Design cleanup

- unknown_unit added
- examples refactored
- base_type renamed to downcast_base_type
- scaled_unit renamed to named_scaled_unit
- detail::reference_unit renamed to scaled_unit
- quantity_test cleanup

src/include/units/physical/dimensions.h

@@ -35,7 +35,7 @@ template<typename Dim, template<typename...> typename DimTemplate>
 concept DimensionOf = (Dimension<Dim> || BaseDimension<Dim>) && is_derived_from_instantiation<Dim, DimTemplate>;
 
 template<typename Q, template<typename...> typename DimTemplate>
-concept QuantityOf = Quantity<Q> && is_derived_from_instantiation<Q::dimension, DimTemplate>;
+concept QuantityOf = Quantity<Q> && is_derived_from_instantiation<typename Q::dimension, DimTemplate>;
 
 // ------------------------ base dimensions -----------------------------
 

src/include/units/physical/si/acceleration.h

@@ -0,0 +1,44 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#pragma once
+
+#include <units/physical/dimensions.h>
+#include <units/physical/si/velocity.h>
+
+namespace units::si {
+
+struct metre_per_second_sq : unit<metre_per_second_sq> {};
+struct dim_acceleration : physical::dim_acceleration<dim_acceleration, metre_per_second_sq, dim_length, dim_time> {};
+
+template<Unit U, Scalar Rep = double>
+using acceleration = quantity<dim_acceleration, U, Rep>;
+
+inline namespace literals {
+
+// mps_sq
+constexpr auto operator""mps_sq(unsigned long long l) { return acceleration<metre_per_second_sq, std::int64_t>(l); }
+constexpr auto operator""mps_sq(long double l) { return acceleration<metre_per_second_sq, long double>(l); }
+
+}  // namespace literals
+
+}  // namespace units::si

src/include/units/physical/si/length.h

@@ -58,10 +58,10 @@ constexpr auto operator"" km(long double l) { return length<kilometre, long doub
 }  // namespace literals
 
 // US customary units
-struct yard : scaled_unit<yard, "yd", no_prefix, ratio<9'144, 10'000>, metre> {};
-struct foot : scaled_unit<foot, "ft", no_prefix, ratio<1, 3>, yard> {};
-struct inch : scaled_unit<inch, "in", no_prefix, ratio<1, 12>, foot> {};
-struct mile : scaled_unit<mile, "mi", no_prefix, ratio<1'760>, yard> {};
+struct yard : named_scaled_unit<yard, "yd", no_prefix, ratio<9'144, 10'000>, metre> {};
+struct foot : named_scaled_unit<foot, "ft", no_prefix, ratio<1, 3>, yard> {};
+struct inch : named_scaled_unit<inch, "in", no_prefix, ratio<1, 12>, foot> {};
+struct mile : named_scaled_unit<mile, "mi", no_prefix, ratio<1'760>, yard> {};
 
 inline namespace literals {
 

src/include/units/physical/si/time.h

@@ -31,8 +31,8 @@ struct second : named_unit<second, "s", prefix> {};
 struct nanosecond : prefixed_unit<nanosecond, nano, second> {};
 struct microsecond : prefixed_unit<microsecond, micro, second> {};
 struct millisecond : prefixed_unit<millisecond, milli, second> {};
-struct minute : scaled_unit<minute, "min", no_prefix, ratio<60>, second> {};
-struct hour : scaled_unit<hour, "h", no_prefix, ratio<3600>, second> {};
+struct minute : named_scaled_unit<minute, "min", no_prefix, ratio<60>, second> {};
+struct hour : named_scaled_unit<hour, "h", no_prefix, ratio<3600>, second> {};
 
 struct dim_time : physical::dim_time<second> {};