From 4dfc4ad2793084c4aeba5d2984bc200f43e13327 Mon Sep 17 00:00:00 2001
From: Fawn <rubiefawn@gmail.com>
Date: Mon, 23 Mar 2026 11:10:09 -0600
Subject: [PATCH 1/3] Resolve all remaining C++20 TODOs

With a single exception, which is already handled by #8321
---
 include/ArrayVector.h | 13 +++----------
 include/Flags.h       |  3 +--
 2 files changed, 4 insertions(+), 12 deletions(-)

diff --git a/include/ArrayVector.h b/include/ArrayVector.h
index 06e09226cf7..08671908186 100644
--- a/include/ArrayVector.h
+++ b/include/ArrayVector.h
@@ -306,8 +306,7 @@ class ArrayVector
 	reference emplace_back(Args&&... args)
 	{
 		assert(!full());
-		// TODO C++20: Use std::construct_at
-		const auto result = new(static_cast<void*>(end())) T(std::forward<Args>(args)...);
+		const auto result = std::construct_at(end(), std::forward<Args>(args)...);
 		++m_size;
 		return *result;
 	}
@@ -362,21 +361,15 @@ class ArrayVector
 		std::swap(a.m_size, b.m_size);
 	}
 
-	// TODO C++20: Replace with operator<=>
-	friend bool operator<(const ArrayVector& l, const ArrayVector& r)
+	friend constexpr auto operator<=>(const ArrayVector& l, const ArrayVector& r)
 	{
-		return std::lexicographical_compare(l.begin(), l.end(), r.begin(), r.end());
+		return std::lexicographical_compare_three_way(l.begin(), l.end(), r.begin(), r.end());
 	}
-	friend bool operator<=(const ArrayVector& l, const ArrayVector& r) { return !(r < l); }
-	friend bool operator>(const ArrayVector& l, const ArrayVector& r) { return r < l; }
-	friend bool operator>=(const ArrayVector& l, const ArrayVector& r) { return !(l < r); }
 
 	friend bool operator==(const ArrayVector& l, const ArrayVector& r)
 	{
 		return std::equal(l.begin(), l.end(), r.begin(), r.end());
 	}
-	// TODO C++20: Remove
-	friend bool operator!=(const ArrayVector& l, const ArrayVector& r) { return !(l == r); }
 
 private:
 	alignas(T) std::byte m_data[std::max(N * sizeof(T), std::size_t{1})]; // Intentionally a raw array
diff --git a/include/Flags.h b/include/Flags.h
index 62a5f8af8ba..37581afaa89 100644
--- a/include/Flags.h
+++ b/include/Flags.h
@@ -68,8 +68,7 @@ class Flags
 	constexpr explicit operator UnderlyingType() const { return m_value; } // TODO C++23: explicit(std::is_scoped_enum<T>)
 	constexpr explicit operator bool() const { return m_value != 0; }
 
-	friend constexpr auto operator==(Flags l, Flags r) -> bool { return l.m_value == r.m_value; } // TODO C++20: = default
-	friend constexpr auto operator!=(Flags l, Flags r) -> bool { return l.m_value != r.m_value; } // TODO C++20: Remove
+	friend constexpr auto operator==(Flags l, Flags r) -> bool = default;
 
 private:
 	UnderlyingType m_value = 0;

From ea0203467d33e6f276803cc50ee04fd71162930b Mon Sep 17 00:00:00 2001
From: Fawn <rubiefawn@gmail.com>
Date: Mon, 23 Mar 2026 14:37:14 -0600
Subject: [PATCH 2/3] Update iterator template constraints

Co-authored-by: Dalton Messmer <messmer.dalton@gmail.com>
---
 include/ArrayVector.h | 17 +++--------------
 1 file changed, 3 insertions(+), 14 deletions(-)

diff --git a/include/ArrayVector.h b/include/ArrayVector.h
index 08671908186..3c31bb5f5bc 100644
--- a/include/ArrayVector.h
+++ b/include/ArrayVector.h
@@ -37,17 +37,6 @@
 
 namespace lmms {
 
-namespace detail {
-
-template<typename T, typename = void>
-constexpr bool is_input_iterator_v = false;
-
-template<typename T>
-constexpr bool is_input_iterator_v<T, std::void_t<typename std::iterator_traits<T>::iterator_category>> =
-	std::is_convertible_v<typename std::iterator_traits<T>::iterator_category, std::input_iterator_tag>;
-
-} // namespace detail
-
 /**
  * A container that stores up to a maximum of `N` elements of type `T` directly
  * within itself, rather than separately on the heap. Useful when a dynamically
@@ -100,7 +89,7 @@ class ArrayVector
 		std::uninitialized_value_construct_n(begin(), count);
 	}
 
-	template<typename It, std::enable_if_t<detail::is_input_iterator_v<It>, int> = 0>
+	template<std::input_iterator It>
 	ArrayVector(It first, It last)
 	{
 		// Can't check the size first as the iterator may not be multipass
@@ -173,7 +162,7 @@ class ArrayVector
 		m_size = count;
 	}
 
-	template<typename It, std::enable_if_t<detail::is_input_iterator_v<It>, int> = 0>
+	template<std::input_iterator It>
 	void assign(It first, It last)
 	{
 		// Can't check the size first as the iterator may not be multipass
@@ -262,7 +251,7 @@ class ArrayVector
 		return mutPos;
 	}
 
-	template<typename It, std::enable_if_t<detail::is_input_iterator_v<It>, int> = 0>
+	template<std::input_iterator It>
 	iterator insert(const_iterator pos, It first, It last)
 	{
 		// Can't check the size first as the iterator may not be multipass

From 5894038cdbe5761214c675f5e973158e020ed4b4 Mon Sep 17 00:00:00 2001
From: Fawn <rubiefawn@gmail.com>
Date: Mon, 23 Mar 2026 14:40:19 -0600
Subject: [PATCH 3/3] TODO C++26: Replace ArrayVector with std::inplace_vector

Co-authored-by: Dalton Messmer <messmer.dalton@gmail.com>
---
 include/ArrayVector.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/ArrayVector.h b/include/ArrayVector.h
index 3c31bb5f5bc..7ea78d8e905 100644
--- a/include/ArrayVector.h
+++ b/include/ArrayVector.h
@@ -43,6 +43,8 @@ namespace lmms {
  * resizeable container is needed for use in real-time code. Can be thought of
  * as a hybrid between `std::array` and `std::vector`. The interface follows
  * that of `std::vector` - see standard C++ documentation.
+ *
+ * TODO C++26: Replace uses of this class with std::inplace_vector
  */
 template<typename T, std::size_t N>
 class ArrayVector