AK: Add global is<T>() and downcast<T>()

Let's unify the is<T>/to<T> implementations that currently exist in
separate versions in LibCore and LibWeb.

AK/StdLibExtras.h

@@ -107,10 +107,12 @@ template<class T>
 struct RemoveConst {
     typedef T Type;
 };
+
 template<class T>
 struct RemoveConst<const T> {
     typedef T Type;
 };
+
 template<class T>
 struct RemoveVolatile {
     typedef T Type;
@@ -441,6 +443,22 @@ inline constexpr T exchange(T& slot, U&& value)
     return old_value;
 }
 
+template<typename T>
+struct IsUnion : public IntegralConstant<bool, __is_union(T)> {
+};
+
+template<typename T>
+struct IsClass : public IntegralConstant<bool, __is_class(T)> {
+};
+
+template<typename Base, typename Derived>
+struct IsBaseOf : public IntegralConstant<bool, __is_base_of(Base, Derived)> {
+};
+
+template<typename ReferenceType, typename T>
+using CopyConst =
+    typename Conditional<IsConst<ReferenceType>::value, typename AddConst<T>::Type, typename RemoveConst<T>::Type>::Type;
+
 }
 
 using AK::AddConst;
@@ -449,8 +467,11 @@ using AK::clamp;
 using AK::Conditional;
 using AK::exchange;
 using AK::forward;
+using AK::IsBaseOf;
+using AK::IsClass;
 using AK::IsConst;
 using AK::IsSame;
+using AK::IsUnion;
 using AK::IsVoid;
 using AK::MakeSigned;
 using AK::MakeUnsigned;

AK/TypeCasts.h

@@ -0,0 +1,93 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in input and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of input code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/StdLibExtras.h>
+
+namespace AK {
+
+template<typename OutputType, typename InputType, bool is_base_type = IsBaseOf<OutputType, InputType>::value>
+struct TypeTraits {
+    static bool has_type(InputType&)
+    {
+        static_assert(IsVoid<OutputType>::value, "No TypeTraits for this type");
+        return false;
+    }
+};
+
+template<typename OutputType, typename InputType>
+struct TypeTraits<OutputType, InputType, true> {
+    static bool has_type(InputType&) { return true; }
+};
+
+template<typename OutputType, typename InputType>
+inline bool is(InputType* input)
+{
+    return input && TypeTraits<const OutputType, const InputType>::has_type(*input);
+}
+
+template<typename OutputType, typename InputType>
+inline bool is(InputType& input)
+{
+    return TypeTraits<const OutputType, const InputType>::has_type(input);
+}
+
+template<typename OutputType, typename InputType>
+inline CopyConst<InputType, OutputType>* downcast(InputType* input)
+{
+    static_assert(IsBaseOf<InputType, OutputType>::value);
+    ASSERT(!input || is<OutputType>(*input));
+    return static_cast<CopyConst<InputType, OutputType>*>(input);
+}
+
+template<typename OutputType, typename InputType>
+inline CopyConst<InputType, OutputType>& downcast(InputType& input)
+{
+    static_assert(IsBaseOf<InputType, OutputType>::value);
+    ASSERT(is<OutputType>(input));
+    return static_cast<CopyConst<InputType, OutputType>&>(input);
+}
+
+#define AK_BEGIN_TYPE_TRAITS(ClassName)                                   \
+    namespace AK {                                                        \
+    template<typename InputType>                                          \
+    class TypeTraits<const ClassName, InputType, false> {                 \
+    public:                                                               \
+        static bool has_type(InputType& input) { return is_type(input); } \
+                                                                          \
+    private:
+
+#define AK_END_TYPE_TRAITS() \
+    }                        \
+    ;                        \
+    }
+
+}
+
+using AK::downcast;
+using AK::is;
+using AK::TypeTraits;