From 57d655062ce88c107209789d9bc6f7a788c75604 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Thu, 22 Dec 2022 21:26:51 +0100
Subject: [PATCH] refactor: `quantity_spec::operator[]` return type constraints
 improved

---
 src/core/include/units/quantity_spec.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/core/include/units/quantity_spec.h b/src/core/include/units/quantity_spec.h
index ce253ebb..5efdd8a6 100644
--- a/src/core/include/units/quantity_spec.h
+++ b/src/core/include/units/quantity_spec.h
@@ -197,7 +197,7 @@ struct derived_quantity_spec : detail::expr_fractions<derived_quantity_spec<>, Q
 
 #ifdef __cpp_explicit_this_parameter
   template<typename Self, Unit U>
-  [[nodiscard]] consteval Reference auto operator[](this const Self, U u)
+  [[nodiscard]] consteval std::same_as<reference<Self{}, u>> auto operator[](this const Self, U u)
     requires(dimension == detail::get_dimension_for(u))
   {
     return reference<Self{}, u>{};
@@ -205,7 +205,7 @@ struct derived_quantity_spec : detail::expr_fractions<derived_quantity_spec<>, Q
 #else
   // TODO can we somehow return an explicit reference type here?
   template<Unit U>
-  [[nodiscard]] consteval Reference auto operator[](U u) const
+  [[nodiscard]] consteval std::same_as<reference<derived_quantity_spec{}, U{}>> auto operator[](U u) const
     requires(dimension == detail::get_dimension_for(u))
   {
     return reference<derived_quantity_spec{}, u>{};
@@ -289,12 +289,12 @@ struct quantity_spec<Self, Dim, Args...> {
 
 #ifdef __cpp_explicit_this_parameter
   template<typename Self, Unit U>
-  [[nodiscard]] consteval Reference auto operator[](this const Self, U u)
+  [[nodiscard]] consteval std::same_as<reference<Self{}, U{}>> auto operator[](this const Self, U u)
     requires(dimension == detail::get_dimension_for(u))
 #else
   template<Unit U>
   // TODO can we somehow return an explicit reference type here?
-  [[nodiscard]] consteval Reference auto operator[](U u) const
+  [[nodiscard]] consteval std::same_as<reference<Self{}, U{}>> auto operator[](U u) const
     requires(dimension == detail::get_dimension_for(u))
 #endif
   {
@@ -364,7 +364,7 @@ struct quantity_spec<Self, Q, Args...> : std::remove_const_t<decltype(Q)> {
 #ifndef __cpp_explicit_this_parameter
   template<Unit U>
   // TODO can we somehow return an explicit reference type here?
-  [[nodiscard]] consteval Reference auto operator[](U u) const
+  [[nodiscard]] consteval std::same_as<reference<Self{}, U{}>> auto operator[](U u) const
     requires(this->dimension == detail::get_dimension_for(u))
   {
     return reference<Self{}, u>{};