From 89d789ac4e2e08aa2bfa06bf76545a9259a93844 Mon Sep 17 00:00:00 2001
From: Jonne Ransijn <jonne@yyny.dev>
Date: Wed, 20 Nov 2024 12:55:39 +0100
Subject: [PATCH 1/2] AK: Allow `Optional<T>` to be used in constant
 expressions

Including its specializations, wherever possible.
---
 AK/FlyString.h                             |  34 ++--
 AK/Optional.h                              | 195 +++++++++++----------
 AK/StdLibExtras.h                          |   2 +
 AK/String.h                                |  34 ++--
 Libraries/LibJS/Runtime/Completion.h       |  16 +-
 Libraries/LibJS/Runtime/Value.h            |  30 ++--
 Libraries/LibTest/Macros.h                 |   5 +
 Libraries/LibWeb/Painting/PaintableBox.cpp |   1 -
 Tests/AK/TestOptional.cpp                  |  70 ++++++--
 9 files changed, 225 insertions(+), 162 deletions(-)

diff --git a/AK/FlyString.h b/AK/FlyString.h
index a0963e6b831..640b7fd18f3 100644
--- a/AK/FlyString.h
+++ b/AK/FlyString.h
@@ -21,7 +21,7 @@ class FlyString {
     AK_MAKE_DEFAULT_COPYABLE(FlyString);
 
 public:
-    FlyString() = default;
+    constexpr FlyString() = default;
 
     static ErrorOr<FlyString> from_utf8(StringView);
     static FlyString from_utf8_without_validation(ReadonlyBytes);
@@ -83,7 +83,7 @@ public:
 private:
     friend class Optional<FlyString>;
 
-    explicit FlyString(nullptr_t)
+    explicit constexpr FlyString(nullptr_t)
         : m_data(Detail::StringBase(nullptr))
     {
     }
@@ -106,38 +106,38 @@ class Optional<FlyString> : public OptionalBase<FlyString> {
 public:
     using ValueType = FlyString;
 
-    Optional() = default;
+    constexpr Optional() = default;
 
     template<SameAs<OptionalNone> V>
-    Optional(V) { }
+    constexpr Optional(V) { }
 
-    Optional(Optional<FlyString> const& other)
+    constexpr Optional(Optional<FlyString> const& other)
     {
         if (other.has_value())
             m_value = other.m_value;
     }
 
     Optional(Optional&& other)
-        : m_value(other.m_value)
+        : m_value(move(other.m_value))
     {
     }
 
     template<typename U = FlyString>
     requires(!IsSame<OptionalNone, RemoveCVReference<U>>)
-    explicit(!IsConvertible<U&&, FlyString>) Optional(U&& value)
+    explicit(!IsConvertible<U&&, FlyString>) constexpr Optional(U&& value)
     requires(!IsSame<RemoveCVReference<U>, Optional<FlyString>> && IsConstructible<FlyString, U &&>)
         : m_value(forward<U>(value))
     {
     }
 
     template<SameAs<OptionalNone> V>
-    Optional& operator=(V)
+    constexpr Optional& operator=(V)
     {
         clear();
         return *this;
     }
 
-    Optional& operator=(Optional const& other)
+    constexpr Optional& operator=(Optional const& other)
     {
         if (this != &other) {
             clear();
@@ -146,7 +146,7 @@ public:
         return *this;
     }
 
-    Optional& operator=(Optional&& other)
+    constexpr Optional& operator=(Optional&& other)
     {
         if (this != &other) {
             clear();
@@ -156,40 +156,40 @@ public:
     }
 
     template<typename O>
-    ALWAYS_INLINE bool operator==(Optional<O> const& other) const
+    ALWAYS_INLINE constexpr bool operator==(Optional<O> const& other) const
     {
         return has_value() == other.has_value() && (!has_value() || value() == other.value());
     }
 
     template<typename O>
-    ALWAYS_INLINE bool operator==(O const& other) const
+    ALWAYS_INLINE constexpr bool operator==(O const& other) const
     {
         return has_value() && value() == other;
     }
 
-    void clear()
+    constexpr void clear()
     {
         m_value = FlyString(nullptr);
     }
 
-    [[nodiscard]] bool has_value() const
+    [[nodiscard]] constexpr bool has_value() const
     {
         return !m_value.is_invalid();
     }
 
-    [[nodiscard]] FlyString& value() &
+    [[nodiscard]] constexpr FlyString& value() &
     {
         VERIFY(has_value());
         return m_value;
     }
 
-    [[nodiscard]] FlyString const& value() const&
+    [[nodiscard]] constexpr FlyString const& value() const&
     {
         VERIFY(has_value());
         return m_value;
     }
 
-    [[nodiscard]] FlyString value() &&
+    [[nodiscard]] constexpr FlyString value() &&
     {
         return release_value();
     }
diff --git a/AK/Optional.h b/AK/Optional.h
index 1bb6441bcbe..caa6f07780d 100644
--- a/AK/Optional.h
+++ b/AK/Optional.h
@@ -46,7 +46,7 @@ template<typename>
 class Optional;
 
 struct OptionalNone {
-    explicit OptionalNone() = default;
+    explicit constexpr OptionalNone() = default;
 };
 
 template<typename T, typename Self = Optional<T>>
@@ -55,30 +55,30 @@ public:
     using ValueType = T;
 
     template<SameAs<OptionalNone> V>
-    Self& operator=(V)
+    constexpr Self& operator=(V)
     {
         static_cast<Self&>(*this).clear();
         return static_cast<Self&>(*this);
     }
 
-    [[nodiscard]] ALWAYS_INLINE T* ptr() &
+    [[nodiscard]] ALWAYS_INLINE constexpr T* ptr() &
     {
         return static_cast<Self&>(*this).has_value() ? __builtin_launder(reinterpret_cast<T*>(&static_cast<Self&>(*this).value())) : nullptr;
     }
 
-    [[nodiscard]] ALWAYS_INLINE T const* ptr() const&
+    [[nodiscard]] ALWAYS_INLINE constexpr T const* ptr() const&
     {
         return static_cast<Self const&>(*this).has_value() ? __builtin_launder(reinterpret_cast<T const*>(&static_cast<Self const&>(*this).value())) : nullptr;
     }
 
-    [[nodiscard]] ALWAYS_INLINE T value_or(T const& fallback) const&
+    [[nodiscard]] ALWAYS_INLINE constexpr T value_or(T const& fallback) const&
     {
         if (static_cast<Self const&>(*this).has_value())
             return static_cast<Self const&>(*this).value();
         return fallback;
     }
 
-    [[nodiscard]] ALWAYS_INLINE T value_or(T&& fallback) &&
+    [[nodiscard]] ALWAYS_INLINE constexpr T value_or(T&& fallback) &&
     {
         if (static_cast<Self&>(*this).has_value())
             return move(static_cast<Self&>(*this).value());
@@ -86,7 +86,7 @@ public:
     }
 
     template<typename Callback, typename O = T>
-    [[nodiscard]] ALWAYS_INLINE O value_or_lazy_evaluated(Callback callback) const
+    [[nodiscard]] ALWAYS_INLINE constexpr O value_or_lazy_evaluated(Callback callback) const
     {
         if (static_cast<Self const&>(*this).has_value())
             return static_cast<Self const&>(*this).value();
@@ -94,7 +94,7 @@ public:
     }
 
     template<typename Callback, typename O = T>
-    [[nodiscard]] ALWAYS_INLINE Optional<O> value_or_lazy_evaluated_optional(Callback callback) const
+    [[nodiscard]] ALWAYS_INLINE constexpr Optional<O> value_or_lazy_evaluated_optional(Callback callback) const
     {
         if (static_cast<Self const&>(*this).has_value())
             return static_cast<Self const&>(*this).value();
@@ -102,7 +102,7 @@ public:
     }
 
     template<typename Callback, typename O = T>
-    [[nodiscard]] ALWAYS_INLINE ErrorOr<O> try_value_or_lazy_evaluated(Callback callback) const
+    [[nodiscard]] ALWAYS_INLINE constexpr ErrorOr<O> try_value_or_lazy_evaluated(Callback callback) const
     {
         if (static_cast<Self const&>(*this).has_value())
             return static_cast<Self const&>(*this).value();
@@ -110,7 +110,7 @@ public:
     }
 
     template<typename Callback, typename O = T>
-    [[nodiscard]] ALWAYS_INLINE ErrorOr<Optional<O>> try_value_or_lazy_evaluated_optional(Callback callback) const
+    [[nodiscard]] ALWAYS_INLINE constexpr ErrorOr<Optional<O>> try_value_or_lazy_evaluated_optional(Callback callback) const
     {
         if (static_cast<Self const&>(*this).has_value())
             return static_cast<Self const&>(*this).value();
@@ -118,7 +118,7 @@ public:
     }
 
     template<typename O>
-    ALWAYS_INLINE bool operator==(Optional<O> const& other) const
+    ALWAYS_INLINE constexpr bool operator==(Optional<O> const& other) const
     {
         return static_cast<Self const&>(*this).has_value() == (other).has_value()
             && (!static_cast<Self const&>(*this).has_value() || static_cast<Self const&>(*this).value() == (other).value());
@@ -126,19 +126,19 @@ public:
 
     template<typename O>
     requires(!Detail::IsBaseOf<OptionalBase<T, Self>, O>)
-    ALWAYS_INLINE bool operator==(O const& other) const
+    ALWAYS_INLINE constexpr bool operator==(O const& other) const
     {
         return static_cast<Self const&>(*this).has_value() && static_cast<Self const&>(*this).value() == other;
     }
 
-    [[nodiscard]] ALWAYS_INLINE T const& operator*() const { return static_cast<Self const&>(*this).value(); }
-    [[nodiscard]] ALWAYS_INLINE T& operator*() { return static_cast<Self&>(*this).value(); }
+    [[nodiscard]] ALWAYS_INLINE constexpr T const& operator*() const { return static_cast<Self const&>(*this).value(); }
+    [[nodiscard]] ALWAYS_INLINE constexpr T& operator*() { return static_cast<Self&>(*this).value(); }
 
-    ALWAYS_INLINE T const* operator->() const { return &static_cast<Self const&>(*this).value(); }
-    ALWAYS_INLINE T* operator->() { return &static_cast<Self&>(*this).value(); }
+    ALWAYS_INLINE constexpr T const* operator->() const { return &static_cast<Self const&>(*this).value(); }
+    ALWAYS_INLINE constexpr T* operator->() { return &static_cast<Self&>(*this).value(); }
 
     template<typename F, typename MappedType = decltype(declval<F>()(declval<T&>())), auto IsErrorOr = IsSpecializationOf<MappedType, ErrorOr>, typename OptionalType = Optional<ConditionallyResultType<IsErrorOr, MappedType>>>
-    ALWAYS_INLINE Conditional<IsErrorOr, ErrorOr<OptionalType>, OptionalType> map(F&& mapper)
+    ALWAYS_INLINE constexpr Conditional<IsErrorOr, ErrorOr<OptionalType>, OptionalType> map(F&& mapper)
     {
         if constexpr (IsErrorOr) {
             if (static_cast<Self&>(*this).has_value())
@@ -153,7 +153,7 @@ public:
     }
 
     template<typename F, typename MappedType = decltype(declval<F>()(declval<T&>())), auto IsErrorOr = IsSpecializationOf<MappedType, ErrorOr>, typename OptionalType = Optional<ConditionallyResultType<IsErrorOr, MappedType>>>
-    ALWAYS_INLINE Conditional<IsErrorOr, ErrorOr<OptionalType>, OptionalType> map(F&& mapper) const
+    ALWAYS_INLINE constexpr Conditional<IsErrorOr, ErrorOr<OptionalType>, OptionalType> map(F&& mapper) const
     {
         if constexpr (IsErrorOr) {
             if (static_cast<Self const&>(*this).has_value())
@@ -178,13 +178,19 @@ requires(!IsLvalueReference<T>) class [[nodiscard]] Optional<T> : public Optiona
 public:
     using ValueType = T;
 
-    ALWAYS_INLINE Optional() = default;
+    ALWAYS_INLINE constexpr Optional()
+        : m_null()
+    {
+    }
 
     template<SameAs<OptionalNone> V>
-    Optional(V) { }
+    constexpr Optional(V)
+        : m_null()
+    {
+    }
 
     template<SameAs<OptionalNone> V>
-    Optional& operator=(V)
+    constexpr Optional& operator=(V)
     {
         clear();
         return *this;
@@ -194,143 +200,146 @@ public:
     AK_MAKE_CONDITIONALLY_NONMOVABLE(Optional, <T>);
     AK_MAKE_CONDITIONALLY_DESTRUCTIBLE(Optional, <T>);
 
-    ALWAYS_INLINE Optional(Optional const& other)
+    ALWAYS_INLINE constexpr Optional(Optional const& other)
     requires(!IsTriviallyCopyConstructible<T>)
         : m_has_value(other.m_has_value)
     {
         if (other.has_value())
-            new (&m_storage) T(other.value());
+            construct_at(&m_value, other.value());
     }
 
-    ALWAYS_INLINE Optional(Optional&& other)
+    ALWAYS_INLINE constexpr Optional(Optional&& other)
         : m_has_value(other.m_has_value)
     {
         if (other.has_value())
-            new (&m_storage) T(other.release_value());
+            construct_at(&m_value, other.release_value());
     }
 
     template<typename U>
-    requires(IsConstructible<T, U const&> && !IsSpecializationOf<T, Optional> && !IsSpecializationOf<U, Optional>) ALWAYS_INLINE explicit Optional(Optional<U> const& other)
+    requires(IsConstructible<T, U const&> && !IsSpecializationOf<T, Optional> && !IsSpecializationOf<U, Optional>) ALWAYS_INLINE explicit constexpr Optional(Optional<U> const& other)
         : m_has_value(other.has_value())
     {
         if (other.has_value())
-            new (&m_storage) T(other.value());
+            construct_at(&m_value, other.value());
     }
 
     template<typename U>
-    requires(IsConstructible<T, U &&> && !IsSpecializationOf<T, Optional> && !IsSpecializationOf<U, Optional>) ALWAYS_INLINE explicit Optional(Optional<U>&& other)
+    requires(IsConstructible<T, U &&> && !IsSpecializationOf<T, Optional> && !IsSpecializationOf<U, Optional>) ALWAYS_INLINE explicit constexpr Optional(Optional<U>&& other)
         : m_has_value(other.has_value())
     {
         if (other.has_value())
-            new (&m_storage) T(other.release_value());
+            construct_at(&m_value, other.value());
     }
 
     template<typename U = T>
     requires(!IsSame<OptionalNone, RemoveCVReference<U>>)
-    ALWAYS_INLINE explicit(!IsConvertible<U&&, T>) Optional(U&& value)
+    ALWAYS_INLINE explicit(!IsConvertible<U&&, T>) constexpr Optional(U&& value)
     requires(!IsSame<RemoveCVReference<U>, Optional<T>> && IsConstructible<T, U &&>)
         : m_has_value(true)
     {
-        new (&m_storage) T(forward<U>(value));
+        construct_at(&m_value, forward<U>(value));
     }
 
-    ALWAYS_INLINE Optional& operator=(Optional const& other)
+    ALWAYS_INLINE constexpr Optional& operator=(Optional const& other)
     requires(!IsTriviallyCopyConstructible<T> || !IsTriviallyDestructible<T>)
     {
         if (this != &other) {
             clear();
             m_has_value = other.m_has_value;
             if (other.has_value()) {
-                new (&m_storage) T(other.value());
+                construct_at(&m_value, other.value());
             }
         }
         return *this;
     }
 
-    ALWAYS_INLINE Optional& operator=(Optional&& other)
+    ALWAYS_INLINE constexpr Optional& operator=(Optional&& other)
     {
         if (this != &other) {
             clear();
             m_has_value = other.m_has_value;
             if (other.has_value()) {
-                new (&m_storage) T(other.release_value());
+                construct_at(&m_value, other.release_value());
             }
         }
         return *this;
     }
 
     template<typename O>
-    ALWAYS_INLINE bool operator==(Optional<O> const& other) const
+    ALWAYS_INLINE constexpr bool operator==(Optional<O> const& other) const
     {
         return has_value() == other.has_value() && (!has_value() || value() == other.value());
     }
 
     template<typename O>
-    ALWAYS_INLINE bool operator==(O const& other) const
+    ALWAYS_INLINE constexpr bool operator==(O const& other) const
     {
         return has_value() && value() == other;
     }
 
-    ALWAYS_INLINE ~Optional()
+    ALWAYS_INLINE constexpr ~Optional()
     requires(!IsTriviallyDestructible<T> && IsDestructible<T>)
     {
         clear();
     }
 
-    ALWAYS_INLINE void clear()
+    ALWAYS_INLINE constexpr void clear()
     {
         if (m_has_value) {
-            value().~T();
+            m_value.~T();
             m_has_value = false;
         }
     }
 
     template<typename... Parameters>
-    ALWAYS_INLINE void emplace(Parameters&&... parameters)
+    ALWAYS_INLINE constexpr void emplace(Parameters&&... parameters)
     {
         clear();
         m_has_value = true;
-        new (&m_storage) T(forward<Parameters>(parameters)...);
+        construct_at(&m_value, forward<Parameters>(parameters)...);
     }
 
     template<typename Callable>
-    ALWAYS_INLINE void lazy_emplace(Callable callable)
+    ALWAYS_INLINE constexpr void lazy_emplace(Callable callable)
     {
         clear();
         m_has_value = true;
-        new (&m_storage) T { callable() };
+        construct_at(&m_value, callable());
     }
 
-    [[nodiscard]] ALWAYS_INLINE bool has_value() const { return m_has_value; }
+    [[nodiscard]] ALWAYS_INLINE constexpr bool has_value() const { return m_has_value; }
 
-    [[nodiscard]] ALWAYS_INLINE T& value() &
+    [[nodiscard]] ALWAYS_INLINE constexpr T& value() &
     {
         VERIFY(m_has_value);
-        return *__builtin_launder(reinterpret_cast<T*>(&m_storage));
+        return m_value;
     }
 
-    [[nodiscard]] ALWAYS_INLINE T const& value() const&
+    [[nodiscard]] ALWAYS_INLINE constexpr T const& value() const&
     {
         VERIFY(m_has_value);
-        return *__builtin_launder(reinterpret_cast<T const*>(&m_storage));
+        return m_value;
     }
 
-    [[nodiscard]] ALWAYS_INLINE T value() &&
+    [[nodiscard]] ALWAYS_INLINE constexpr T value() &&
     {
         return release_value();
     }
 
-    [[nodiscard]] ALWAYS_INLINE T release_value()
+    [[nodiscard]] ALWAYS_INLINE constexpr T release_value()
     {
         VERIFY(m_has_value);
-        T released_value = move(value());
-        value().~T();
+        T released_value = move(m_value);
+        m_value.~T();
         m_has_value = false;
         return released_value;
     }
 
 private:
-    alignas(T) u8 m_storage[sizeof(T)];
+    union {
+        char m_null;
+        T m_value;
+    };
     bool m_has_value { false };
 };
 
@@ -345,76 +354,76 @@ requires(IsLvalueReference<T>) class [[nodiscard]] Optional<T> {
 public:
     using ValueType = T;
 
-    ALWAYS_INLINE Optional() = default;
+    ALWAYS_INLINE constexpr Optional() = default;
 
     template<SameAs<OptionalNone> V>
-    Optional(V) { }
+    constexpr Optional(V) { }
 
     template<SameAs<OptionalNone> V>
-    Optional& operator=(V)
+    constexpr Optional& operator=(V)
     {
         clear();
         return *this;
     }
 
     template<typename U = T>
-    ALWAYS_INLINE Optional(U& value)
+    ALWAYS_INLINE constexpr Optional(U& value)
     requires(CanBePlacedInOptional<U&>)
         : m_pointer(&value)
     {
     }
 
-    ALWAYS_INLINE Optional(RemoveReference<T>& value)
+    ALWAYS_INLINE constexpr Optional(RemoveReference<T>& value)
         : m_pointer(&value)
     {
     }
 
-    ALWAYS_INLINE Optional(Optional const& other)
+    ALWAYS_INLINE constexpr Optional(Optional const& other)
         : m_pointer(other.m_pointer)
     {
     }
 
-    ALWAYS_INLINE Optional(Optional&& other)
+    ALWAYS_INLINE constexpr Optional(Optional&& other)
         : m_pointer(other.m_pointer)
     {
         other.m_pointer = nullptr;
     }
 
     template<typename U>
-    ALWAYS_INLINE Optional(Optional<U>& other)
+    ALWAYS_INLINE constexpr Optional(Optional<U>& other)
     requires(CanBePlacedInOptional<U>)
         : m_pointer(other.ptr())
     {
     }
 
     template<typename U>
-    ALWAYS_INLINE Optional(Optional<U> const& other)
+    ALWAYS_INLINE constexpr Optional(Optional<U> const& other)
     requires(CanBePlacedInOptional<U const>)
         : m_pointer(other.ptr())
     {
     }
 
     template<typename U>
-    ALWAYS_INLINE Optional(Optional<U>&& other)
+    ALWAYS_INLINE constexpr Optional(Optional<U>&& other)
     requires(CanBePlacedInOptional<U>)
         : m_pointer(other.ptr())
     {
         other.m_pointer = nullptr;
     }
 
-    ALWAYS_INLINE Optional& operator=(Optional& other)
+    ALWAYS_INLINE constexpr Optional& operator=(Optional& other)
     {
         m_pointer = other.m_pointer;
         return *this;
     }
 
-    ALWAYS_INLINE Optional& operator=(Optional const& other)
+    ALWAYS_INLINE constexpr Optional& operator=(Optional const& other)
     {
         m_pointer = other.m_pointer;
         return *this;
     }
 
-    ALWAYS_INLINE Optional& operator=(Optional&& other)
+    ALWAYS_INLINE constexpr Optional& operator=(Optional&& other)
     {
         m_pointer = other.m_pointer;
         other.m_pointer = nullptr;
@@ -422,7 +431,7 @@ public:
     }
 
     template<typename U>
-    ALWAYS_INLINE Optional& operator=(Optional<U>& other)
+    ALWAYS_INLINE constexpr Optional& operator=(Optional<U>& other)
     requires(CanBePlacedInOptional<U>)
     {
         m_pointer = other.ptr();
@@ -430,7 +439,7 @@ public:
     }
 
     template<typename U>
-    ALWAYS_INLINE Optional& operator=(Optional<U> const& other)
+    ALWAYS_INLINE constexpr Optional& operator=(Optional<U> const& other)
     requires(CanBePlacedInOptional<U const>)
     {
         m_pointer = other.ptr();
@@ -438,7 +447,7 @@ public:
     }
 
     template<typename U>
-    ALWAYS_INLINE Optional& operator=(Optional<U>&& other)
+    ALWAYS_INLINE constexpr Optional& operator=(Optional<U>&& other)
     requires(CanBePlacedInOptional<U> && IsLvalueReference<U>)
     {
         m_pointer = other.m_pointer;
@@ -449,44 +458,44 @@ public:
     // Note: Disallows assignment from a temporary as this does not do any lifetime extension.
     template<typename U>
     requires(!IsSame<OptionalNone, RemoveCVReference<U>>)
-    ALWAYS_INLINE Optional& operator=(U&& value)
+    ALWAYS_INLINE constexpr Optional& operator=(U&& value)
     requires(CanBePlacedInOptional<U> && IsLvalueReference<U>)
     {
         m_pointer = &value;
         return *this;
     }
 
-    ALWAYS_INLINE void clear()
+    ALWAYS_INLINE constexpr void clear()
     {
         m_pointer = nullptr;
     }
 
-    [[nodiscard]] ALWAYS_INLINE bool has_value() const { return m_pointer != nullptr; }
+    [[nodiscard]] ALWAYS_INLINE constexpr bool has_value() const { return m_pointer != nullptr; }
 
-    [[nodiscard]] ALWAYS_INLINE RemoveReference<T>* ptr()
+    [[nodiscard]] ALWAYS_INLINE constexpr RemoveReference<T>* ptr()
     {
         return m_pointer;
     }
 
-    [[nodiscard]] ALWAYS_INLINE RemoveReference<T> const* ptr() const
+    [[nodiscard]] ALWAYS_INLINE constexpr RemoveReference<T> const* ptr() const
     {
         return m_pointer;
     }
 
-    [[nodiscard]] ALWAYS_INLINE T value()
+    [[nodiscard]] ALWAYS_INLINE constexpr T value()
     {
         VERIFY(m_pointer);
         return *m_pointer;
     }
 
-    [[nodiscard]] ALWAYS_INLINE AddConstToReferencedType<T> value() const
+    [[nodiscard]] ALWAYS_INLINE constexpr AddConstToReferencedType<T> value() const
     {
         VERIFY(m_pointer);
         return *m_pointer;
     }
 
     template<typename U>
-    requires(IsBaseOf<RemoveCVReference<T>, U>) [[nodiscard]] ALWAYS_INLINE AddConstToReferencedType<T> value_or(U& fallback) const
+    requires(IsBaseOf<RemoveCVReference<T>, U>) [[nodiscard]] ALWAYS_INLINE constexpr AddConstToReferencedType<T> value_or(U& fallback) const
     {
         if (m_pointer)
             return value();
@@ -494,38 +503,38 @@ public:
     }
 
     // Note that this ends up copying the value.
-    [[nodiscard]] ALWAYS_INLINE RemoveCVReference<T> value_or(RemoveCVReference<T> fallback) const
+    [[nodiscard]] ALWAYS_INLINE constexpr RemoveCVReference<T> value_or(RemoveCVReference<T> fallback) const
     {
         if (m_pointer)
             return value();
         return fallback;
     }
 
-    [[nodiscard]] ALWAYS_INLINE T release_value()
+    [[nodiscard]] ALWAYS_INLINE constexpr T release_value()
     {
         return *exchange(m_pointer, nullptr);
     }
 
     template<typename U>
-    ALWAYS_INLINE bool operator==(Optional<U> const& other) const
+    ALWAYS_INLINE constexpr bool operator==(Optional<U> const& other) const
     {
         return has_value() == other.has_value() && (!has_value() || value() == other.value());
     }
 
     template<typename U>
-    ALWAYS_INLINE bool operator==(U const& other) const
+    ALWAYS_INLINE constexpr bool operator==(U const& other) const
     {
         return has_value() && value() == other;
     }
 
-    ALWAYS_INLINE AddConstToReferencedType<T> operator*() const { return value(); }
-    ALWAYS_INLINE T operator*() { return value(); }
+    ALWAYS_INLINE constexpr AddConstToReferencedType<T> operator*() const { return value(); }
+    ALWAYS_INLINE constexpr T operator*() { return value(); }
 
-    ALWAYS_INLINE RawPtr<AddConst<RemoveReference<T>>> operator->() const { return &value(); }
-    ALWAYS_INLINE RawPtr<RemoveReference<T>> operator->() { return &value(); }
+    ALWAYS_INLINE constexpr RawPtr<AddConst<RemoveReference<T>>> operator->() const { return &value(); }
+    ALWAYS_INLINE constexpr RawPtr<RemoveReference<T>> operator->() { return &value(); }
 
     // Conversion operators from Optional<T&> -> Optional<T>
-    ALWAYS_INLINE operator Optional<RemoveCVReference<T>>() const
+    ALWAYS_INLINE constexpr operator Optional<RemoveCVReference<T>>() const
     {
         if (has_value())
             return Optional<RemoveCVReference<T>>(value());
@@ -533,7 +542,7 @@ public:
     }
 
     template<typename Callback>
-    [[nodiscard]] ALWAYS_INLINE T value_or_lazy_evaluated(Callback callback) const
+    [[nodiscard]] ALWAYS_INLINE constexpr T value_or_lazy_evaluated(Callback callback) const
     {
         if (m_pointer != nullptr)
             return value();
@@ -541,7 +550,7 @@ public:
     }
 
     template<typename Callback>
-    [[nodiscard]] ALWAYS_INLINE Optional<T> value_or_lazy_evaluated_optional(Callback callback) const
+    [[nodiscard]] ALWAYS_INLINE constexpr Optional<T> value_or_lazy_evaluated_optional(Callback callback) const
     {
         if (m_pointer != nullptr)
             return value();
@@ -549,7 +558,7 @@ public:
     }
 
     template<typename Callback>
-    [[nodiscard]] ALWAYS_INLINE ErrorOr<T> try_value_or_lazy_evaluated(Callback callback) const
+    [[nodiscard]] ALWAYS_INLINE constexpr ErrorOr<T> try_value_or_lazy_evaluated(Callback callback) const
     {
         if (m_pointer != nullptr)
             return value();
@@ -557,7 +566,7 @@ public:
     }
 
     template<typename Callback>
-    [[nodiscard]] ALWAYS_INLINE ErrorOr<Optional<T>> try_value_or_lazy_evaluated_optional(Callback callback) const
+    [[nodiscard]] ALWAYS_INLINE constexpr ErrorOr<Optional<T>> try_value_or_lazy_evaluated_optional(Callback callback) const
     {
         if (m_pointer != nullptr)
             return value();
@@ -565,7 +574,7 @@ public:
     }
 
     template<typename F, typename MappedType = decltype(declval<F>()(declval<T&>())), auto IsErrorOr = IsSpecializationOf<MappedType, ErrorOr>, typename OptionalType = Optional<ConditionallyResultType<IsErrorOr, MappedType>>>
-    ALWAYS_INLINE Conditional<IsErrorOr, ErrorOr<OptionalType>, OptionalType> map(F&& mapper)
+    ALWAYS_INLINE constexpr Conditional<IsErrorOr, ErrorOr<OptionalType>, OptionalType> map(F&& mapper)
     {
         if constexpr (IsErrorOr) {
             if (m_pointer != nullptr)
@@ -580,7 +589,7 @@ public:
     }
 
     template<typename F, typename MappedType = decltype(declval<F>()(declval<T&>())), auto IsErrorOr = IsSpecializationOf<MappedType, ErrorOr>, typename OptionalType = Optional<ConditionallyResultType<IsErrorOr, MappedType>>>
-    ALWAYS_INLINE Conditional<IsErrorOr, ErrorOr<OptionalType>, OptionalType> map(F&& mapper) const
+    ALWAYS_INLINE constexpr Conditional<IsErrorOr, ErrorOr<OptionalType>, OptionalType> map(F&& mapper) const
     {
         if constexpr (IsErrorOr) {
             if (m_pointer != nullptr)
diff --git a/AK/StdLibExtras.h b/AK/StdLibExtras.h
index d0ba945aaf8..523c3812187 100644
--- a/AK/StdLibExtras.h
+++ b/AK/StdLibExtras.h
@@ -10,6 +10,7 @@
 #include <AK/Platform.h>
 #include <AK/StdLibExtraDetails.h>
 
+#include <memory>
 #include <utility>
 
 namespace AK {
@@ -31,6 +32,7 @@ requires(AK::Detail::IsIntegral<T>)
 template<typename... Args>
 void compiletime_fail(Args...);
 
+using std::construct_at;
 using std::forward;
 using std::move;
 }
diff --git a/AK/String.h b/AK/String.h
index 825677b9408..4c4ba6548a5 100644
--- a/AK/String.h
+++ b/AK/String.h
@@ -236,38 +236,38 @@ class Optional<String> : public OptionalBase<String> {
 public:
     using ValueType = String;
 
-    Optional() = default;
+    constexpr Optional() = default;
 
     template<SameAs<OptionalNone> V>
-    Optional(V) { }
+    constexpr Optional(V) { }
 
-    Optional(Optional<String> const& other)
+    constexpr Optional(Optional<String> const& other)
     {
         if (other.has_value())
             m_value = other.m_value;
     }
 
-    Optional(Optional&& other)
+    constexpr Optional(Optional&& other)
         : m_value(move(other.m_value))
     {
     }
 
     template<typename U = String>
     requires(!IsSame<OptionalNone, RemoveCVReference<U>>)
-    explicit(!IsConvertible<U&&, String>) Optional(U&& value)
+    explicit(!IsConvertible<U&&, String>) constexpr Optional(U&& value)
     requires(!IsSame<RemoveCVReference<U>, Optional<String>> && IsConstructible<String, U &&>)
         : m_value(forward<U>(value))
     {
     }
 
     template<SameAs<OptionalNone> V>
-    Optional& operator=(V)
+    constexpr Optional& operator=(V)
     {
         clear();
         return *this;
     }
 
-    Optional& operator=(Optional const& other)
+    constexpr Optional& operator=(Optional const& other)
     {
         if (this != &other) {
             m_value = other.m_value;
@@ -275,7 +275,7 @@ public:
         return *this;
     }
 
-    Optional& operator=(Optional&& other)
+    constexpr Optional& operator=(Optional&& other)
     {
         if (this != &other) {
             m_value = move(other.m_value);
@@ -284,48 +284,48 @@ public:
     }
 
     template<typename O>
-    ALWAYS_INLINE bool operator==(Optional<O> const& other) const
+    ALWAYS_INLINE constexpr bool operator==(Optional<O> const& other) const
     {
         return has_value() == other.has_value() && (!has_value() || value() == other.value());
     }
 
     template<typename O>
-    ALWAYS_INLINE bool operator==(O const& other) const
+    ALWAYS_INLINE constexpr bool operator==(O const& other) const
     {
         return has_value() && value() == other;
     }
 
-    void clear()
+    constexpr void clear()
     {
         m_value = String(nullptr);
     }
 
-    [[nodiscard]] bool has_value() const
+    [[nodiscard]] constexpr bool has_value() const
     {
         return !m_value.is_invalid();
     }
 
-    [[nodiscard]] String& value() &
+    [[nodiscard]] constexpr String& value() &
     {
         VERIFY(has_value());
         return m_value;
     }
 
-    [[nodiscard]] String const& value() const&
+    [[nodiscard]] constexpr String const& value() const&
     {
         VERIFY(has_value());
         return m_value;
     }
 
-    [[nodiscard]] String value() &&
+    [[nodiscard]] constexpr String value() &&
     {
         return release_value();
     }
 
-    [[nodiscard]] String release_value()
+    [[nodiscard]] constexpr String release_value()
     {
         VERIFY(has_value());
-        String released_value = m_value;
+        String released_value = move(m_value);
         clear();
         return released_value;
     }
diff --git a/Libraries/LibJS/Runtime/Completion.h b/Libraries/LibJS/Runtime/Completion.h
index b599a0911d1..70c70a3bce2 100644
--- a/Libraries/LibJS/Runtime/Completion.h
+++ b/Libraries/LibJS/Runtime/Completion.h
@@ -164,7 +164,7 @@ class Optional<JS::Completion> : public OptionalBase<JS::Completion> {
 public:
     using ValueType = JS::Completion;
 
-    Optional() = default;
+    constexpr Optional() = default;
 
     Optional(Optional<JS::Completion> const& other)
     {
@@ -172,19 +172,19 @@ public:
             m_value = other.m_value;
     }
 
-    Optional(Optional&& other)
+    constexpr Optional(Optional&& other)
         : m_value(move(other.m_value))
     {
     }
 
     template<typename U = JS::Completion>
-    explicit(!IsConvertible<U&&, JS::Completion>) Optional(U&& value)
+    explicit(!IsConvertible<U&&, JS::Completion>) constexpr Optional(U&& value)
     requires(!IsSame<RemoveCVReference<U>, Optional<JS::Completion>> && IsConstructible<JS::Completion, U &&>)
         : m_value(forward<U>(value))
     {
     }
 
-    Optional& operator=(Optional const& other)
+    constexpr Optional& operator=(Optional const& other)
     {
         if (this != &other) {
             clear();
@@ -193,7 +193,7 @@ public:
         return *this;
     }
 
-    Optional& operator=(Optional&& other)
+    constexpr Optional& operator=(Optional&& other)
     {
         if (this != &other) {
             clear();
@@ -207,18 +207,18 @@ public:
         m_value = JS::Completion(JS::Completion::EmptyTag {});
     }
 
-    [[nodiscard]] bool has_value() const
+    [[nodiscard]] constexpr bool has_value() const
     {
         return !m_value.is_empty();
     }
 
-    [[nodiscard]] JS::Completion& value() &
+    [[nodiscard]] constexpr JS::Completion& value() &
     {
         VERIFY(has_value());
         return m_value;
     }
 
-    [[nodiscard]] JS::Completion const& value() const&
+    [[nodiscard]] constexpr JS::Completion const& value() const&
     {
         VERIFY(has_value());
         return m_value;
diff --git a/Libraries/LibJS/Runtime/Value.h b/Libraries/LibJS/Runtime/Value.h
index 11cc0cb8908..9b43ea836e8 100644
--- a/Libraries/LibJS/Runtime/Value.h
+++ b/Libraries/LibJS/Runtime/Value.h
@@ -154,7 +154,7 @@ public:
         return !is_nan() && !is_infinity();
     }
 
-    Value()
+    constexpr Value()
         : Value(EMPTY_TAG << GC::TAG_SHIFT, (u64)0)
     {
     }
@@ -419,7 +419,7 @@ private:
     ThrowCompletionOr<Value> to_numeric_slow_case(VM&) const;
     ThrowCompletionOr<Value> to_primitive_slow_case(VM&, PreferredType) const;
 
-    Value(u64 tag, u64 val)
+    constexpr Value(u64 tag, u64 val)
     {
         ASSERT(!(tag & val));
         m_value.encoded = tag | val;
@@ -544,10 +544,10 @@ class Optional<JS::Value> : public OptionalBase<JS::Value> {
 public:
     using ValueType = JS::Value;
 
-    Optional() = default;
+    constexpr Optional() = default;
 
     template<SameAs<OptionalNone> V>
-    Optional(V) { }
+    constexpr Optional(V) { }
 
     Optional(Optional<JS::Value> const& other)
     {
@@ -555,27 +555,27 @@ public:
             m_value = other.m_value;
     }
 
-    Optional(Optional&& other)
+    constexpr Optional(Optional&& other)
         : m_value(other.m_value)
     {
     }
 
     template<typename U = JS::Value>
     requires(!IsSame<OptionalNone, RemoveCVReference<U>>)
-    explicit(!IsConvertible<U&&, JS::Value>) Optional(U&& value)
+    explicit(!IsConvertible<U&&, JS::Value>) constexpr Optional(U&& value)
     requires(!IsSame<RemoveCVReference<U>, Optional<JS::Value>> && IsConstructible<JS::Value, U &&>)
         : m_value(forward<U>(value))
     {
     }
 
     template<SameAs<OptionalNone> V>
-    Optional& operator=(V)
+    constexpr Optional& operator=(V)
     {
         clear();
         return *this;
     }
 
-    Optional& operator=(Optional const& other)
+    constexpr Optional& operator=(Optional const& other)
     {
         if (this != &other) {
             clear();
@@ -584,7 +584,7 @@ public:
         return *this;
     }
 
-    Optional& operator=(Optional&& other)
+    constexpr Optional& operator=(Optional&& other)
     {
         if (this != &other) {
             clear();
@@ -594,34 +594,34 @@ public:
     }
 
     template<typename O>
-    ALWAYS_INLINE bool operator==(Optional<O> const& other) const
+    ALWAYS_INLINE constexpr bool operator==(Optional<O> const& other) const
     {
         return has_value() == other.has_value() && (!has_value() || value() == other.value());
     }
 
     template<typename O>
-    ALWAYS_INLINE bool operator==(O const& other) const
+    ALWAYS_INLINE constexpr bool operator==(O const& other) const
     {
         return has_value() && value() == other;
     }
 
-    void clear()
+    constexpr void clear()
     {
         m_value = {};
     }
 
-    [[nodiscard]] bool has_value() const
+    [[nodiscard]] constexpr bool has_value() const
     {
         return !m_value.is_empty();
     }
 
-    [[nodiscard]] JS::Value& value() &
+    [[nodiscard]] constexpr JS::Value& value() &
     {
         VERIFY(has_value());
         return m_value;
     }
 
-    [[nodiscard]] JS::Value const& value() const&
+    [[nodiscard]] constexpr JS::Value const& value() const&
     {
         VERIFY(has_value());
         return m_value;
diff --git a/Libraries/LibTest/Macros.h b/Libraries/LibTest/Macros.h
index 1be6c82075e..8a4a81bc190 100644
--- a/Libraries/LibTest/Macros.h
+++ b/Libraries/LibTest/Macros.h
@@ -130,6 +130,9 @@ bool assume(T const& expression, StringView expression_string, SourceLocation lo
     return true;
 }
 
+template<typename T>
+consteval void expect_consteval(T) { }
+
 }
 
 #define EXPECT(x)                  \
@@ -179,6 +182,8 @@ bool assume(T const& expression, StringView expression_string, SourceLocation lo
         ::Test::set_current_test_result(::Test::TestResult::Failed);                       \
     } while (false)
 
+#define EXPECT_CONSTEVAL(...) ::Test::expect_consteval(__VA_ARGS__)
+
 // To use, specify the lambda to execute in a sub process and verify it exits:
 //  EXPECT_CRASH("This should fail", []{
 //      return Test::Crash::Failure::DidNotCrash;
diff --git a/Libraries/LibWeb/Painting/PaintableBox.cpp b/Libraries/LibWeb/Painting/PaintableBox.cpp
index aaff8189b74..d218d78351a 100644
--- a/Libraries/LibWeb/Painting/PaintableBox.cpp
+++ b/Libraries/LibWeb/Painting/PaintableBox.cpp
@@ -706,7 +706,6 @@ void PaintableWithLines::paint(PaintContext& context, PaintPhase phase) const
         return;
 
     bool should_clip_overflow = computed_values().overflow_x() != CSS::Overflow::Visible && computed_values().overflow_y() != CSS::Overflow::Visible;
-    Optional<u32> corner_clip_id;
 
     auto clip_box = absolute_padding_box_rect();
     if (get_clip_rect().has_value()) {
diff --git a/Tests/AK/TestOptional.cpp b/Tests/AK/TestOptional.cpp
index cb17d62debd..84dc0a9c92f 100644
--- a/Tests/AK/TestOptional.cpp
+++ b/Tests/AK/TestOptional.cpp
@@ -13,6 +13,17 @@
 #include <AK/String.h>
 #include <AK/Vector.h>
 
+struct DontCopyMe {
+    constexpr DontCopyMe() { }
+    ~DontCopyMe() = default;
+    DontCopyMe(DontCopyMe&&) = default;
+    DontCopyMe& operator=(DontCopyMe&&) = default;
+    DontCopyMe(DontCopyMe const&) = delete;
+    DontCopyMe& operator=(DontCopyMe const&) = delete;
+
+    int x { 13 };
+};
+
 TEST_CASE(basic_optional)
 {
     Optional<int> x;
@@ -39,17 +50,6 @@ TEST_CASE(move_optional)
 
 TEST_CASE(optional_rvalue_ref_qualified_getters)
 {
-    struct DontCopyMe {
-        DontCopyMe() { }
-        ~DontCopyMe() = default;
-        DontCopyMe(DontCopyMe&&) = default;
-        DontCopyMe& operator=(DontCopyMe&&) = default;
-        DontCopyMe(DontCopyMe const&) = delete;
-        DontCopyMe& operator=(DontCopyMe const&) = delete;
-
-        int x { 13 };
-    };
-
     auto make_an_optional = []() -> Optional<DontCopyMe> {
         return DontCopyMe {};
     };
@@ -122,6 +122,54 @@ TEST_CASE(comparison_with_numeric_types)
     EXPECT_NE(opt1, -2);
 }
 
+TEST_CASE(test_constexpr)
+{
+    int i = 13;
+    DontCopyMe dcm {};
+
+    EXPECT_CONSTEVAL(Optional<int> {});
+    EXPECT_CONSTEVAL(Optional<DontCopyMe> {});
+    EXPECT_CONSTEVAL(Optional<int const> {});
+    EXPECT_CONSTEVAL(Optional<DontCopyMe const> {});
+    EXPECT_CONSTEVAL(Optional<int&> {});
+    EXPECT_CONSTEVAL(Optional<DontCopyMe&> {});
+    EXPECT_CONSTEVAL(Optional<int const&> {});
+    EXPECT_CONSTEVAL(Optional<DontCopyMe const&> {});
+
+    EXPECT_CONSTEVAL(Optional<int> { 13 });
+    EXPECT_CONSTEVAL(Optional<DontCopyMe> { DontCopyMe {} });
+    EXPECT_CONSTEVAL(Optional<int const> { 13 });
+    EXPECT_CONSTEVAL(Optional<DontCopyMe const> { DontCopyMe {} });
+    EXPECT_CONSTEVAL(Optional<int&> { i });
+    EXPECT_CONSTEVAL(Optional<DontCopyMe&> { dcm });
+    EXPECT_CONSTEVAL(Optional<int const&> { 13 });
+    EXPECT_CONSTEVAL(Optional<DontCopyMe const&> { DontCopyMe {} });
+
+    static_assert(!Optional<int> {}.has_value());
+    static_assert(!Optional<DontCopyMe> {}.has_value());
+    static_assert(!Optional<int const> {}.has_value());
+    static_assert(!Optional<DontCopyMe const> {}.has_value());
+    static_assert(!Optional<int&> {}.has_value());
+    static_assert(!Optional<DontCopyMe&> {}.has_value());
+    static_assert(!Optional<int const&> {}.has_value());
+    static_assert(!Optional<DontCopyMe const&> {}.has_value());
+
+    static_assert(Optional<int> { 13 }.has_value());
+    static_assert(Optional<DontCopyMe> { DontCopyMe {} }.has_value());
+    static_assert(Optional<int const> { 13 }.has_value());
+    static_assert(Optional<DontCopyMe const> { DontCopyMe {} }.has_value());
+    static_assert(Optional<int&> { i }.has_value());
+    static_assert(Optional<DontCopyMe&> { dcm }.has_value());
+    static_assert(Optional<int const&> { 13 }.has_value());
+    static_assert(Optional<DontCopyMe const&> { DontCopyMe {} }.has_value());
+
+    static_assert(Optional<int> { 13 }.value() == 13);
+    static_assert(Optional<DontCopyMe> { DontCopyMe {} }.value().x == 13);
+    static_assert(Optional<int const> { 13 }.value() == 13);
+    static_assert(Optional<int const&> { 13 }.value() == 13);
+    static_assert(Optional<DontCopyMe const&> { DontCopyMe {} }.value().x == 13);
+}
+
 TEST_CASE(test_copy_ctor_and_dtor_called)
 {
 #ifdef AK_HAVE_CONDITIONALLY_TRIVIAL

From 2c9860cd602007b7fb4cea2769d8aade9c66b0e3 Mon Sep 17 00:00:00 2001
From: Jonne Ransijn <jonne@yyny.dev>
Date: Wed, 20 Nov 2024 21:07:58 +0100
Subject: [PATCH 2/2] Tests: Fix missing argument removed by `-cc1`

This argument is removed (or rather, never added) by `-cc1`, which
breaks any builds using GNU libc++, including CI.
---
 Tests/ClangPlugins/CMakeLists.txt | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Tests/ClangPlugins/CMakeLists.txt b/Tests/ClangPlugins/CMakeLists.txt
index e6a3c3c309d..45f0ea40a2d 100644
--- a/Tests/ClangPlugins/CMakeLists.txt
+++ b/Tests/ClangPlugins/CMakeLists.txt
@@ -11,6 +11,7 @@ list(APPEND CLANG_PLUGINS_COMPILE_OPTIONS_FOR_TESTS
     -Wno-literal-range
     -Wno-unknown-warning-option
     -Wno-unqualified-std-cast-call
+    -fgnuc-version=4.2.1 # NOTE: Clang default as of 10.0.0
 )
 
 # Ensure we always check for invalid function field types regardless of the value of ENABLE_CLANG_PLUGINS_INVALID_FUNCTION_MEMBERS