import std.traits : Signed;
immutable uint u = 42;
auto s1 = signed(u); //not qualified
static assert(is(typeof(s1) == int));
Signed!(typeof(u)) s2 = signed(u); //same qualification
static assert(is(typeof(s2) == immutable int));
immutable s3 = signed(u); //explicitly qualified
Returns the corresponding signed value for x (e.g. if x has type uint, it returns cast(int) x). The advantage compared to the cast is that you do not need to rewrite the cast if x later changes type (e.g from uint to ulong).
Note that the result is always mutable even if the original type was const or immutable. In order to retain the constness, use std.traits.Signed.