diff --git a/src/from_fn.rs b/src/from_fn.rs
index b10f87d..0edde10 100644
--- a/src/from_fn.rs
+++ b/src/from_fn.rs
@@ -24,6 +24,7 @@ where
     ///
     /// Propagates the `E` type returned from the provided `F` in the event of error.
     pub fn try_from_fn<E>(f: impl FnMut(usize) -> Result<T, E>) -> Result<Self, E> {
+        core::convert::identity(U::__CHECK_INVARIANT);
         let mut array = Array::<MaybeUninit<T>, U>::uninit();
         try_from_fn_erased(array.0.as_mut(), f)?;
 
diff --git a/src/lib.rs b/src/lib.rs
index 4c8e037..28c2212 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -107,7 +107,7 @@ use core::{
     ptr,
     slice::{self, Iter, IterMut},
 };
-use typenum::{Diff, Sum};
+use typenum::{Diff, Sum, Unsigned};
 
 #[cfg(feature = "zeroize")]
 use zeroize::{Zeroize, ZeroizeOnDrop};
@@ -207,7 +207,7 @@ where
         U: Add<N>,
         Sum<U, N>: ArraySize,
     {
-        self.into_iter().chain(other.into_iter()).collect()
+        self.into_iter().chain(other).collect()
     }
 
     /// Splits `self` at index `N` in two arrays.
@@ -223,7 +223,7 @@ where
         unsafe {
             let array = ManuallyDrop::new(self);
             let head = ptr::read(array.as_ptr().cast());
-            let tail = ptr::read(array.as_ptr().add(N::USIZE).cast());
+            let tail = ptr::read(array.as_ptr().add(<N::Size as Unsigned>::USIZE).cast());
             (head, tail)
         }
     }
@@ -239,7 +239,7 @@ where
         unsafe {
             let array_ptr = self.as_ptr();
             let head = &*array_ptr.cast();
-            let tail = &*array_ptr.add(N::USIZE).cast();
+            let tail = &*array_ptr.add(<N::Size as Unsigned>::USIZE).cast();
             (head, tail)
         }
     }
@@ -255,7 +255,7 @@ where
         unsafe {
             let array_ptr = self.as_mut_ptr();
             let head = &mut *array_ptr.cast();
-            let tail = &mut *array_ptr.add(N::USIZE).cast();
+            let tail = &mut *array_ptr.add(<N::Size as Unsigned>::USIZE).cast();
             (head, tail)
         }
     }
@@ -268,12 +268,16 @@ where
     #[allow(clippy::arithmetic_side_effects)]
     #[inline]
     pub fn slice_as_chunks(buf: &[T]) -> (&[Self], &[T]) {
-        assert_ne!(U::USIZE, 0, "chunk size must be non-zero");
+        assert_ne!(
+            <U::Size as Unsigned>::USIZE,
+            0,
+            "chunk size must be non-zero"
+        );
         // Arithmetic safety: we have checked that `N::USIZE` is not zero, thus
         // division always returns correct result. `tail_pos` can not be bigger than `buf.len()`,
         // thus overflow on multiplication and underflow on substraction are impossible.
-        let chunks_len = buf.len() / U::USIZE;
-        let tail_pos = U::USIZE * chunks_len;
+        let chunks_len = buf.len() / <U::Size as Unsigned>::USIZE;
+        let tail_pos = <U::Size as Unsigned>::USIZE * chunks_len;
         let tail_len = buf.len() - tail_pos;
         unsafe {
             let ptr = buf.as_ptr();
@@ -291,12 +295,16 @@ where
     #[allow(clippy::arithmetic_side_effects)]
     #[inline]
     pub fn slice_as_chunks_mut(buf: &mut [T]) -> (&mut [Self], &mut [T]) {
-        assert_ne!(U::USIZE, 0, "chunk size must be non-zero");
+        assert_ne!(
+            <U::Size as Unsigned>::USIZE,
+            0,
+            "chunk size must be non-zero"
+        );
         // Arithmetic safety: we have checked that `N::USIZE` is not zero, thus
         // division always returns correct result. `tail_pos` can not be bigger than `buf.len()`,
         // thus overflow on multiplication and underflow on substraction are impossible.
-        let chunks_len = buf.len() / U::USIZE;
-        let tail_pos = U::USIZE * chunks_len;
+        let chunks_len = buf.len() / <U::Size as Unsigned>::USIZE;
+        let tail_pos = <U::Size as Unsigned>::USIZE * chunks_len;
         let tail_len = buf.len() - tail_pos;
         unsafe {
             let ptr = buf.as_mut_ptr();
@@ -605,6 +613,7 @@ where
 {
     #[inline]
     fn from(arr: [T; N]) -> Array<T, U> {
+        core::convert::identity(U::__CHECK_INVARIANT);
         Array(arr)
     }
 }
@@ -766,7 +775,8 @@ where
 
     #[inline]
     fn try_from(slice: &'a [T]) -> Result<Array<T, U>, TryFromSliceError> {
-        <&'a Self>::try_from(slice).map(Clone::clone)
+        core::convert::identity(U::__CHECK_INVARIANT);
+        <&'a Self>::try_from(slice).cloned()
     }
 }
 
@@ -778,6 +788,7 @@ where
 
     #[inline]
     fn try_from(slice: &'a [T]) -> Result<Self, TryFromSliceError> {
+        core::convert::identity(U::__CHECK_INVARIANT);
         check_slice_length::<T, U>(slice)?;
 
         // SAFETY: `Array<T, U>` is a `repr(transparent)` newtype for a core
@@ -794,6 +805,7 @@ where
 
     #[inline]
     fn try_from(slice: &'a mut [T]) -> Result<Self, TryFromSliceError> {
+        core::convert::identity(U::__CHECK_INVARIANT);
         check_slice_length::<T, U>(slice)?;
 
         // SAFETY: `Array<T, U>` is a `repr(transparent)` newtype for a core
@@ -825,9 +837,12 @@ where
 /// Generate a [`TryFromSliceError`] if the slice doesn't match the given length.
 #[cfg_attr(debug_assertions, allow(clippy::panic_in_result_fn))]
 fn check_slice_length<T, U: ArraySize>(slice: &[T]) -> Result<(), TryFromSliceError> {
-    debug_assert_eq!(Array::<(), U>::default().len(), U::USIZE);
+    debug_assert_eq!(
+        Array::<(), U>::default().len(),
+        <U::Size as Unsigned>::USIZE
+    );
 
-    if slice.len() != U::USIZE {
+    if slice.len() != <U::Size as Unsigned>::USIZE {
         // Hack: `TryFromSliceError` lacks a public constructor
         <&[T; 1]>::try_from([].as_slice())?;
 
diff --git a/src/sizes.rs b/src/sizes.rs
index 28888eb..d5f1ec9 100644
--- a/src/sizes.rs
+++ b/src/sizes.rs
@@ -22,6 +22,7 @@ macro_rules! impl_array_sizes {
     ($($len:expr => $ty:ident),+ $(,)?) => {
         $(
             unsafe impl ArraySize for $ty {
+                type Size = $ty;
                 type ArrayType<T> = [T; $len];
             }
 
diff --git a/src/traits.rs b/src/traits.rs
index d5824a9..91a50a9 100644
--- a/src/traits.rs
+++ b/src/traits.rs
@@ -3,6 +3,7 @@
 use crate::Array;
 use core::{
     borrow::{Borrow, BorrowMut},
+    mem::size_of,
     ops::{Index, IndexMut, Range},
 };
 use typenum::Unsigned;
@@ -13,17 +14,23 @@ use typenum::Unsigned;
 /// # Safety
 ///
 /// `ArrayType` MUST be an array with a number of elements exactly equal to
-/// [`Unsigned::USIZE`]. Breaking this requirement will cause undefined behavior.
-///
-/// NOTE: This trait is effectively sealed and can not be implemented by third-party crates.
-/// It is implemented only for a number of types defined in [`typenum::consts`].
-pub unsafe trait ArraySize: Unsigned {
+/// [`Size::USIZE`](Unsigned::USIZE). Breaking this requirement will cause undefined behavior.
+pub unsafe trait ArraySize: Sized + 'static {
+    #[doc(hidden)]
+    const __CHECK_INVARIANT: () = {
+        let a = <Self::Size as Unsigned>::USIZE;
+        let b = size_of::<Self::ArrayType<u8>>();
+        assert!(a == b, "ArraySize invariant violated");
+    };
+
+    /// The size underlying the array.
+    type Size: Unsigned;
+
     /// Array type which corresponds to this size.
     ///
     /// This is always defined to be `[T; N]` where `N` is the same as
-    /// [`ArraySize::USIZE`][`typenum::Unsigned::USIZE`].
-    type ArrayType<T>: AssocArraySize<Size = Self>
-        + AsRef<[T]>
+    /// [`ArraySize::Size::USIZE`][`typenum::Unsigned::USIZE`].
+    type ArrayType<T>: AsRef<[T]>
         + AsMut<[T]>
         + Borrow<[T]>
         + BorrowMut<[T]>
diff --git a/tests/custom_size.rs b/tests/custom_size.rs
new file mode 100644
index 0000000..5c52cc7
--- /dev/null
+++ b/tests/custom_size.rs
@@ -0,0 +1,23 @@
+use hybrid_array::Array;
+
+struct Size54321;
+
+// This macro constructs a UInt type from a sequence of bits.  The bits are interpreted as the
+// little-endian representation of the integer in question.  For example, uint!(1 1 0 1 0 0 1) is
+// U75 (not U105).
+macro_rules! uint {
+    () => { typenum::UTerm };
+    (0 $($bs:tt)*) => { typenum::UInt< uint!($($bs)*), typenum::B0 > };
+    (1 $($bs:tt)*) => { typenum::UInt< uint!($($bs)*), typenum::B1 > };
+}
+
+unsafe impl hybrid_array::ArraySize for Size54321 {
+    type Size = uint!(1 0 0 0 1 1 0 0 0 0 1 0 1 0 1 1);
+    type ArrayType<T> = [T; 54321];
+}
+
+#[test]
+fn from_fn() {
+    let array = Array::<u8, Size54321>::from_fn(|n| (n + 1) as u8);
+    assert_eq!(array.as_slice().len(), 54321);
+}