refactor: system_reference moved to a dedicated header file

src/core/CMakeLists.txt

@@ -56,7 +56,8 @@ add_library(
     # include/units/random.h
     include/units/ratio.h
     include/units/reference.h
-    # include/units/symbol_text.h
+    include/units/symbol_text.h
+    include/units/system_reference.h
     include/units/unit.h
 )
 target_compile_features(mp-units-core INTERFACE cxx_std_20)

src/core/include/units/reference.h

@@ -107,21 +107,6 @@ template<Reference R1, Reference R2>
   return interconvertible(R1::dimension, R2::dimension) && interconvertible(R1::unit, R2::unit);
 }
 
-
-template<Dimension auto Dim, Unit auto CoU>
-  requires(!detail::associated_unit<std::remove_const_t<decltype(CoU)>>)
-struct system_reference {
-  static constexpr auto dimension = Dim;
-  static constexpr auto coherent_unit = CoU;
-
-  template<Unit U>
-    requires(interconvertible(coherent_unit, U{}))
-  [[nodiscard]] constexpr reference<dimension, U{}> operator[](U) const
-  {
-    return {};
-  }
-};
-
 }  // namespace units
 
 namespace std {

src/core/include/units/system_reference.h

@@ -0,0 +1,45 @@
+// 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/concepts.h>
+#include <units/reference.h>
+#include <units/unit.h>
+
+namespace units {
+
+template<Dimension auto Dim, Unit auto CoU>
+  requires(!detail::associated_unit<std::remove_const_t<decltype(CoU)>>)
+struct system_reference {
+  static constexpr auto dimension = Dim;
+  static constexpr auto coherent_unit = CoU;
+
+  template<Unit U>
+    requires(interconvertible(coherent_unit, U{}))
+  [[nodiscard]] constexpr reference<dimension, U{}> operator[](U) const
+  {
+    return {};
+  }
+};
+
+}  // namespace units

test/unit_test/static/reference_test.cpp

@@ -25,6 +25,7 @@
 #include <units/quantity.h>
 #include <units/reference.h>
 #include <units/si/prefixes.h>
+#include <units/system_reference.h>
 #include <units/unit.h>
 
 namespace {