it is defined as deleted immediately - not only when it's odr-used. Therefore, an instantiation shouldn't be required to make the program ill-formed.