boost::fusion::map or boost::fusion::map surprising behavior

Hi!

I found surprising behavior with `boost::fusion::map`.
Given the following set of types:
```cpp
struct Foo {};
struct Bar {};
using Child = boost::fusion::map>;
using Parent = boost::fusion::map>;
```
The following example fails to compile:
```cpp
auto example()
{
return Parent{
boost::fusion::make_pair(
boost::fusion::make_pair(42))};
}
```
This can be fixed by doing:
```cpp
auto example()
{
return Parent{
boost::fusion::pair(
boost::fusion::make_pair(42))};
}
```
While I do understand while the former fails and the latter doesn't. I do find this suprising behavior. Specially, considering the errors produced by the failing example.

Start with the minimal examples in the issue and inspect the behavior of boost::fusion::map, boost::fusion::pair, and make_pair during compilation. The issue names no source files or tests; done would require an agreed resolution for the surprising construction behavior, with its expected behavior documented or verified by project tests.

Written by the indexing model from the issue text.