feat: conversion helpers from mp-units to std::chrono types added

Resolves #316

docs/CHANGELOG.md

@@ -5,6 +5,7 @@
   - refactor: `quantity` `op+()` and `op-()` reimplemented in terms of `reference` rather then `quantity` types
   - feat: `std::format` support for compliant compilers added
   - feat: HEP system support added (thanks [@RalphSteinhagen](https://github.com/RalphSteinhagen))
+  - feat: conversion helpers from `mp-units` to `std::chrono` types added
   - (!) fix: add `quantity_point::origin`, like `std::chrono::time_point::clock`
   - fix: account for different dimensions in `quantity_point_cast`'s constraint
   - build: Minimum Conan version changed to 1.40

docs/use_cases/interoperability.rst

@@ -65,3 +65,26 @@ provide a deduction guide from `QuantityPointLike`::
     using namespace std::chrono_literals;
 
     static_assert((quantity_point{std::chrono::sys_seconds{1s}} + 1 * s).relative() == 2s);
+
+
+Interoperability with the C++ Standard Library
+----------------------------------------------
+
+The above ``std::chrono``-specific customization points are already predefined in the ``mp-units``
+library for both ``std::chrono::duration`` as well as ``std::chrono::time_point``.
+They are provided in a dedicated header file::
+
+    #include <units/chrono.h>
+
+The same header file provides additional conversion helpers from ``mp-units`` to
+the C++ Standard Library types:
+
+- an explicit conversion of `quantity` to ``std::chrono::duration``::
+
+    template<QuantityOf<isq::si::dim_time> Q>
+    constexpr auto to_std_duration(const Q& q);
+
+- an alias that provides a conversion from `ratio` to ``std::ratio``::
+
+    template<ratio R>
+    using to_std_ratio = /* ... */

src/core/include/units/chrono.h

@@ -58,6 +58,36 @@ namespace detail {
 template<typename C, typename Rep, typename Period>
 inline constexpr bool is_quantity_point_like<std::chrono::time_point<C, std::chrono::duration<Rep, Period>>> = true;
 
+constexpr std::intmax_t pow_10(std::intmax_t v)
+{
+  gsl_Expects(v > 0);
+  std::intmax_t res = 1;
+  for(std::intmax_t i = 0; i < v; i++)
+    res *= 10;
+  return res;
+}
+
+template<ratio R>
+constexpr auto to_std_ratio_impl()
+{
+  if constexpr(R.exp == 0)
+    return std::ratio<R.num, R.den>{};
+  else if constexpr(R.exp > 0)
+    return std::ratio<R.num * pow_10(R.exp), R.den>{};
+  else
+    return std::ratio<R.num, R.den * pow_10(-R.exp)>{};
+}
+
 }  // namespace detail
 
+// TODO ICE below on gcc-11 when `ratio` is used instead of `auto`
+template<auto R>
+using to_std_ratio = decltype(detail::to_std_ratio_impl<R>());
+
+template<QuantityOf<isq::si::dim_time> Q>
+constexpr auto to_std_duration(const Q& q)
+{
+  return std::chrono::duration<typename Q::rep, to_std_ratio<Q::unit::ratio>>(q.number());
+}
+
 } // namespace units

test/unit_test/static/chrono_test.cpp

@@ -90,4 +90,12 @@ static_assert(10_q_m / quantity{2s} == 5_q_m_per_s);
 static_assert(quantity_point{sys_seconds{1s}} + 1_q_s == time_point<std::chrono::system_clock, si::second>{2_q_s});
 static_assert(quantity_point{sys_seconds{1s}} + 1_q_min == time_point<std::chrono::system_clock, si::second>{61_q_s});
 
+// to_duration
+static_assert(to_std_duration(1_q_s) == 1s);
+static_assert(to_std_duration(2_q_h) == 2h);
+static_assert(to_std_duration(3_q_ns) == 3ns);
+static_assert(is_same_v<decltype(to_std_duration(1_q_s))::period, std::ratio<1>>);
+static_assert(is_same_v<decltype(to_std_duration(2_q_h))::period, std::ratio<3600>>);
+static_assert(is_same_v<decltype(to_std_duration(3_q_ns))::period, std::nano>);
+
 }  // namespace