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Permit arithmetic operations with more types
The goal of these `std::is_arithmetic` SFINAE uses was to avoid ambiguous overloads when we multiply a `Quantity` with another `Quantity`. It was only ever a shortcut. It seems to work well in most cases, but recently, a user pointed out that it doesn't help `std::complex`. (To be clear, we could _form_ a `Quantity` with `std::complex` rep, but we couldn't multiply or divide a `Quantity` with a `std::complex` scalar.) This PR takes a different approach. First, we define what constitutes a valid "rep". We're very permissive here, using a "deny-list" approach that filters out empty types, known Au types, and other libraries' units types (which we detect via our `CorrespondingQuantity` trait). Incidentally, this should give us a nice head start on #52. Next, we permit an operation (multiplication or division) depending on how the "candidate scalar" type would interact with the Quantity's rep: simply put, the result must itself both exist, and be a valid rep. The net result should be that we automatically consider a much wider variety of types --- including complex number types from outside the standard library --- to be valid scalars. Crucially, this should also be able to avoid breaking existing code: we are introducing potentially a _lot_ of new overloads for these operators, and we can't allow any to create an ambiguity with existing overloads.
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| // Copyright 2024 Aurora Operations, Inc. | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 (the "License"); | ||
| // you may not use this file except in compliance with the License. | ||
| // You may obtain a copy of the License at | ||
| // | ||
| // http://www.apache.org/licenses/LICENSE-2.0 | ||
| // | ||
| // Unless required by applicable law or agreed to in writing, software | ||
| // distributed under the License is distributed on an "AS IS" BASIS, | ||
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| // See the License for the specific language governing permissions and | ||
| // limitations under the License. | ||
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| #pragma once | ||
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| #include <type_traits> | ||
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| #include "au/stdx/experimental/is_detected.hh" | ||
| #include "au/stdx/type_traits.hh" | ||
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| namespace au { | ||
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| // | ||
| // A type trait that determines if a type is a valid representation type for `Quantity` or | ||
| // `QuantityPoint`. | ||
| // | ||
| template <typename T> | ||
| struct IsValidRep; | ||
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| // | ||
| // A type trait to indicate whether the product of two types is a valid rep. | ||
| // | ||
| // Will validly return `false` if the product does not exist. | ||
| // | ||
| template <typename T, typename U> | ||
| struct IsProductValidRep; | ||
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| // | ||
| // A type trait to indicate whether the quotient of two types is a valid rep. | ||
| // | ||
| // Will validly return `false` if the quotient does not exist. | ||
| // | ||
| template <typename T, typename U> | ||
| struct IsQuotientValidRep; | ||
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| //////////////////////////////////////////////////////////////////////////////////////////////////// | ||
| // Implementation details below. | ||
| //////////////////////////////////////////////////////////////////////////////////////////////////// | ||
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| // Forward declarations for main Au container types. | ||
| template <typename U, typename R> | ||
| class Quantity; | ||
| template <typename U, typename R> | ||
| class QuantityPoint; | ||
| template <typename T> | ||
| struct CorrespondingQuantity; | ||
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| namespace detail { | ||
| template <typename T> | ||
| struct IsAuType : std::false_type {}; | ||
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| template <typename U, typename R> | ||
| struct IsAuType<::au::Quantity<U, R>> : std::true_type {}; | ||
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| template <typename U, typename R> | ||
| struct IsAuType<::au::QuantityPoint<U, R>> : std::true_type {}; | ||
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| template <typename T> | ||
| using CorrespondingUnit = typename CorrespondingQuantity<T>::Unit; | ||
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| template <typename T> | ||
| using CorrespondingRep = typename CorrespondingQuantity<T>::Rep; | ||
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| template <typename T> | ||
| struct HasCorrespondingQuantity | ||
| : stdx::conjunction<stdx::experimental::is_detected<CorrespondingUnit, T>, | ||
| stdx::experimental::is_detected<CorrespondingRep, T>> {}; | ||
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| template <typename T> | ||
| using LooksLikeAuOrOtherQuantity = stdx::disjunction<IsAuType<T>, HasCorrespondingQuantity<T>>; | ||
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| // We need a way to form an "operation on non-quantity types only". That is: it's some operation, | ||
| // but _if either input is a quantity_, then we _don't even form the type_. | ||
| // | ||
| // The reason this very specific machinery lives in `rep.hh` is because when we're dealing with | ||
| // operations on "types that might be a rep", we know we can exclude quantity types right away. | ||
| // (Note that we're using the term "quantity" in an expansive sense, which includes not just | ||
| // `au::Quantity`, but also `au::QuantityPoint`, and "quantity-like" types from other libraries | ||
| // (which we consider as "anything that has a `CorrespondingQuantity`". | ||
| template <template <class...> class Op, typename... Ts> | ||
| struct ResultIfNoneAreQuantity; | ||
| template <template <class...> class Op, typename... Ts> | ||
| using ResultIfNoneAreQuantityT = typename ResultIfNoneAreQuantity<Op, Ts...>::type; | ||
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| // Default implementation where we know that none are quantities. | ||
| template <bool AreAnyQuantity, template <class...> class Op, typename... Ts> | ||
| struct ResultIfNoneAreQuantityImpl : stdx::type_identity<Op<Ts...>> {}; | ||
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| // Implementation if any of the types are quantities. | ||
| template <template <class...> class Op, typename... Ts> | ||
| struct ResultIfNoneAreQuantityImpl<true, Op, Ts...> : stdx::type_identity<void> {}; | ||
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| // The main implementation. | ||
| template <template <class...> class Op, typename... Ts> | ||
| struct ResultIfNoneAreQuantity | ||
| : ResultIfNoneAreQuantityImpl<stdx::disjunction<LooksLikeAuOrOtherQuantity<Ts>...>::value, | ||
| Op, | ||
| Ts...> {}; | ||
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| // The `std::is_empty` is a good way to catch all of the various unit and other monovalue types in | ||
| // our library, which have little else in common. It's also just intrinsically true that it | ||
| // wouldn't make much sense to use an empty type as a rep. | ||
| template <typename T> | ||
| struct IsKnownInvalidRep | ||
| : stdx::disjunction<std::is_empty<T>, LooksLikeAuOrOtherQuantity<T>, std::is_same<void, T>> {}; | ||
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| // The type of the product of two types. | ||
| template <typename T, typename U> | ||
| using ProductType = decltype(std::declval<T>() * std::declval<U>()); | ||
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| template <typename T, typename U> | ||
| using ProductTypeOrVoid = stdx::experimental::detected_or_t<void, ProductType, T, U>; | ||
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| // The type of the quotient of two types. | ||
| template <typename T, typename U> | ||
| using QuotientType = decltype(std::declval<T>() / std::declval<U>()); | ||
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| template <typename T, typename U> | ||
| using QuotientTypeOrVoid = stdx::experimental::detected_or_t<void, QuotientType, T, U>; | ||
| } // namespace detail | ||
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| // Implementation for `IsValidRep`. | ||
| // | ||
| // For now, we'll accept anything that isn't explicitly known to be invalid. We may tighten this up | ||
| // later, but this seems like a reasonable starting point. | ||
| template <typename T> | ||
| struct IsValidRep : stdx::negation<detail::IsKnownInvalidRep<T>> {}; | ||
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| template <typename T, typename U> | ||
| struct IsProductValidRep | ||
| : IsValidRep<detail::ResultIfNoneAreQuantityT<detail::ProductTypeOrVoid, T, U>> {}; | ||
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| template <typename T, typename U> | ||
| struct IsQuotientValidRep | ||
| : IsValidRep<detail::ResultIfNoneAreQuantityT<detail::QuotientTypeOrVoid, T, U>> {}; | ||
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| } // namespace au |
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