diff --git a/crates/compiler/test_derive/src/eq.rs b/crates/compiler/test_derive/src/eq.rs
new file mode 100644
index 0000000000..75d12e409e
--- /dev/null
+++ b/crates/compiler/test_derive/src/eq.rs
@@ -0,0 +1,38 @@
+#![cfg(test)]
+// Even with #[allow(non_snake_case)] on individual idents, rust-analyzer issues diagnostics.
+// See https://github.com/rust-lang/rust-analyzer/issues/6541.
+// For the `v!` macro we use uppercase variables when constructing tag unions.
+#![allow(non_snake_case)]
+
+use crate::{util::check_single_lset_immediate, v};
+use roc_module::symbol::Symbol;
+use roc_types::subs::Variable;
+
+use roc_derive_key::DeriveBuiltin::IsEq;
+
+#[test]
+fn immediates() {
+    // Everything is an immediate for `Eq`.
+    check_single_lset_immediate(IsEq, v!(U8), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!(U16), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!(U32), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!(U64), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!(U128), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!(I8), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!(I16), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!(I32), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!(I64), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!(I128), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!(STR), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!(Symbol::LIST_LIST v!(U8)), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!(Symbol::LIST_LIST v!(STR)), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!({ a: v!(U8), }), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!(EMPTY_RECORD), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!([ A v!(U8) v!(STR), B v!(STR) ]), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!([ A v!(U8) v!(STR), B v!(STR) ]), Symbol::BOOL_EQ);
+    check_single_lset_immediate(IsEq, v!([ Nil, Cons v!(^lst)] as lst), Symbol::BOOL_EQ);
+
+    // NOTE: despite this reaching an immediate, `F64`s will never actually be allowed to be
+    // compared, because obligation checking will rule them out from `isEq`!
+    check_single_lset_immediate(IsEq, v!(F64), Symbol::BOOL_EQ);
+}