From 25f986d32cd3f7863a67bca547a2639457d0c343 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Wed, 4 Oct 2023 10:00:17 -0600
Subject: [PATCH] fix: MSVC-related fixes and workarounds

---
 example/measurement.cpp                       |  8 +-
 src/core-fmt/include/mp-units/bits/fmt.h      |  9 ++-
 .../include/mp-units/bits/get_common_base.h   |  4 +-
 src/core/include/mp-units/quantity_point.h    |  3 +-
 src/core/include/mp-units/quantity_spec.h     | 74 ++++++++++---------
 src/core/include/mp-units/reference.h         | 17 +++++
 src/core/include/mp-units/system_reference.h  |  5 ++
 src/core/include/mp-units/unit.h              | 10 +--
 8 files changed, 78 insertions(+), 52 deletions(-)

diff --git a/example/measurement.cpp b/example/measurement.cpp
index b67416273..c20d82249 100644
--- a/example/measurement.cpp
+++ b/example/measurement.cpp
@@ -134,11 +134,11 @@ void example()
   using namespace mp_units;
   using namespace mp_units::si::unit_symbols;
 
-  const auto a = isq::acceleration(measurement{9.8, 0.1} * (m / s2));
-  const auto t = measurement{1.2, 0.1} * s;
+  const auto acceleration = isq::acceleration(measurement{9.8, 0.1} * (m / s2));
+  const auto time = measurement{1.2, 0.1} * s;
 
-  const QuantityOf<isq::velocity> auto v = a * t;
-  std::cout << a << " * " << t << " = " << v << " = " << v.in(km / h) << '\n';
+  const QuantityOf<isq::velocity> auto velocity = acceleration * time;
+  std::cout << acceleration << " * " << time << " = " << velocity << " = " << velocity.in(km / h) << '\n';
 
   const auto length = measurement{123., 1.} * m;
   std::cout << "10 * " << length << " = " << 10 * length << '\n';
diff --git a/src/core-fmt/include/mp-units/bits/fmt.h b/src/core-fmt/include/mp-units/bits/fmt.h
index 95ef4549f..973f01ad0 100644
--- a/src/core-fmt/include/mp-units/bits/fmt.h
+++ b/src/core-fmt/include/mp-units/bits/fmt.h
@@ -103,9 +103,9 @@ struct width_checker {
         if (value < 0) MP_UNITS_THROW(MP_UNITS_STD_FMT::format_error("negative width"));
       }
       return static_cast<unsigned long long>(value);
-    } else {
-      MP_UNITS_THROW(MP_UNITS_STD_FMT::format_error("width is not integer"));
     }
+    MP_UNITS_THROW(MP_UNITS_STD_FMT::format_error("width is not integer"));
+    return 0;  // should never happen
   }
 };
 
@@ -118,9 +118,9 @@ struct precision_checker {
         if (value < 0) MP_UNITS_THROW(MP_UNITS_STD_FMT::format_error("negative precision"));
       }
       return static_cast<unsigned long long>(value);
-    } else {
-      MP_UNITS_THROW(MP_UNITS_STD_FMT::format_error("precision is not integer"));
     }
+    MP_UNITS_THROW(MP_UNITS_STD_FMT::format_error("precision is not integer"));
+    return 0;  // should never happen
   }
 };
 
@@ -230,6 +230,7 @@ template<std::input_iterator It, std::sentinel_for<It> S, typename IDHandler>
     return begin;
   }
   MP_UNITS_THROW(MP_UNITS_STD_FMT::format_error("invalid format string"));
+  return begin;  // should never happen
 }
 
 template<std::input_iterator It, std::sentinel_for<It> S, typename IDHandler>
diff --git a/src/core/include/mp-units/bits/get_common_base.h b/src/core/include/mp-units/bits/get_common_base.h
index 0c232dc19..97ad5fbdd 100644
--- a/src/core/include/mp-units/bits/get_common_base.h
+++ b/src/core/include/mp-units/bits/get_common_base.h
@@ -58,8 +58,8 @@ template<QuantitySpec A, QuantitySpec B>
 template<QuantitySpec A, QuantitySpec B>
 [[nodiscard]] consteval auto have_common_base(A a, B b)
 {
-  constexpr int a_length = hierarchy_path_length(A{});
-  constexpr int b_length = hierarchy_path_length(B{});
+  constexpr std::size_t a_length = hierarchy_path_length(A{});
+  constexpr std::size_t b_length = hierarchy_path_length(B{});
   if constexpr (a_length > b_length)
     return have_common_base_in_hierarchy_of_equal_length(hierarchy_path_advance<a_length - b_length>(a), b);
   else
diff --git a/src/core/include/mp-units/quantity_point.h b/src/core/include/mp-units/quantity_point.h
index 22e37ac90..6cf8308bd 100644
--- a/src/core/include/mp-units/quantity_point.h
+++ b/src/core/include/mp-units/quantity_point.h
@@ -142,7 +142,8 @@ class quantity_point {
   quantity_point& operator=(quantity_point&&) = default;
 
   template<PointOriginFor<quantity_spec> NewPO>
-  [[nodiscard]] constexpr QuantityPointOf<NewPO{}> auto point_for(NewPO new_origin) const
+  [[nodiscard]] constexpr MP_UNITS_CONSTRAINED_AUTO_WORKAROUND(QuantityPointOf<NewPO{}>) auto point_for(
+    NewPO new_origin) const
   {
     if constexpr (is_same_v<NewPO, std::remove_const_t<decltype(point_origin)>>)
       return *this;
diff --git a/src/core/include/mp-units/quantity_spec.h b/src/core/include/mp-units/quantity_spec.h
index b94e476c5..22cc47bb9 100644
--- a/src/core/include/mp-units/quantity_spec.h
+++ b/src/core/include/mp-units/quantity_spec.h
@@ -41,12 +41,12 @@ namespace detail {
 
 template<QuantitySpec QS, Unit U>
   requires(!AssociatedUnit<U>) || UnitOf<U, QS{}>
-[[nodiscard]] consteval Reference auto make_reference(QS qs, U u)
+[[nodiscard]] consteval Reference auto make_reference(QS, U u)
 {
   if constexpr (detail::QuantityKindSpec<QS>)
     return u;
   else
-    return reference<qs, u>{};
+    return reference<QS{}, U{}>{};
 }
 
 // TODO revise the note in the below comment
@@ -494,9 +494,10 @@ template<QuantitySpec auto... From, QuantitySpec Q>
 
 // Operators
 
-[[nodiscard]] consteval QuantitySpec auto operator*(QuantitySpec auto lhs, QuantitySpec auto rhs)
+template<QuantitySpec Lhs, QuantitySpec Rhs>
+[[nodiscard]] consteval QuantitySpec auto operator*(Lhs lhs, Rhs rhs)
 {
-  return clone_kind_of<lhs, rhs>(
+  return clone_kind_of<Lhs{}, Rhs{}>(
     detail::expr_multiply<derived_quantity_spec, struct dimensionless, detail::type_list_of_quantity_spec_less>(
       remove_kind(lhs), remove_kind(rhs)));
 }
@@ -504,15 +505,16 @@ template<QuantitySpec auto... From, QuantitySpec Q>
 template<QuantitySpec Lhs, QuantitySpec Rhs>
 [[nodiscard]] consteval QuantitySpec auto operator/(Lhs lhs, Rhs rhs)
 {
-  return clone_kind_of<lhs, rhs>(
+  return clone_kind_of<Lhs{}, Rhs{}>(
     detail::expr_divide<derived_quantity_spec, struct dimensionless, detail::type_list_of_quantity_spec_less>(
       remove_kind(lhs), remove_kind(rhs)));
 }
 
-[[nodiscard]] consteval QuantitySpec auto operator/(int value, QuantitySpec auto q)
+template<QuantitySpec QS>
+[[nodiscard]] consteval QuantitySpec auto operator/(int value, QS qs)
 {
   gsl_Expects(value == 1);
-  return clone_kind_of<q>(detail::expr_invert<derived_quantity_spec, struct dimensionless>(q));
+  return clone_kind_of<QS{}>(detail::expr_invert<derived_quantity_spec, struct dimensionless>(qs));
 }
 
 [[nodiscard]] consteval QuantitySpec auto operator/(QuantitySpec auto, int) = delete;
@@ -554,7 +556,7 @@ 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 (Num == 0 || q == dimensionless)
+  if constexpr (Num == 0 || Q{} == dimensionless)
     return dimensionless;
   else if constexpr (ratio{Num, Den} == 1)
     return q;
@@ -779,7 +781,7 @@ template<int Complexity, QuantitySpec Q>
 template<int Complexity, IntermediateDerivedQuantitySpec Q>
 [[nodiscard]] consteval auto explode(Q q)
 {
-  constexpr auto c = get_complexity(q);
+  constexpr auto c = get_complexity(Q{});
   if constexpr (c > Complexity)
     return explode<Complexity>(q, type_list_sort<typename Q::_num_, type_list_of_ingredients_less>{},
                                type_list_sort<typename Q::_den_, type_list_of_ingredients_less>{});
@@ -790,9 +792,9 @@ template<int Complexity, IntermediateDerivedQuantitySpec Q>
 template<int Complexity, NamedQuantitySpec Q>
 [[nodiscard]] consteval auto explode(Q q)
 {
-  constexpr auto c = get_complexity(q);
+  constexpr auto c = get_complexity(Q{});
   if constexpr (c > Complexity && requires { Q::_equation_; }) {
-    constexpr auto res = explode_to_equation(q);
+    constexpr auto res = explode_to_equation(Q{});
     return explode<Complexity>(res.equation).common_convertibility_with(res);
   } else
     return explode_result{q};
@@ -887,8 +889,8 @@ extract_results(bool, From = {}, To = {}, prepend_rest = {}, Elem = {}) -> extra
 template<typename From, typename To>
 [[nodiscard]] consteval auto extract_convertible_quantities(From from, To to)
 {
-  constexpr auto qfrom = map_power(from);
-  constexpr auto qto = map_power(to);
+  constexpr auto qfrom = map_power(From{});
+  constexpr auto qto = map_power(To{});
   if constexpr (qfrom.dimension == qto.dimension) {
     if constexpr (is_specialization_of_power<From> && is_specialization_of_power<To>) {
       constexpr auto cr = common_ratio(From::exponent, To::exponent);
@@ -905,8 +907,8 @@ template<typename From, typename To>
       else
         return std::tuple{Q{}, ratio{1}};
     };
-    constexpr auto from_norm = normalize(from);
-    constexpr auto to_norm = normalize(to);
+    constexpr auto from_norm = normalize(From{});
+    constexpr auto to_norm = normalize(To{});
     constexpr auto from_factor = std::get<0>(from_norm);
     constexpr auto from_exp = std::get<1>(from_norm);
     constexpr auto to_factor = std::get<0>(to_norm);
@@ -1335,11 +1337,11 @@ template<QuantitySpec From, QuantitySpec To>
 
   if constexpr (From::dimension != To::dimension)
     return no;
-  else if constexpr (from == to)
+  else if constexpr (From{} == To{})
     return yes;
   else if constexpr (QuantityKindSpec<From> || QuantityKindSpec<To>) {
-    constexpr auto from_kind = get_kind(from);
-    constexpr auto to_kind = get_kind(to);
+    constexpr auto from_kind = get_kind(From{});
+    constexpr auto to_kind = get_kind(To{});
     constexpr auto exploded_kind_result = [](specs_convertible_result res) {
       using enum specs_convertible_result;
       return res == no ? no : yes;
@@ -1354,21 +1356,21 @@ template<QuantitySpec From, QuantitySpec To>
     else
       return exploded_kind_result(
         convertible_impl(from_kind, get_kind(explode<get_complexity(from_kind)>(to_kind).quantity)));
-  } else if constexpr (NestedQuantityKindSpecOf<get_kind(to), from> && get_kind(to) == to)
+  } else if constexpr (NestedQuantityKindSpecOf<get_kind(To{}), from> && get_kind(To{}) == To{})
     return yes;
   else if constexpr (NamedQuantitySpec<From> && NamedQuantitySpec<To>) {
-    if constexpr (have_common_base(from, to)) {
+    if constexpr (have_common_base(From{}, To{})) {
       if (std::derived_from<From, To>)
         return yes;
       else
         return std::derived_from<To, From> ? explicit_conversion : (get_kind(from) == get_kind(to) ? cast : no);
-    } else if constexpr (get_kind(from) != get_kind(to))
+    } else if constexpr (get_kind(From{}) != get_kind(To{}))
       return no;
-    else if constexpr (get_complexity(from) != get_complexity(to)) {
-      if constexpr (get_complexity(from) > get_complexity(to))
+    else if constexpr (get_complexity(From{}) != get_complexity(To{})) {
+      if constexpr (get_complexity(From{}) > get_complexity(To{}))
         return convertible_impl(explode<get_complexity(to)>(from).quantity, to);
       else {
-        constexpr auto res = explode<get_complexity(from)>(to);
+        auto res = explode<get_complexity(from)>(to);
         return min(res.result, convertible_impl(from, res.quantity));
       }
     }
@@ -1379,7 +1381,7 @@ template<QuantitySpec From, QuantitySpec To>
     if constexpr (NamedQuantitySpec<std::remove_const_t<decltype(res.quantity)>>)
       return convertible_impl(res.quantity, to);
     else if constexpr (requires { to._equation_; }) {
-      constexpr auto eq = explode_to_equation(to);
+      auto eq = explode_to_equation(to);
       return min(eq.result, convertible_impl(res.quantity, eq.equation));
     } else
       return are_ingredients_convertible(from, to);
@@ -1449,16 +1451,16 @@ template<QuantitySpec Q>
 template<QuantitySpec Q>
 [[nodiscard]] consteval QuantitySpec auto get_kind(Q q)
 {
-  auto defined_as_kind = [](auto qq) {
+  auto defined_as_kind = []<typename QQ>(QQ qq) {
     if constexpr (requires { detail::defined_as_kind(qq); })
-      return detail::defined_as_kind(qq);
+      return detail::defined_as_kind(QQ{});
     else
       return false;
   };
 
   if constexpr (detail::QuantityKindSpec<Q>) {
     return remove_kind(q);
-  } else if constexpr (defined_as_kind(q)) {
+  } else if constexpr (defined_as_kind(Q{})) {
     return q;
   } else if constexpr (requires { Q::_parent_; }) {
     return get_kind(Q::_parent_);
@@ -1481,25 +1483,25 @@ template<QuantitySpec Q1, QuantitySpec Q2>
   using QQ2 = std::remove_const_t<decltype(remove_kind(q2))>;
   if constexpr (is_same_v<Q1, Q2>)
     return q1;
-  else if constexpr (detail::NestedQuantityKindSpecOf<q1, q2>)
+  else if constexpr (detail::NestedQuantityKindSpecOf<Q1{}, Q2{}>)
     return remove_kind(q1);
-  else if constexpr (detail::NestedQuantityKindSpecOf<q2, q1>)
+  else if constexpr (detail::NestedQuantityKindSpecOf<Q2{}, Q1{}>)
     return remove_kind(q2);
   else if constexpr ((detail::QuantityKindSpec<Q1> && !detail::QuantityKindSpec<Q2>) ||
                      (detail::IntermediateDerivedQuantitySpec<QQ1> && detail::NamedQuantitySpec<QQ2> &&
-                      implicitly_convertible(q1, q2)))
+                      implicitly_convertible(Q1{}, Q2{})))
     return q2;
   else if constexpr ((!detail::QuantityKindSpec<Q1> && detail::QuantityKindSpec<Q2>) ||
                      (detail::NamedQuantitySpec<QQ1> && detail::IntermediateDerivedQuantitySpec<QQ2> &&
-                      implicitly_convertible(q2, q1)))
+                      implicitly_convertible(Q2{}, Q1{})))
     return q1;
-  else if constexpr (detail::have_common_base(q1, q2))
+  else if constexpr (detail::have_common_base(Q1{}, Q2{}))
     return detail::get_common_base(q1, q2);
-  else if constexpr (implicitly_convertible(q1, q2))
+  else if constexpr (implicitly_convertible(Q1{}, Q2{}))
     return q2;
-  else if constexpr (implicitly_convertible(q2, q1))
+  else if constexpr (implicitly_convertible(Q2{}, Q1{}))
     return q1;
-  else if constexpr (implicitly_convertible(get_kind(q1), get_kind(q2)))
+  else if constexpr (implicitly_convertible(get_kind(Q1{}), get_kind(Q2{})))
     return get_kind(q2);
   else
     return get_kind(q1);
diff --git a/src/core/include/mp-units/reference.h b/src/core/include/mp-units/reference.h
index 032c4192a..1633336bd 100644
--- a/src/core/include/mp-units/reference.h
+++ b/src/core/include/mp-units/reference.h
@@ -22,6 +22,7 @@
 
 #pragma once
 
+#include <mp-units/bits/external/hacks.h>
 #include <mp-units/bits/get_associated_quantity.h>
 #include <mp-units/bits/quantity_concepts.h>
 #include <mp-units/bits/reference_concepts.h>
@@ -73,13 +74,21 @@ struct reference {
   }
 
   template<AssociatedUnit U2>
+#if MP_UNITS_COMP_MSVC
+  [[nodiscard]] friend consteval decltype(reference<Q * get_quantity_spec(U2{}), U * U2{}>{}) operator*(reference, U2)
+#else
   [[nodiscard]] friend consteval reference<Q * get_quantity_spec(U2{}), U * U2{}> operator*(reference, U2)
+#endif
   {
     return {};
   }
 
   template<AssociatedUnit U1>
+#if MP_UNITS_COMP_MSVC
+  [[nodiscard]] friend consteval decltype(reference<get_quantity_spec(U1{}) * Q, U1{} * U>{}) operator*(U1, reference)
+#else
   [[nodiscard]] friend consteval reference<get_quantity_spec(U1{}) * Q, U1{} * U> operator*(U1, reference)
+#endif
   {
     return {};
   }
@@ -91,13 +100,21 @@ struct reference {
   }
 
   template<AssociatedUnit U2>
+#if MP_UNITS_COMP_MSVC
+  [[nodiscard]] friend consteval decltype(reference<Q / get_quantity_spec(U2{}), U / U2{}>{}) operator/(reference, U2)
+#else
   [[nodiscard]] friend consteval reference<Q / get_quantity_spec(U2{}), U / U2{}> operator/(reference, U2)
+#endif
   {
     return {};
   }
 
   template<AssociatedUnit U1>
+#if MP_UNITS_COMP_MSVC
+  [[nodiscard]] friend consteval decltype(reference<get_quantity_spec(U1{}) / Q, U1{} / U>{}) operator/(U1, reference)
+#else
   [[nodiscard]] friend consteval reference<get_quantity_spec(U1{}) / Q, U1{} / U> operator/(U1, reference)
+#endif
   {
     return {};
   }
diff --git a/src/core/include/mp-units/system_reference.h b/src/core/include/mp-units/system_reference.h
index 32568569a..59f4769c0 100644
--- a/src/core/include/mp-units/system_reference.h
+++ b/src/core/include/mp-units/system_reference.h
@@ -22,6 +22,7 @@
 
 #pragma once
 
+#include <mp-units/bits/external/hacks.h>
 #include <mp-units/quantity_spec.h>
 #include <mp-units/reference.h>
 #include <mp-units/unit.h>
@@ -63,7 +64,11 @@ struct system_reference {
 
   template<Unit U>
     requires(convertible(coherent_unit, U{}))
+#if MP_UNITS_COMP_MSVC
+  [[nodiscard]] constexpr decltype(reference<quantity_spec, U{}>{}) operator[](U) const
+#else
   [[nodiscard]] constexpr reference<quantity_spec, U{}> operator[](U) const
+#endif
   {
     return {};
   }
diff --git a/src/core/include/mp-units/unit.h b/src/core/include/mp-units/unit.h
index c4343c452..ca3068cff 100644
--- a/src/core/include/mp-units/unit.h
+++ b/src/core/include/mp-units/unit.h
@@ -354,9 +354,9 @@ template<typename T, typename F, int Num, int... Den>
 template<typename... Us>
 [[nodiscard]] consteval auto get_canonical_unit_impl(const type_list<Us...>&)
 {
-  auto mag = (mp_units::mag<1> * ... * get_canonical_unit_impl(Us{}, Us{}).mag);
+  auto m = (mp_units::mag<1> * ... * get_canonical_unit_impl(Us{}, Us{}).mag);
   auto u = (one * ... * get_canonical_unit_impl(Us{}, Us{}).reference_unit);
-  return canonical_unit{mag, u};
+  return canonical_unit{m, u};
 }
 
 template<Unit T, typename... Expr>
@@ -402,12 +402,12 @@ using type_list_of_unit_less = expr_less<T1, T2, unit_less>;
  * Multiplication by `1` returns the same unit, otherwise `scaled_unit` is being returned.
  */
 template<Magnitude M, Unit U>
-[[nodiscard]] MP_UNITS_CONSTEVAL Unit auto operator*(M mag, const U u)
+[[nodiscard]] MP_UNITS_CONSTEVAL Unit auto operator*(M, const U u)
 {
-  if constexpr (mag == mp_units::mag<1>)
+  if constexpr (std::is_same_v<M, std::remove_cvref_t<decltype(mp_units::mag<1>)>>)
     return u;
   else
-    return scaled_unit<mag, U>{};
+    return scaled_unit<M{}, U>{};
 }
 
 [[nodiscard]] consteval Unit auto operator*(Unit auto, Magnitude auto) = delete;