diff --git a/crates/fayalite/tests/ui/simvalue_is_not_internable.stderr b/crates/fayalite/tests/ui/simvalue_is_not_internable.stderr
index 6550b5f8..030500bb 100644
--- a/crates/fayalite/tests/ui/simvalue_is_not_internable.stderr
+++ b/crates/fayalite/tests/ui/simvalue_is_not_internable.stderr
@@ -45,6 +45,64 @@ note: required by a bound in `fayalite::intern::Interned`
    | pub struct Interned<T: ?Sized + 'static + Send + Sync> {
    |                                                  ^^^^ required by this bound in `Interned`
 
+error[E0277]: `NonNull<()>` cannot be sent between threads safely
+  --> tests/ui/simvalue_is_not_internable.rs:11:26
+   |
+11 | fn f(v: SimValue<()>) -> Interned<SimValue<()>> {
+   |                          ^^^^^^^^^^^^^^^^^^^^^^ `NonNull<()>` cannot be sent between threads safely
+   |
+   = help: within `SimValue<()>`, the trait `Send` is not implemented for `NonNull<()>`
+note: required because it appears within the type `DynSimOnlyValue`
+  --> src/sim/value/sim_only_value_unsafe.rs
+   |
+   | pub struct DynSimOnlyValue {
+   |            ^^^^^^^^^^^^^^^
+note: required because it appears within the type `PhantomData<DynSimOnlyValue>`
+  --> $RUST/core/src/marker.rs
+   |
+   | pub struct PhantomData<T: PointeeSized>;
+   |            ^^^^^^^^^^^
+note: required because it appears within the type `alloc::raw_vec::RawVec<DynSimOnlyValue>`
+  --> $RUST/alloc/src/raw_vec/mod.rs
+   |
+   | pub(crate) struct RawVec<T, A: Allocator = Global> {
+   |                   ^^^^^^
+note: required because it appears within the type `Vec<DynSimOnlyValue>`
+  --> $RUST/alloc/src/vec/mod.rs
+   |
+   | pub struct Vec<T, #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global> {
+   |            ^^^
+note: required because it appears within the type `OpaqueSimValue`
+  --> src/ty.rs
+   |
+   | pub struct OpaqueSimValue {
+   |            ^^^^^^^^^^^^^^
+note: required because it appears within the type `value::SimValueInner<()>`
+  --> src/sim/value.rs
+   |
+   | struct SimValueInner<T: Type> {
+   |        ^^^^^^^^^^^^^
+note: required because it appears within the type `UnsafeCell<value::SimValueInner<()>>`
+  --> $RUST/core/src/cell.rs
+   |
+   | pub struct UnsafeCell<T: ?Sized> {
+   |            ^^^^^^^^^^
+note: required because it appears within the type `util::alternating_cell::AlternatingCell<value::SimValueInner<()>>`
+  --> src/util/alternating_cell.rs
+   |
+   | pub(crate) struct AlternatingCell<T: ?Sized> {
+   |                   ^^^^^^^^^^^^^^^
+note: required because it appears within the type `SimValue<()>`
+  --> src/sim/value.rs
+   |
+   | pub struct SimValue<T: Type> {
+   |            ^^^^^^^^
+note: required by a bound in `fayalite::intern::Interned`
+  --> src/intern.rs
+   |
+   | pub struct Interned<T: ?Sized + 'static + Send + Sync> {
+   |                                           ^^^^ required by this bound in `Interned`
+
 error[E0277]: the trait bound `SimValue<()>: Intern` is not satisfied
   --> tests/ui/simvalue_is_not_internable.rs:12:26
    |
@@ -138,6 +196,73 @@ help: consider dereferencing here
 12 |     Intern::intern_sized(*v)
    |                          +
 
+error[E0277]: `NonNull<()>` cannot be sent between threads safely
+  --> tests/ui/simvalue_is_not_internable.rs:12:26
+   |
+12 |     Intern::intern_sized(v)
+   |     -------------------- ^ `NonNull<()>` cannot be sent between threads safely
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: within `SimValue<()>`, the trait `Send` is not implemented for `NonNull<()>`
+note: required because it appears within the type `DynSimOnlyValue`
+  --> src/sim/value/sim_only_value_unsafe.rs
+   |
+   | pub struct DynSimOnlyValue {
+   |            ^^^^^^^^^^^^^^^
+note: required because it appears within the type `PhantomData<DynSimOnlyValue>`
+  --> $RUST/core/src/marker.rs
+   |
+   | pub struct PhantomData<T: PointeeSized>;
+   |            ^^^^^^^^^^^
+note: required because it appears within the type `alloc::raw_vec::RawVec<DynSimOnlyValue>`
+  --> $RUST/alloc/src/raw_vec/mod.rs
+   |
+   | pub(crate) struct RawVec<T, A: Allocator = Global> {
+   |                   ^^^^^^
+note: required because it appears within the type `Vec<DynSimOnlyValue>`
+  --> $RUST/alloc/src/vec/mod.rs
+   |
+   | pub struct Vec<T, #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global> {
+   |            ^^^
+note: required because it appears within the type `OpaqueSimValue`
+  --> src/ty.rs
+   |
+   | pub struct OpaqueSimValue {
+   |            ^^^^^^^^^^^^^^
+note: required because it appears within the type `value::SimValueInner<()>`
+  --> src/sim/value.rs
+   |
+   | struct SimValueInner<T: Type> {
+   |        ^^^^^^^^^^^^^
+note: required because it appears within the type `UnsafeCell<value::SimValueInner<()>>`
+  --> $RUST/core/src/cell.rs
+   |
+   | pub struct UnsafeCell<T: ?Sized> {
+   |            ^^^^^^^^^^
+note: required because it appears within the type `util::alternating_cell::AlternatingCell<value::SimValueInner<()>>`
+  --> src/util/alternating_cell.rs
+   |
+   | pub(crate) struct AlternatingCell<T: ?Sized> {
+   |                   ^^^^^^^^^^^^^^^
+note: required because it appears within the type `SimValue<()>`
+  --> src/sim/value.rs
+   |
+   | pub struct SimValue<T: Type> {
+   |            ^^^^^^^^
+note: required by a bound in `intern_sized`
+  --> src/intern.rs
+   |
+   | pub trait Intern: Any + Send + Sync {
+   |                         ^^^^ required by this bound in `Intern::intern_sized`
+   |     fn intern(&self) -> Interned<Self>;
+   |     fn intern_sized(self) -> Interned<Self>
+   |        ------------ required by a bound in this associated function
+help: consider dereferencing here
+   |
+12 |     Intern::intern_sized(*v)
+   |                          +
+
 error[E0277]: `Cell<util::alternating_cell::State>` cannot be shared between threads safely
   --> tests/ui/simvalue_is_not_internable.rs:12:5
    |
@@ -184,3 +309,61 @@ note: required by a bound in `fayalite::intern::Interned`
    |
    | pub struct Interned<T: ?Sized + 'static + Send + Sync> {
    |                                                  ^^^^ required by this bound in `Interned`
+
+error[E0277]: `NonNull<()>` cannot be sent between threads safely
+  --> tests/ui/simvalue_is_not_internable.rs:12:5
+   |
+12 |     Intern::intern_sized(v)
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ `NonNull<()>` cannot be sent between threads safely
+   |
+   = help: within `SimValue<()>`, the trait `Send` is not implemented for `NonNull<()>`
+note: required because it appears within the type `DynSimOnlyValue`
+  --> src/sim/value/sim_only_value_unsafe.rs
+   |
+   | pub struct DynSimOnlyValue {
+   |            ^^^^^^^^^^^^^^^
+note: required because it appears within the type `PhantomData<DynSimOnlyValue>`
+  --> $RUST/core/src/marker.rs
+   |
+   | pub struct PhantomData<T: PointeeSized>;
+   |            ^^^^^^^^^^^
+note: required because it appears within the type `alloc::raw_vec::RawVec<DynSimOnlyValue>`
+  --> $RUST/alloc/src/raw_vec/mod.rs
+   |
+   | pub(crate) struct RawVec<T, A: Allocator = Global> {
+   |                   ^^^^^^
+note: required because it appears within the type `Vec<DynSimOnlyValue>`
+  --> $RUST/alloc/src/vec/mod.rs
+   |
+   | pub struct Vec<T, #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global> {
+   |            ^^^
+note: required because it appears within the type `OpaqueSimValue`
+  --> src/ty.rs
+   |
+   | pub struct OpaqueSimValue {
+   |            ^^^^^^^^^^^^^^
+note: required because it appears within the type `value::SimValueInner<()>`
+  --> src/sim/value.rs
+   |
+   | struct SimValueInner<T: Type> {
+   |        ^^^^^^^^^^^^^
+note: required because it appears within the type `UnsafeCell<value::SimValueInner<()>>`
+  --> $RUST/core/src/cell.rs
+   |
+   | pub struct UnsafeCell<T: ?Sized> {
+   |            ^^^^^^^^^^
+note: required because it appears within the type `util::alternating_cell::AlternatingCell<value::SimValueInner<()>>`
+  --> src/util/alternating_cell.rs
+   |
+   | pub(crate) struct AlternatingCell<T: ?Sized> {
+   |                   ^^^^^^^^^^^^^^^
+note: required because it appears within the type `SimValue<()>`
+  --> src/sim/value.rs
+   |
+   | pub struct SimValue<T: Type> {
+   |            ^^^^^^^^
+note: required by a bound in `fayalite::intern::Interned`
+  --> src/intern.rs
+   |
+   | pub struct Interned<T: ?Sized + 'static + Send + Sync> {
+   |                                           ^^^^ required by this bound in `Interned`