With function templates the combination of the types of template arguments and
template parameters shows some interesting contractions. What happens, for
example if a template type parameter is specified as an rvalue reference but
an lvalue reference argument type is provided?

In such cases the compiler performs type contractions. Doubling identical
reference types results in a simple contraction: the type is deduced to be a
single reference type. Example: if the template parameter type is specified as
a tt(Type &&) and the actual parameter is an tt(int &&) then tt(Type) is
deduced to be an tt(int), rather than an tt(int &&).

This is fairly intuitive. But what happens if the actual type is tt(int &)?
There is no such thing as an tt(int & &&param) and so the compiler contracts
the double reference by removing the rvalue reference, keeping the lvalue
reference. Here the following rules are applied:

    quote(
    1. A function template parameter defined as an lvalue reference to
    a template's type parameter (e.g., tt(Type &)) receiving an lvalue
    reference argument results in a single lvalue reference.

    2. A function template parameter defined as an rvalue reference to a
    template's type parameter (e.g., tt(Type &&)) receiving any kind of
    reference argument uses the reference type of the argument.  )
Examples:
    itemization(
    it() When providing an tt(Actual &) argument then tt(Type &) becomes an
    tt(Actual &) and tt(Type) is inferred as tt(Actual);

    it() When providing an tt(Actual &) then tt(Type &&) becomes an
    tt(Actual &) and tt(Type) is inferred as tt(Actual);

    it() When providing an tt(Actual &&) then tt(Type &) also becomes
    tt(Actual &)  and tt(Type) is inferred as tt(Actual);

    it() When providing an tt(Actual &&) then tt(Type &&)  becomes tt(Actual
    &&) and tt(Type) is inferred as tt(Actual);
    )

Let's look at a concrete example where contraction occurs. Consider the
following function template where a function parameter is defined as an rvalue
references to some template type parameter:
        verb(    template <typename Type>
    void function(Type &&param)
    {
        callee(static_cast<Type &&>(param));
    })

In this situation, when tt(function) is called with an (lvalue) argument of
type tt(TP &) the template type parameter tt(Type) is deduced to be tt(Tp
&). Therefore, tt(Type &&param) is instantiated as tt(Tp &param), tt(Type)
becomes tt(Tp) and the rvalue reference is replaced by an lvalue reference.

Likewise, when tt(callee) is called using the tt(static_cast) the same
contraction occurs, so tt(Type &&param) operates on tt(Tp &param). Therefore
(using contraction) the static cast em(also) uses type tt(Tp &param). If
tt(param) happened to be of type tt(Tp &&) then the static cast uses type
tt(Tp &&param).

This characteristic allows us to pass a function argument to a nested function
em(without) changing its type: lvalues remain lvalues, rvalues remain
rvalues. This characteristic is therefore also known as
 emi(perfect forwarding) which is discussed in greater detail in section
ref(PERFECT). Perfect forwarding prevents the template author from having to
define multiple overloaded versions of a function template.
