Remove custom implementations of optional and string_view

Summary:
Now that we're using C++17, we don't need to maintain custom implementations of `optional` and `string_view`.

The standard optional has an implicit ctor that causes ambiguities in some reduce_util.h functions. Add explicit overrides to resolve the ambiguities.

Note that this is technically an API break, since exec_aten::optional is changing in a way that can break some existing code. There is no way to roll this out incrementally, though, and there should not be many examples of user code that would break because of this change.

Differential Revision: D64634702

Author: dbort

kernels/portable/cpu/util/reduce_util.h

@@ -565,6 +565,15 @@ size_t compute_reduced_out_size(
     bool keepdim,
     exec_aten::SizesType* sizes_arr);
 
+inline size_t compute_reduced_out_size(
+    const exec_aten::Tensor& in,
+    int64_t dim,
+    bool keepdim,
+    exec_aten::SizesType* sizes_arr) {
+  return compute_reduced_out_size(
+      in, exec_aten::optional<int64_t>(dim), keepdim, sizes_arr);
+}
+
 inline ssize_t compute_reduced_out_dim(
     const exec_aten::Tensor& in,
     const exec_aten::optional<int64_t>& dim,
@@ -588,6 +597,14 @@ inline ssize_t compute_reduced_out_dim(
           : 0);
 }
 
+inline ssize_t compute_reduced_out_dim(
+    const exec_aten::Tensor& in,
+    int64_t dim,
+    bool keepdim) {
+  return compute_reduced_out_dim(
+      in, exec_aten::optional<int64_t>(dim), keepdim);
+}
+
 //
 // Resize out tensor of reduction op
 //
@@ -604,6 +621,15 @@ Error resize_reduction_out(
     bool keepdim,
     exec_aten::Tensor& out);
 
+inline Error resize_reduction_out(
+    const exec_aten::Tensor& in,
+    int64_t dim,
+    bool keepdim,
+    exec_aten::Tensor& out) {
+  return resize_reduction_out(
+      in, exec_aten::optional<int64_t>(dim), keepdim, out);
+}
+
 #ifndef USE_ATEN_LIB
 bool check_reduction_args(
     const Tensor& in,

runtime/core/exec_aten/exec_aten.h

@@ -104,8 +104,7 @@ using ArrayRef = torch::executor::ArrayRef<T>;
 template <typename T>
 using optional = torch::executor::optional<T>;
 using nullopt_t = torch::executor::nullopt_t;
-// NOLINTNEXTLINE(facebook-hte-NamespaceScopedStaticDeclaration)
-static constexpr nullopt_t nullopt{0};
+using torch::executor::nullopt;
 using ScalarType = torch::executor::ScalarType;
 using TensorList = ArrayRef<Tensor>;
 using Scalar = torch::executor::Scalar;

runtime/core/portable_type/optional.h

@@ -8,175 +8,15 @@
 
 #pragma once
 
-#include <executorch/runtime/platform/assert.h>
-#include <new>
-#include <utility> // std::forward and other template magic checks
+#include <optional>
 
 namespace executorch {
 namespace runtime {
 namespace etensor {
 
-/// Used to indicate an optional type with uninitialized state.
-struct nullopt_t final {
-  constexpr explicit nullopt_t(int32_t) {}
-};
-
-/// A constant of type nullopt_t that is used to indicate an optional type with
-/// uninitialized state.
-constexpr nullopt_t nullopt{0};
-
-/// Leaner optional class, subset of c10, std, and boost optional APIs.
-template <class T>
-class optional final {
- public:
-  /// The type wrapped by the optional class.
-  using value_type = T;
-
-  /// Constructs an optional object that does not contain a value.
-  /* implicit */ optional() noexcept : storage_(trivial_init), init_(false) {}
-
-  /// Constructs an optional object that does not contain a value.
-  /* implicit */ optional(nullopt_t) noexcept
-      : storage_(trivial_init), init_(false) {}
-
-  /// Constructs an optional object that matches the state of v.
-  /* implicit */ optional(const optional<T>& v)
-      : storage_(trivial_init), init_(v.init_) {
-    if (init_) {
-      new (&storage_.value_) T(v.storage_.value_);
-    }
-  }
-
-  /// Constructs an optional object that contains the specified value.
-  /* implicit */ optional(const T& v) : storage_(v), init_(true) {}
-
-  /// Constructs an optional object from v.
-  /* implicit */ optional(optional<T>&& v) noexcept(
-      std::is_nothrow_move_constructible<T>::value)
-      : storage_(trivial_init), init_(v.init_) {
-    if (init_) {
-      new (&storage_.value_) T(std::forward<T>(v.storage_.value_));
-    }
-  }
-
-  /// Constructs an optional object that contains the specified value.
-  /* implicit */ optional(T&& v) : storage_(std::forward<T>(v)), init_(true) {}
-
-  optional& operator=(const optional& rhs) {
-    if (init_ && !rhs.init_) {
-      clear();
-    } else if (!init_ && rhs.init_) {
-      init_ = true;
-      new (&storage_.value_) T(rhs.storage_.value_);
-    } else if (init_ && rhs.init_) {
-      storage_.value_ = rhs.storage_.value_;
-    }
-    return *this;
-  }
-
-  optional& operator=(optional&& rhs) noexcept(
-      std::is_nothrow_move_assignable<T>::value &&
-      std::is_nothrow_move_constructible<T>::value) {
-    if (init_ && !rhs.init_) {
-      clear();
-    } else if (!init_ && rhs.init_) {
-      init_ = true;
-      new (&storage_.value_) T(std::forward<T>(rhs.storage_.value_));
-    } else if (init_ && rhs.init_) {
-      storage_.value_ = std::forward<T>(rhs.storage_.value_);
-    }
-    return *this;
-  }
-
-  /// Destroys the stored value if there is one
-  ~optional() {
-    if (init_) {
-      storage_.value_.~T();
-    }
-  }
-
-  optional& operator=(nullopt_t) noexcept {
-    clear();
-    return *this;
-  }
-
-  /// Returns true if the object contains a value, false otherwise
-  explicit operator bool() const noexcept {
-    return init_;
-  }
-
-  /// Returns true if the object contains a value, false otherwise
-  bool has_value() const noexcept {
-    return init_;
-  }
-
-  /// Returns a constant reference to the contained value. Calls ET_CHECK if
-  /// the object does not contain a value.
-  T const& value() const& {
-    ET_CHECK(init_);
-    return contained_val();
-  }
-
-  /// Returns a mutable reference to the contained value. Calls ET_CHECK if the
-  /// object does not contain a value.
-  T& value() & {
-    ET_CHECK(init_);
-    return contained_val();
-  }
-
-  /// Returns an rvalue of the contained value. Calls ET_CHECK if the object
-  /// does not contain a value.
-  T&& value() && {
-    ET_CHECK(init_);
-    return std::forward<T>(contained_val());
-  }
-
- private:
-  // Used to invoke the dummy ctor of storage_t in the initializer lists of
-  // optional_base as default ctor is implicitly deleted because T is nontrivial
-  struct trivial_init_t {
-  } trivial_init{};
-
-  /**
-   * A wrapper type that lets us avoid constructing a T when there is no value.
-   * If there is a value present, the optional class must destroy it.
-   */
-  union storage_t {
-    /// A small, trivially-constructable alternative to T.
-    unsigned char dummy_;
-    /// The constructed value itself, if optional::has_value_ is true.
-    T value_;
-
-    /* implicit */ storage_t(trivial_init_t) {
-      dummy_ = 0;
-    }
-
-    template <class... Args>
-    storage_t(Args&&... args) : value_(std::forward<Args>(args)...) {}
-
-    ~storage_t() {}
-  };
-
-  const T& contained_val() const& {
-    return storage_.value_;
-  }
-  T&& contained_val() && {
-    return std::move(storage_.value_);
-  }
-  T& contained_val() & {
-    return storage_.value_;
-  }
-
-  void clear() noexcept {
-    if (init_) {
-      storage_.value_.~T();
-    }
-    init_ = false;
-  }
-
-  storage_t storage_;
-  bool init_;
-};
+using std::nullopt;
+using std::nullopt_t;
+using std::optional;
 
 } // namespace etensor
 } // namespace runtime