fix: prefixed_alias_unit constraints fixed

docs/reference/core/concepts.rst

@@ -45,6 +45,11 @@ Concepts
     A concept matching all unit types that have an atomic text symbol that can be used to aggregate it with
     other named units to form a final symbol of a derived unit.
 
+.. concept:: template<typename T> AliasUnit
+
+    A concept matching all alias unit types in the library. Satisfied by all unit types derived
+    from the instantiation of :class:`alias_unit`.
+
 .. concept:: template<typename U, typename D> UnitOf
 
     A concept matching only units of a specified dimension. Satisfied by all unit types that

src/core/include/units/bits/basic_concepts.h

@@ -29,6 +29,7 @@
 #include <units/bits/external/type_traits.h>
 #include <units/customization_points.h>
 #include <units/magnitude.h>
+#include <units/symbol_text.h>
 // IWYU pragma: end_exports
 
 #include <cstdint>
@@ -86,6 +87,20 @@ inline constexpr bool is_named = false;
 template<typename T>
 concept NamedUnit = Unit<T> && detail::is_named<T>;
 
+template<Unit U, basic_symbol_text Symbol>
+struct alias_unit;
+
+// TODO: Remove when P1985 accepted
+namespace detail {
+
+template<Unit U, basic_symbol_text Symbol>
+void to_base_alias_unit(const volatile alias_unit<U, Symbol>*);
+
+}  // namespace detail
+
+template<typename T>
+concept AliasUnit = requires(T* t) { detail::to_base_alias_unit(t); };
+
 // BaseDimension
 template<basic_fixed_string Symbol, NamedUnit U>
 struct base_dimension;

src/core/include/units/unit.h

@@ -58,8 +58,10 @@ inline constexpr bool can_be_prefixed = false;
  *
  * @tparam M a Magnitude representing the (relative) size of this unit
  * @tparam U a unit to use as a reference for this dimension
+ *
+ * @note U cannot be constrained with Unit as for some specializations (i.e. named_unit)
+ *       it gets the incomplete child's type with the CRTP idiom.
  */
-// TODO Replace `typename` with `Unit`
 template<Magnitude auto M, typename U>
 struct scaled_unit : downcast_base<scaled_unit<M, U>> {
   static constexpr UNITS_MSVC_WORKAROUND(Magnitude) auto mag = M;
@@ -175,12 +177,8 @@ struct alias_unit : U {
  * @tparam P prefix to be appied to the reference unit
  * @tparam AU reference alias unit
  */
-// TODO gcc bug: 95015
+template<Unit U, Prefix P, AliasUnit AU>
-// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95015
+  requires(!AliasUnit<U>)
-// template<Unit U, Prefix P, AliasUnit AU>
-//   requires (!AliasUnit<U>)
-template<Unit U, Prefix P, NamedUnit AU>
-  requires detail::can_be_prefixed<AU>
 struct prefixed_alias_unit : U {
   static constexpr auto symbol = P::symbol + AU::symbol;
 };