Scalar concept extended

Scalar concept extended with types that are not constructible from and integral type

docs/reference/concepts.rst

@@ -70,4 +70,11 @@ Concepts
 
 .. concept:: template<typename T> Scalar
 
-    A concept matching non-Quantity types. Satisfied by types that satisfy :expr:`(!Quantity<T>) && (!WrappedQuantity<T>) && std::regular<T>`.
+    A concept matching non-Quantity types. Satisfied by types that match
+    :expr:`(!Quantity<T>) && (!WrappedQuantity<T>) && std::regular<T>` and satisfy one of the
+    following:
+
+    - if type :expr:`T` is constructible from ``std::int64_t`` (which is the type that stores
+      the elements of `ratio`), :expr:`T * T` and :expr:`T / T` must be valid,
+    - otherwise, :expr:`T * std::int64_t`, :expr:`std::int64_t * T`, and :expr:`T / std::int64_t`
+      must be valid.

src/include/units/concepts.h

@@ -248,6 +248,29 @@ template<typename T>
 concept WrappedQuantity = detail::is_wrapped_quantity<T>;
 
 // Scalar
+
+namespace detail {
+
+template<typename T>
+concept constructible_from_integral =
+  // construction from an integral type
+  std::constructible_from<T, std::int64_t> && 
+  // unit scaling
+  std::regular_invocable<std::multiplies<>, T, T> &&
+  std::regular_invocable<std::divides<>, T, T>;
+
+template<typename T>
+concept not_constructible_from_integral =
+  // not construction from an integral type
+  (!std::constructible_from<T, std::int64_t>) && 
+
+  // scaling by the value from ratio
+  std::regular_invocable<std::multiplies<>, T, std::int64_t> &&
+  std::regular_invocable<std::multiplies<>, std::int64_t, T>; // &&
+  // std::regular_invocable<std::divides<>, T, std::int64_t>;  // TODO Uncomment when a bug in LA is fixed
+
+}  // namesapce detail
+
 /**
  * @brief A concept matching non-Quantity types.
  * 
@@ -258,10 +281,6 @@ concept Scalar =
   (!Quantity<T>) &&
   (!WrappedQuantity<T>) &&
   std::regular<T> &&
-  // construction from an integral type
-  std::constructible_from<T, std::int64_t> &&
-  // unit scaling
-  std::regular_invocable<std::multiplies<>, T, T> &&
-  std::regular_invocable<std::divides<>, T, T>;
+  (detail::constructible_from_integral<T> || detail::not_constructible_from_integral<T>);
 
 }  // namespace units