diff --git a/crates/fayalite/tests/ui/simvalue_is_not_internable.rs b/crates/fayalite/tests/ui/simvalue_is_not_internable.rs
new file mode 100644
index 0000000..d40990f
--- /dev/null
+++ b/crates/fayalite/tests/ui/simvalue_is_not_internable.rs
@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: LGPL-3.0-or-later
+// See Notices.txt for copyright information
+
+//! check that SimValue can't be interned, since equality may ignore types
+
+use fayalite::{
+    intern::{Intern, Interned},
+    sim::value::SimValue,
+};
+
+fn f(v: SimValue<()>) -> Interned<SimValue<()>> {
+    Intern::intern_sized(v)
+}
+
+fn main() {}
diff --git a/crates/fayalite/tests/ui/simvalue_is_not_internable.stderr b/crates/fayalite/tests/ui/simvalue_is_not_internable.stderr
new file mode 100644
index 0000000..eb8877b
--- /dev/null
+++ b/crates/fayalite/tests/ui/simvalue_is_not_internable.stderr
@@ -0,0 +1,178 @@
+error[E0277]: `Cell<util::alternating_cell::State>` cannot be shared between threads safely
+  --> tests/ui/simvalue_is_not_internable.rs:11:26
+   |
+11 | fn f(v: SimValue<()>) -> Interned<SimValue<()>> {
+   |                          ^^^^^^^^^^^^^^^^^^^^^^ `Cell<util::alternating_cell::State>` cannot be shared between threads safely
+   |
+   = help: within `SimValue<()>`, the trait `Sync` is not implemented for `Cell<util::alternating_cell::State>`, which is required by `SimValue<()>: Sync`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock`
+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]: `UnsafeCell<value::SimValueInner<()>>` cannot be shared between threads safely
+  --> tests/ui/simvalue_is_not_internable.rs:11:26
+   |
+11 | fn f(v: SimValue<()>) -> Interned<SimValue<()>> {
+   |                          ^^^^^^^^^^^^^^^^^^^^^^ `UnsafeCell<value::SimValueInner<()>>` cannot be shared between threads safely
+   |
+   = help: within `SimValue<()>`, the trait `Sync` is not implemented for `UnsafeCell<value::SimValueInner<()>>`, which is required by `SimValue<()>: Sync`
+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
+   |
+12 |     Intern::intern_sized(v)
+   |     -------------------- ^ the trait `Hash` is not implemented for `SimValue<()>`, which is required by `SimValue<()>: Intern`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the following other types implement trait `Intern`:
+             BitSlice
+             [T]
+             str
+   = note: required for `SimValue<()>` to implement `Intern`
+
+error[E0277]: the trait bound `SimValue<()>: Intern` is not satisfied
+  --> tests/ui/simvalue_is_not_internable.rs:12:26
+   |
+12 |     Intern::intern_sized(v)
+   |     -------------------- ^ the trait `std::cmp::Eq` is not implemented for `SimValue<()>`, which is required by `SimValue<()>: Intern`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the following other types implement trait `Intern`:
+             BitSlice
+             [T]
+             str
+   = note: required for `SimValue<()>` to implement `Intern`
+
+error[E0277]: `Cell<util::alternating_cell::State>` cannot be shared between threads safely
+  --> tests/ui/simvalue_is_not_internable.rs:12:26
+   |
+12 |     Intern::intern_sized(v)
+   |     -------------------- ^ `Cell<util::alternating_cell::State>` cannot be shared between threads safely
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: within `SimValue<()>`, the trait `Sync` is not implemented for `Cell<util::alternating_cell::State>`, which is required by `SimValue<()>: Sync`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock`
+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
+
+error[E0277]: `UnsafeCell<value::SimValueInner<()>>` cannot be shared between threads safely
+  --> tests/ui/simvalue_is_not_internable.rs:12:26
+   |
+12 |     Intern::intern_sized(v)
+   |     -------------------- ^ `UnsafeCell<value::SimValueInner<()>>` cannot be shared between threads safely
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: within `SimValue<()>`, the trait `Sync` is not implemented for `UnsafeCell<value::SimValueInner<()>>`, which is required by `SimValue<()>: Sync`
+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
+
+error[E0277]: `Cell<util::alternating_cell::State>` cannot be shared between threads safely
+  --> tests/ui/simvalue_is_not_internable.rs:12:5
+   |
+12 |     Intern::intern_sized(v)
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ `Cell<util::alternating_cell::State>` cannot be shared between threads safely
+   |
+   = help: within `SimValue<()>`, the trait `Sync` is not implemented for `Cell<util::alternating_cell::State>`, which is required by `SimValue<()>: Sync`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock`
+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]: `UnsafeCell<value::SimValueInner<()>>` cannot be shared between threads safely
+  --> tests/ui/simvalue_is_not_internable.rs:12:5
+   |
+12 |     Intern::intern_sized(v)
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ `UnsafeCell<value::SimValueInner<()>>` cannot be shared between threads safely
+   |
+   = help: within `SimValue<()>`, the trait `Sync` is not implemented for `UnsafeCell<value::SimValueInner<()>>`, which is required by `SimValue<()>: Sync`
+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`