Add 'xo-unit/' from commit 'b531e382c2'

git-subtree-dir: xo-unit
git-subtree-mainline: e9ee6992ca
git-subtree-split: b531e382c2

xo-unit/example/ex4/ex4.cpp

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+/** @file ex4.cpp **/
+
+#include "xo/unit/quantity.hpp"
+#include "xo/unit/quantity_iostream.hpp"
+#include <iostream>
+
+int
+main () {
+    namespace q = xo::qty::qty;
+
+    auto t1 = q::milliseconds(1);
+    auto t2 = q::minutes(1);
+
+    auto r1 = t1 / with_repr<double>(t2);
+
+    static_assert(r1.is_dimensionless());
+    static_assert(!t2.is_dimensionless());
+
+    static_assert(std::same_as<decltype(static_cast<double>(r1)), double>);
+
+    /* r1_value: assignment compiles,  since r1 dimensionless */
+    double r1_value = r1;
+
+    /* r2_value: assignment won't compile,  'cannot convert' error */
+    //double r2_value = t2;
+
+    std::cerr << "t1: " << t1 << ", t2: " << t2 << ", t1/t2: " << r1 << std::endl;
+}
+
+/** end ex4.cpp */