[cleanup] Simplity SFINAE of child() member functions
Putting enable_if in an additional argument is easier
to read and allows to rely on return type deduction.
Hence the _lazy_member_child_decltype
-hack is no longer
needed.
Putting enable_if in an additional argument is easier
to read and allows to rely on return type deduction.
Hence the _lazy_member_child_decltype
-hack is no longer
needed.