From 9ab8136c982f76d824648bb51a291871cd736a11 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Tue, 13 Jun 2023 14:52:14 +0300
Subject: [PATCH] refactor: `has_unit_symbol()` added

---
 src/core-fmt/include/mp-units/format.h  |  2 +-
 src/core-io/include/mp-units/iostream.h |  2 +-
 src/core/include/mp-units/unit.h        | 12 +++++++++---
 3 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/src/core-fmt/include/mp-units/format.h b/src/core-fmt/include/mp-units/format.h
index c5832340..1234296c 100644
--- a/src/core-fmt/include/mp-units/format.h
+++ b/src/core-fmt/include/mp-units/format.h
@@ -396,7 +396,7 @@ private:
     if (begin == end || *begin == '}') {
       // default format should print value followed by the unit separated with 1 space
       out = mp_units::detail::format_units_quantity_value<CharT>(out, q.number(), specs.rep, ctx.locale());
-      if constexpr (!std::derived_from<decltype(get_unit(Reference)), mp_units::derived_unit<>>) {
+      if constexpr (mp_units::detail::has_unit_symbol(get_unit(Reference))) {
         *out++ = CharT(' ');
         out = unit_symbol_to<CharT>(out, get_unit(Reference));
       }
diff --git a/src/core-io/include/mp-units/iostream.h b/src/core-io/include/mp-units/iostream.h
index 3426a98d..d9958fae 100644
--- a/src/core-io/include/mp-units/iostream.h
+++ b/src/core-io/include/mp-units/iostream.h
@@ -35,7 +35,7 @@ template<typename CharT, class Traits, auto R, typename Rep>
 void to_stream(std::basic_ostream<CharT, Traits>& os, const quantity<R, Rep>& q)
 {
   os << q.number();
-  if constexpr (!std::derived_from<decltype(get_unit(R)), derived_unit<>>) {
+  if constexpr (has_unit_symbol(get_unit(R))) {
     os << " ";
     unit_symbol_to<CharT>(std::ostream_iterator<CharT>(os), get_unit(R));
   }
diff --git a/src/core/include/mp-units/unit.h b/src/core/include/mp-units/unit.h
index 76d9d815..ca433c39 100644
--- a/src/core/include/mp-units/unit.h
+++ b/src/core/include/mp-units/unit.h
@@ -684,6 +684,12 @@ constexpr Out print_separator(Out out, unit_symbol_formatting fmt)
   return out;
 }
 
+template<Unit U>
+[[nodiscard]] consteval bool has_unit_symbol(U)
+{
+  return !std::derived_from<U, derived_unit<>>;
+}
+
 template<typename CharT, std::output_iterator<CharT> Out, Unit U>
   requires requires { U::symbol; }
 constexpr Out unit_symbol_impl(Out out, U, unit_symbol_formatting fmt, bool negative_power)
@@ -706,11 +712,11 @@ constexpr Out unit_symbol_impl(Out out, const scaled_unit<M, U>& u, unit_symbol_
     constexpr auto mag_txt = magnitude_text<M>();
     out = copy<CharT>(mag_txt, fmt.encoding, out);
 
-    if constexpr (std::derived_from<std::remove_const_t<decltype(u.reference_unit)>, derived_unit<>>)
-      return out;
-    else {
+    if constexpr (has_unit_symbol(u.reference_unit)) {
       *out++ = ' ';
       return unit_symbol_impl<CharT>(out, u.reference_unit, fmt, negative_power);
+    } else {
+      return out;
     }
   }
 }