fix: common_unit handling fixed for some corner cases

test/static/unit_symbol_test.cpp

@@ -20,6 +20,7 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#include <mp-units/systems/angular.h>
 #include <mp-units/systems/iau.h>
 #include <mp-units/systems/iec.h>
 #include <mp-units/systems/international.h>
@@ -209,6 +210,10 @@ static_assert(unit_symbol(get_common_unit(kilo<metre> / hour, metre / second) /
               "EQUIV{[1/5 km/h], [1/18 m/s]}/s");
 static_assert(unit_symbol(get_common_unit(kilo<metre> / hour, metre / second) * second) ==
               "EQUIV{[1/5 km/h], [1/18 m/s]} s");
+static_assert(unit_symbol(get_common_unit(radian, degree)) == "EQUIV{[1/𝜋°], [1/180 rad]}");
+static_assert(unit_symbol(get_common_unit(angular::radian, angular::revolution)) == "EQUIV{rad, [2⁻¹ 𝜋⁻¹ rev]}");
+static_assert(unit_symbol<usf{.solidus = always}>(get_common_unit(angular::radian, angular::revolution)) ==
+              "EQUIV{rad, [1/(2 𝜋) rev]}");
 
 // derived units
 static_assert(unit_symbol(one) == "");  // NOLINT(readability-container-size-empty)