[Support] Add ErrorOr support

Signed-off-by: Richard Qin <richard.qin.x@gmail.com>

1 files changed, 293 insertions, 0 deletions

diff --git a/rcc/include/rcc/Support/ErrorOr.h b/rcc/include/rcc/Support/ErrorOr.h
new file mode 100644
index 0000000..3a95fc9
--- /dev/null
+++ b/rcc/include/rcc/Support/ErrorOr.h
@@ -0,0 +1,293 @@
+/*
+ RCC-Project
+
+ Copyright (C) 2026 Richard Qin
+
+ This file is part of RCC-Project.
+
+ RCC-Project is free software; you can redistribute it and/or modify it under
+ the terms of the GNU General Public License as published by the Free
+ Software Foundation; either version 3 of the License, or (at your option) any
+ later version.
+
+ RCC-Project is distributed in the hope that it will be useful, but WITHOUT ANY
+ WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License along with
+ RCC-Project; see the file COPYING.  If not see
+ <http://www.gnu.org/licenses/>.
+*/
+
+//===- llvm/Support/ErrorOr.h - Error Smart Pointer -------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+///
+/// Provides ErrorOr<T> smart pointer.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef RCC_SUPPORT_ERROROR_H
+#define RCC_SUPPORT_ERROROR_H
+
+#include <cassert>
+#include <functional> // for std::reference_wrapper
+#include <system_error>
+#include <type_traits>
+#include <utility>
+
+namespace rcc {
+
+/// Represents either an error or a value T.
+///
+/// ErrorOr<T> is a pointer-like class that represents the result of an
+/// operation. The result is either an error, or a value of type T. This is
+/// designed to emulate the usage of returning a pointer where nullptr indicates
+/// failure. However instead of just knowing that the operation failed, we also
+/// have an error_code that describes why it failed.
+///
+/// It is used like the following.
+/// \code
+///   ErrorOr<Buffer> getBuffer();
+///
+///   auto buffer = getBuffer();
+///   if (error_code ec = buffer.getError())
+///     return ec;
+///   buffer->write("adena");
+/// \endcode
+///
+///
+/// Implicit conversion to bool returns true if there is a usable value. The
+/// unary * and -> operators provide pointer like access to the value. Accessing
+/// the value when there is an error has undefined behavior.
+///
+/// When T is a reference type the behavior is slightly different. The reference
+/// is held in a std::reference_wrapper<std::remove_reference<T>::type>, and
+/// there is special handling to make operator -> work as if T was not a
+/// reference.
+///
+/// T cannot be a rvalue reference.
+template<class T>
+class ErrorOr {
+  template <class OtherT> friend class ErrorOr;
+
+  static constexpr bool isRef = std::is_reference_v<T>;
+
+  using wrap = std::reference_wrapper<std::remove_reference_t<T>>;
+
+public:
+  using storage_type = std::conditional_t<isRef, wrap, T>;
+
+private:
+  using reference = std::remove_reference_t<T> &;
+  using const_reference = const std::remove_reference_t<T> &;
+  using pointer = std::remove_reference_t<T> *;
+  using const_pointer = const std::remove_reference_t<T> *;
+
+public:
+  template <class E>
+  ErrorOr(E ErrorCode,
+          std::enable_if_t<std::is_error_code_enum<E>::value ||
+                               std::is_error_condition_enum<E>::value,
+                           void *> = nullptr)
+      : HasError(true) {
+    new (getErrorStorage()) std::error_code(make_error_code(ErrorCode));
+  }
+
+  ErrorOr(std::error_code EC) : HasError(true) {
+    new (getErrorStorage()) std::error_code(EC);
+  }
+
+  template <class OtherT>
+  ErrorOr(OtherT &&Val,
+          std::enable_if_t<std::is_convertible_v<OtherT, T>> * = nullptr)
+      : HasError(false) {
+    new (getStorage()) storage_type(std::forward<OtherT>(Val));
+  }
+
+  ErrorOr(const ErrorOr &Other) {
+    copyConstruct(Other);
+  }
+
+  template <class OtherT>
+  ErrorOr(const ErrorOr<OtherT> &Other,
+          std::enable_if_t<std::is_convertible_v<OtherT, T>> * = nullptr) {
+    copyConstruct(Other);
+  }
+
+  template <class OtherT>
+  explicit ErrorOr(
+      const ErrorOr<OtherT> &Other,
+      std::enable_if_t<!std::is_convertible_v<OtherT, const T &>> * = nullptr) {
+    copyConstruct(Other);
+  }
+
+  ErrorOr(ErrorOr &&Other) {
+    moveConstruct(std::move(Other));
+  }
+
+  template <class OtherT>
+  ErrorOr(ErrorOr<OtherT> &&Other,
+          std::enable_if_t<std::is_convertible_v<OtherT, T>> * = nullptr) {
+    moveConstruct(std::move(Other));
+  }
+
+  // This might eventually need SFINAE but it's more complex than is_convertible
+  // & I'm too lazy to write it right now.
+  template <class OtherT>
+  explicit ErrorOr(
+      ErrorOr<OtherT> &&Other,
+      std::enable_if_t<!std::is_convertible_v<OtherT, T>> * = nullptr) {
+    moveConstruct(std::move(Other));
+  }
+
+  ErrorOr &operator=(const ErrorOr &Other) {
+    copyAssign(Other);
+    return *this;
+  }
+
+  ErrorOr &operator=(ErrorOr &&Other) {
+    moveAssign(std::move(Other));
+    return *this;
+  }
+
+  ~ErrorOr() {
+    if (!HasError)
+      getStorage()->~storage_type();
+  }
+
+  /// Return false if there is an error.
+  explicit operator bool() const {
+    return !HasError;
+  }
+
+  reference get() { return *getStorage(); }
+  const_reference get() const { return const_cast<ErrorOr<T> *>(this)->get(); }
+
+  std::error_code getError() const {
+    return HasError ? *getErrorStorage() : std::error_code();
+  }
+
+  pointer operator ->() {
+    return toPointer(getStorage());
+  }
+
+  const_pointer operator->() const { return toPointer(getStorage()); }
+
+  reference operator *() {
+    return *getStorage();
+  }
+
+  const_reference operator*() const { return *getStorage(); }
+
+private:
+  template <class OtherT>
+  void copyConstruct(const ErrorOr<OtherT> &Other) {
+    if (!Other.HasError) {
+      // Get the other value.
+      HasError = false;
+      new (getStorage()) storage_type(*Other.getStorage());
+    } else {
+      // Get other's error.
+      HasError = true;
+      new (getErrorStorage()) std::error_code(Other.getError());
+    }
+  }
+
+  template <class T1>
+  static bool compareThisIfSameType(const T1 &a, const T1 &b) {
+    return &a == &b;
+  }
+
+  template <class T1, class T2>
+  static bool compareThisIfSameType(const T1 &, const T2 &) {
+    return false;
+  }
+
+  template <class OtherT>
+  void copyAssign(const ErrorOr<OtherT> &Other) {
+    if (compareThisIfSameType(*this, Other))
+      return;
+
+    this->~ErrorOr();
+    new (this) ErrorOr(Other);
+  }
+
+  template <class OtherT>
+  void moveConstruct(ErrorOr<OtherT> &&Other) {
+    if (!Other.HasError) {
+      // Get the other value.
+      HasError = false;
+      new (getStorage()) storage_type(std::move(*Other.getStorage()));
+    } else {
+      // Get other's error.
+      HasError = true;
+      new (getErrorStorage()) std::error_code(Other.getError());
+    }
+  }
+
+  template <class OtherT>
+  void moveAssign(ErrorOr<OtherT> &&Other) {
+    if (compareThisIfSameType(*this, Other))
+      return;
+
+    this->~ErrorOr();
+    new (this) ErrorOr(std::move(Other));
+  }
+
+  pointer toPointer(pointer Val) {
+    return Val;
+  }
+
+  const_pointer toPointer(const_pointer Val) const { return Val; }
+
+  pointer toPointer(wrap *Val) {
+    return &Val->get();
+  }
+
+  const_pointer toPointer(const wrap *Val) const { return &Val->get(); }
+
+  storage_type *getStorage() {
+    assert(!HasError && "Cannot get value when an error exists!");
+    return &TStorage;
+  }
+
+  const storage_type *getStorage() const {
+    assert(!HasError && "Cannot get value when an error exists!");
+    return &TStorage;
+  }
+
+  std::error_code *getErrorStorage() {
+    assert(HasError && "Cannot get error when a value exists!");
+    return &ErrorStorage;
+  }
+
+  const std::error_code *getErrorStorage() const {
+    assert(HasError && "Cannot get error when a value exists!");
+    return &ErrorStorage;
+  }
+
+  union {
+    storage_type TStorage;
+    std::error_code ErrorStorage;
+  };
+  bool HasError : 1;
+};
+
+template <class T, class E>
+std::enable_if_t<std::is_error_code_enum<E>::value ||
+                     std::is_error_condition_enum<E>::value,
+                 bool>
+operator==(const ErrorOr<T> &Err, E Code) {
+  return Err.getError() == Code;
+}
+
+} // end namespace rcc
+
+#endif // RCC_SUPPORT_ERROROR_H