diff --git a/artiq/compiler/transforms/inferencer.py b/artiq/compiler/transforms/inferencer.py
index 1e1686fc3..5733ac900 100644
--- a/artiq/compiler/transforms/inferencer.py
+++ b/artiq/compiler/transforms/inferencer.py
@@ -767,14 +767,6 @@ class Inferencer(algorithm.Visitor):
                         arg.loc, default.loc)
 
     def visit_FunctionDefT(self, node):
-        old_function, self.function = self.function, node
-        old_in_loop, self.in_loop = self.in_loop, False
-        old_has_return, self.has_return = self.has_return, False
-        self.generic_visit(node)
-        self.function = old_function
-        self.in_loop = old_in_loop
-        self.has_return = old_has_return
-
         if any(node.decorator_list):
             diag = diagnostic.Diagnostic("error",
                 "decorators are not supported", {},
@@ -782,13 +774,30 @@ class Inferencer(algorithm.Visitor):
             self.engine.process(diag)
             return
 
+        old_function, self.function = self.function, node
+        old_in_loop, self.in_loop = self.in_loop, False
+        old_has_return, self.has_return = self.has_return, False
+
+        self.generic_visit(node)
+
         # Lack of return statements is not the only case where the return
         # type cannot be inferred. The other one is infinite (possibly mutual)
         # recursion. Since Python functions don't have to return a value,
         # we ignore that one.
         if not self.has_return:
+            def makenotes(printer, typea, typeb, loca, locb):
+                return [
+                    diagnostic.Diagnostic("note",
+                        "function with return type {typea}",
+                        {"typea": printer.name(typea)},
+                        node.name_loc),
+                ]
             self._unify(node.return_type, builtins.TNone(),
-                        node.name_loc, None)
+                        node.name_loc, None, makenotes)
+
+        self.function = old_function
+        self.in_loop = old_in_loop
+        self.has_return = old_has_return
 
         signature_type = self._type_from_arguments(node.args, node.return_type)
         if signature_type: