Change BitfieldExtract to use a pointer to the bitfield member

Source/Core/Common/BitField.h

@@ -155,8 +155,8 @@ public:
 
   constexpr T Value() const { return Value(std::is_signed<T>()); }
   constexpr operator T() const { return Value(); }
-  constexpr std::size_t StartBit() const { return position; }
-  constexpr std::size_t NumBits() const { return bits; }
+  static constexpr std::size_t StartBit() { return position; }
+  static constexpr std::size_t NumBits() { return bits; }
 
 private:
   // Unsigned version of StorageType

Source/Core/Common/CMakeLists.txt

@@ -125,6 +125,7 @@ add_library(common
   TraversalClient.cpp
   TraversalClient.h
   TraversalProto.h
+  TypeUtils.h
   UPnP.cpp
   UPnP.h
   VariantUtil.h

Source/Core/Common/TypeUtils.h

@@ -0,0 +1,70 @@
+// Copyright 2021 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <type_traits>
+
+namespace Common
+{
+template <typename>
+struct MemberPointerInfo;
+// Helper to get information about a pointer to a data member.
+// See https://en.cppreference.com/w/cpp/language/pointer#Pointers_to_members
+// This template takes the type for a member pointer.
+template <typename M, typename O>
+struct MemberPointerInfo<M O::*>
+{
+  using MemberType = M;
+  using ObjectType = O;
+};
+
+// This template takes a specific member pointer.
+template <auto member_pointer>
+using MemberType = typename MemberPointerInfo<decltype(member_pointer)>::MemberType;
+
+// This template takes a specific member pointer.
+template <auto member_pointer>
+using ObjectType = typename MemberPointerInfo<decltype(member_pointer)>::ObjectType;
+
+namespace detail
+{
+template <int x>
+struct Data
+{
+  static constexpr int GetX() { return x; }
+};
+struct Foo
+{
+  Data<1> a;
+  Data<2> b;
+  int c;
+};
+struct Bar : Foo
+{
+  int d;
+};
+
+static_assert(std::is_same_v<MemberType<&Foo::a>, Data<1>>);
+static_assert(MemberType<&Foo::a>::GetX() == 1);
+static_assert(std::is_same_v<MemberType<&Foo::b>, Data<2>>);
+static_assert(MemberType<&Foo::b>::GetX() == 2);
+static_assert(std::is_same_v<MemberType<&Foo::c>, int>);
+
+static_assert(std::is_same_v<ObjectType<&Foo::a>, Foo>);
+static_assert(std::is_same_v<ObjectType<&Foo::b>, Foo>);
+static_assert(std::is_same_v<ObjectType<&Foo::c>, Foo>);
+
+static_assert(std::is_same_v<MemberType<&Foo::c>, MemberPointerInfo<int Foo::*>::MemberType>);
+static_assert(std::is_same_v<ObjectType<&Foo::c>, MemberPointerInfo<int Foo::*>::ObjectType>);
+
+static_assert(std::is_same_v<MemberType<&Bar::c>, int>);
+static_assert(std::is_same_v<MemberType<&Bar::d>, int>);
+
+static_assert(std::is_same_v<ObjectType<&Bar::d>, Bar>);
+// Somewhat unexpected behavior:
+static_assert(std::is_same_v<ObjectType<&Bar::c>, Foo>);
+static_assert(!std::is_same_v<ObjectType<&Bar::c>, Bar>);
+}  // namespace detail
+}  // namespace Common