diff --git a/src/extension/infer.rs b/src/extension/infer.rs
index 7a1b03cfb..cd41a3cfe 100644
--- a/src/extension/infer.rs
+++ b/src/extension/infer.rs
@@ -285,6 +285,10 @@ impl UnificationContext {
                     // The function that it contains, however, may have an
                     // extension delta, so its output shouldn't be equal to the
                     // FuncDefn's output.
+                    //
+                    // TODO: Add a constraint that the extensions of the output
+                    // node of a FuncDefn should be those of the input node plus
+                    // the extension delta specified in the function signature.
                     if node_type.tag() != OpTag::FuncDefn {
                         self.add_constraint(m_output_node, Constraint::Equal(m_output));
                     }
diff --git a/src/extension/infer/test.rs b/src/extension/infer/test.rs
index 8f51e6236..f748c4570 100644
--- a/src/extension/infer/test.rs
+++ b/src/extension/infer/test.rs
@@ -966,6 +966,11 @@ fn sccs() {
 }
 
 #[test]
+/// Note: This test is relying on the builder's `define_function` doing the
+/// right thing: it takes input resources via a [`Signature`], which it passes
+/// to `create_with_io`, creating concrete resource sets.
+/// Inference can still fail for a valid FuncDefn hugr created without using
+/// the builder API.
 fn simple_funcdefn() -> Result<(), Box<dyn Error>> {
     let mut builder = ModuleBuilder::new();
     let mut func_builder = builder.define_function(