Finish fleshing out make_ratio

src/core/include/units/magnitude.h

@@ -81,7 +81,7 @@ using prime_factorization_t = typename prime_factorization<N>::type;
 } // namespace detail
 
 template <std::intmax_t N, std::intmax_t D = 1>
-auto make_ratio() {
+constexpr auto make_ratio() {
   return quotient_t<detail::prime_factorization_t<N>, detail::prime_factorization_t<D>>{};
 }
 
@@ -95,10 +95,14 @@ struct pi {
 
 template <BasePower... Bs>
 struct magnitude {
-  bool operator==(magnitude) const { return true; }
+  template <Magnitude M>
+  constexpr bool operator==(M) const { return std::is_same_v<magnitude, M>; }
 
-  template <BasePower... OtherBs>
+  template <Magnitude M>
-  bool operator==(magnitude<OtherBs...>) const { return false; }
+  constexpr friend auto operator*(magnitude, M) { return product_t<magnitude, M>{}; }
+
+  template <Magnitude M>
+  constexpr friend auto operator/(magnitude, M) { return quotient_t<magnitude, M>{}; }
 };
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////

test/unit_test/runtime/magnitude_test.cpp

@@ -248,6 +248,51 @@ TEST_CASE("Equality works for magnitudes")
     CHECK(make_ratio<3>() != make_ratio<5>());
     CHECK(make_ratio<3>() != make_ratio<3, 2>());
   }
+
+  SECTION("Supports constexpr")
+  {
+    constexpr auto eq = make_ratio<4, 5>() == make_ratio<4, 3>();
+    CHECK(!eq);
+  }
+}
+
+TEST_CASE("Multiplication works for magnitudes")
+{
+  SECTION("Reciprocals reduce to null magnitude")
+  {
+    CHECK(make_ratio<3, 4>() * make_ratio<4, 3>() == make_ratio<1>());
+  }
+
+  SECTION("Products work as expected")
+  {
+    CHECK(make_ratio<4, 5>() * make_ratio<4, 3>() == make_ratio<16, 15>());
+  }
+
+  SECTION("Supports constexpr")
+  {
+    constexpr auto p = make_ratio<4, 5>() * make_ratio<4, 3>();
+    CHECK(p == make_ratio<16, 15>());
+  }
+}
+
+TEST_CASE("Division works for magnitudes")
+{
+  SECTION("Dividing anything by itself reduces to null magnitude")
+  {
+    CHECK(make_ratio<3, 4>() / make_ratio<3, 4>() == make_ratio<1>());
+    CHECK(make_ratio<15>() / make_ratio<15>() == make_ratio<1>());
+  }
+
+  SECTION("Quotients work as expected")
+  {
+    CHECK(make_ratio<4, 5>() / make_ratio<4, 3>() == make_ratio<3, 5>());
+  }
+
+  SECTION("Supports constexpr")
+  {
+    constexpr auto q = make_ratio<4, 5>() / make_ratio<4, 3>();
+    CHECK(q == make_ratio<3, 5>());
+  }
 }
 
 namespace detail