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diff --git a/libsupport/include/CoreLib/Error/ErrorOr.h b/libsupport/include/CoreLib/Error/ErrorOr.h
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+++ b/libsupport/include/CoreLib/Error/ErrorOr.h
@@ -0,0 +1,378 @@
+/*
+ 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