From de4fefde3f93980fa00d980874922597aaba3f43 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Tue, 13 Jun 2023 09:14:01 +0300
Subject: [PATCH] fix: `pow()` fixed for quantity specs and units

---
 src/core/include/mp-units/quantity_spec.h |  6 ++---
 src/core/include/mp-units/unit.h          | 28 ++++++++++++++++-------
 2 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/src/core/include/mp-units/quantity_spec.h b/src/core/include/mp-units/quantity_spec.h
index d3029c92..4412c099 100644
--- a/src/core/include/mp-units/quantity_spec.h
+++ b/src/core/include/mp-units/quantity_spec.h
@@ -545,9 +545,9 @@ template<std::intmax_t Num, std::intmax_t Den = 1, QuantitySpec Q>
   // `2 * one * (2 * one)` should compare to `4 * one`
   // `2 * rad * (2 * rad)` should compare to `4 * rad^2`
   // all are dimensionless quantities :-(
-  if constexpr (q == dimensionless)
-    return q;
-  else if constexpr (Num == 1 && Den == 1)
+  if constexpr (Num == 0 || q == dimensionless)
+    return dimensionless;
+  else if constexpr (ratio{Num, Den} == 1)
     return q;
   else if constexpr (detail::IntermediateDerivedQuantitySpec<Q>)
     return detail::clone_kind_of<Q{}>(
diff --git a/src/core/include/mp-units/unit.h b/src/core/include/mp-units/unit.h
index 12257032..76d9d815 100644
--- a/src/core/include/mp-units/unit.h
+++ b/src/core/include/mp-units/unit.h
@@ -501,6 +501,16 @@ template<typename... Expr1, typename... Expr2>
          canonical_lhs.mag == canonical_rhs.mag;
 }
 
+namespace detail {
+
+template<typename T>
+inline constexpr bool is_specialization_of_derived_unit = false;
+
+template<typename... Expr>
+inline constexpr bool is_specialization_of_derived_unit<derived_unit<Expr...>> = true;
+
+}  // namespace detail
+
 /**
  * @brief Computes the value of a unit raised to the `Num/Den` power
  *
@@ -514,16 +524,18 @@ template<std::intmax_t Num, std::intmax_t Den = 1, Unit U>
   requires detail::non_zero<Den>
 [[nodiscard]] consteval Unit auto pow(U u)
 {
-  if constexpr (requires { U::symbol; }) {
-    if constexpr (Den == 1)
-      return derived_unit<power<U, Num>>{};
-    else
-      return derived_unit<power<U, Num, Den>>{};
-  } else if constexpr (detail::is_specialization_of_scaled_unit<U>) {
+  if constexpr (Num == 0 || is_same_v<U, struct one>)
+    return one;
+  else if constexpr (ratio{Num, Den} == 1)
+    return u;
+  else if constexpr (detail::is_specialization_of_scaled_unit<U>)
     return scaled_unit<pow<Num, Den>(U::mag), std::remove_const_t<decltype(pow<Num, Den>(U::reference_unit))>>{};
-  } else {
+  else if constexpr (detail::is_specialization_of_derived_unit<U>)
     return detail::expr_pow<Num, Den, derived_unit, struct one, detail::type_list_of_unit_less>(u);
-  }
+  else if constexpr (Den == 1)
+    return derived_unit<power<U, Num>>{};
+  else
+    return derived_unit<power<U, Num, Den>>{};
 }
 
 /**