vec

import std.algorithm : equal;
import std.traits : Unqual;

const double[] arr = [1, 2, 4, 0];  // arr.length known at runtime
const v1 = vec4 (arr);             // asserts that arr.length == 4
static assert( is(Unqual!(typeof(v1)) == DVec4) );
assert(equal(v1.data, [1, 2, 4, 0]));

const int comp = 2;
const v2 = vec4 (comp);
static assert( is(Unqual!(typeof(v2)) == IVec4) );
assert(equal(v2.data, [2, 2, 2, 2]));