From Jason Turner

[simd.expos.defn]

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+ #### Exposition-only helpers <a id="simd.expos.defn">[[simd.expos.defn]]</a>
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+
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+ ``` cpp
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+ using simd-size-type = see below;
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+ ```
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+
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+ *simd-size-type* is an alias for a signed integer type.
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+
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+ ``` cpp
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+ template<size_t Bytes> using integer-from = see below;
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+ ```
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+
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+ *`integer-from`*`<Bytes>` is an alias for a signed integer type `T` such
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+ that `sizeof(T)` equals `Bytes`.
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+
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+ ``` cpp
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+ template<class T, class Abi>
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+ constexpr simd-size-type simd-size-v = see below;
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+ ```
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+
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+ *`simd-size-v`*`<T, Abi>` denotes the width of `basic_vec<T, Abi>` if
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+ the specialization `basic_vec<T, Abi>` is enabled, or `0` otherwise.
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+
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+ ``` cpp
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+ template<class T> constexpr size_t mask-element-size = see below;
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+ ```
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+
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+ *`mask-element-size`*`<basic_mask<Bytes, Abi>>` has the value `Bytes`.
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+
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+ ``` cpp
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+ template<class T> using deduced-vec-t = see below;
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+ ```
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+
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+ Let `x` denote an lvalue of type `const T`.
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+
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+ *`deduced-vec-t`*`<T>` is an alias for
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+
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+ - `decltype(x + x)`, if the type of `x + x` is an enabled specialization
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+ of `basic_vec`; otherwise
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+ - `void`.
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+
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+ ``` cpp
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+ template<class V, class T> using make-compatible-simd-t = see below;
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+ ```
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+
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+ Let `x` denote an lvalue of type `const T`.
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+
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+ *`make-compatible-simd-t`*`<V, T>` is an alias for
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+
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+ - *`deduced-vec-t`*`<T>`, if that type is not `void`, otherwise
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+ - `vec<decltype(x + x), V::size()>`.
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+
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+ ``` cpp
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+ template<class... Ts>
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+ requires math-floating-point<Ts...>
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+ using math-common-simd-t = see below;
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+ ```
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+
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+ Let `T0` denote `Ts...[0]`. Let `T1` denote `Ts...[1]`. Let `TRest`
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+ denote a pack such that `T0, T1, TRest...` is equivalent to `Ts...`.
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+
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+ Let *`math-common-simd-t`*`<Ts...>` be an alias for
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+
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+ - *`deduced-vec-t`*`<T0>`, if `sizeof...(Ts)` equals 1; otherwise
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+ - `common_type_t<`*`deduced-vec-t`*`<T0>, `*`deduced-vec-t`*`<T1>>`, if
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+ `sizeof...(Ts)` equals 2 and
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+ `math-floating-point<T0> && math-floating-point<T1>` is `true`;
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+ otherwise
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+ - `common_type_t<`*`deduced-vec-t`*`<T0>, T1>`, if `sizeof...(Ts)`
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+ equals 2 and *`math-floating-point`*`<T0>` is `true`; otherwise
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+ - `common_type_t<T0, `*`deduced-vec-t`*`<T1>>`, if `sizeof...(Ts)`
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+ equals 2; otherwise
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+ - `common_type_t<`*`math-common-simd-t`*`<T0, T1>, TRest...>`, if
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+ *`math-common-simd-t`*`<T0, T1>` is valid and denotes a type;
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+ otherwise
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+ - `common_type_t<`*`math-common-simd-t`*`<TRest...>, T0, T1>`.
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+
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+ ``` cpp
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+ template<class BinaryOperation, class T>
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+ concept reduction-binary-operation =
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+ requires (const BinaryOperation binary_op, const vec<T, 1> v) {
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+ { binary_op(v, v) } -> same_as<vec<T, 1>>;
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+ };
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+ ```
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+
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+ Types `BinaryOperation` and `T` model
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+ `reduction-binary-operation<BinaryOperation, T>` only if:
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+
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+ - `BinaryOperation` is a binary element-wise operation and the operation
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+ is commutative.
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+ - An object of type `BinaryOperation` can be invoked with two arguments
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+ of type `basic_vec<T, Abi>`, with unspecified ABI tag `Abi`, returning
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+ a `basic_vec<T, Abi>`.
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+