The closure type for a non-generic lambda-expression
whose constraints (if any) are satisfied
has a conversion function to pointer to
function with C++ language linkage
the same parameter and return types as the closure type's function call operator.
The conversion is to “pointer to noexcept
if the function call operator
has a non-throwing exception specification.
If the function call operator is a static member function,
then the value returned by this conversion function is
the address of the function call operator.
Otherwise, the value returned by this conversion function
is the address of a function F
that, when invoked,
has the same effect as invoking the closure type's function call operator
on a default-constructed instance of the closure type.
is a constexpr function
if the function call operator is a constexpr function
and is an immediate function
if the function call operator is an immediate function.