struct is_function

is_function()

Determines whether the passed type `T` is a function type.

This metafunction is designed to be a drop-in replacement for the C++11

`std::is_function()` metafunction for platforms that have incomplete C++11

support (such as libstdc++ 4.x).

This metafunction works because appending `const` to a type does nothing to

function types and reference types (and forms a const-qualified type

otherwise).