Macro impl_arg 

macro_rules! impl_arg {
    ( $( $t:ty ),* ) => { ... };
    ( generic@ $( $t:ty ),* ) => { ... };
    ( generic@ $( $t:ty where $($bounds:tt)* ),* ) => { ... };
    (@internal $t:ty $(, $gen:ident)? | $(, $($bounds:tt)*)? ) => { ... };
}

Allows this type to be the target of a map operation from any source type, e.g. Parametric::map(|any: T| -> Self { /* ... */ })

This implementation serves as a workaround until the specialization feature is stabilized in Rust.