diff --git a/compiler/forget/src/__tests__/fixtures/hir/inadvertent-mutability-readonly-class.expect.md b/compiler/forget/src/__tests__/fixtures/hir/inadvertent-mutability-readonly-class.expect.md
new file mode 100644
index 0000000000..df981f7767
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/inadvertent-mutability-readonly-class.expect.md
@@ -0,0 +1,41 @@
+
+## Input
+
+```javascript
+function Component(props) {
+  const env = useRelayEnvironment();
+  // Note: this is a class has no mutable methods, ie it always treats `this` as readonly
+  const mutator = new Mutator(env);
+
+  useOtherHook();
+
+  // `x` should be independently memoizeable, since foo(x, mutator) cannot mutate
+  // the mutator.
+  const x = {};
+  foo(x, mutator);
+  return x;
+}
+
+class Mutator {}
+
+```
+
+## Code
+
+```javascript
+function Component(props) {
+  const env = useRelayEnvironment();
+
+  const mutator = new Mutator(env);
+
+  useOtherHook();
+
+  const x = {};
+  foo(x, mutator);
+  return x;
+}
+
+class Mutator {}
+
+```
+      
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/inadvertent-mutability-readonly-class.js b/compiler/forget/src/__tests__/fixtures/hir/inadvertent-mutability-readonly-class.js
new file mode 100644
index 0000000000..11740ef0a4
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/inadvertent-mutability-readonly-class.js
@@ -0,0 +1,15 @@
+function Component(props) {
+  const env = useRelayEnvironment();
+  // Note: this is a class has no mutable methods, ie it always treats `this` as readonly
+  const mutator = new Mutator(env);
+
+  useOtherHook();
+
+  // `x` should be independently memoizeable, since foo(x, mutator) cannot mutate
+  // the mutator.
+  const x = {};
+  foo(x, mutator);
+  return x;
+}
+
+class Mutator {}
diff --git a/compiler/forget/src/__tests__/fixtures/hir/inadvertent-mutability-readonly-lambda.expect.md b/compiler/forget/src/__tests__/fixtures/hir/inadvertent-mutability-readonly-lambda.expect.md
new file mode 100644
index 0000000000..61e283f619
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/inadvertent-mutability-readonly-lambda.expect.md
@@ -0,0 +1,37 @@
+
+## Input
+
+```javascript
+function Component(props) {
+  const [value, setValue] = useState(null);
+  // NOTE: this lambda does not capture any mutable values (only the state setter)
+  // and thus should be treated as readonly
+  const onChange = (e) => setX((value) => value + e.target.value);
+
+  useOtherHook();
+
+  // x should be independently memoizeable, since foo(x, onChange) cannot modify onChange
+  const x = {};
+  foo(x, onChange);
+  return x;
+}
+
+```
+
+## Code
+
+```javascript
+function Component(props) {
+  useState(null);
+
+  const onChange = (e) => setX((value) => value + e.target.value);
+
+  useOtherHook();
+
+  const x = {};
+  foo(x, onChange);
+  return x;
+}
+
+```
+      
\ No newline at end of file
diff --git a/compiler/forget/src/__tests__/fixtures/hir/inadvertent-mutability-readonly-lambda.js b/compiler/forget/src/__tests__/fixtures/hir/inadvertent-mutability-readonly-lambda.js
new file mode 100644
index 0000000000..0bdca05afd
--- /dev/null
+++ b/compiler/forget/src/__tests__/fixtures/hir/inadvertent-mutability-readonly-lambda.js
@@ -0,0 +1,13 @@
+function Component(props) {
+  const [value, setValue] = useState(null);
+  // NOTE: this lambda does not capture any mutable values (only the state setter)
+  // and thus should be treated as readonly
+  const onChange = (e) => setX((value) => value + e.target.value);
+
+  useOtherHook();
+
+  // x should be independently memoizeable, since foo(x, onChange) cannot modify onChange
+  const x = {};
+  foo(x, onChange);
+  return x;
+}