diff --git a/artiq/py2llvm/typing.py b/artiq/py2llvm/typing.py
index da793f6d3..e93af3225 100644
--- a/artiq/py2llvm/typing.py
+++ b/artiq/py2llvm/typing.py
@@ -130,6 +130,7 @@ class Inferencer(algorithm.Transformer):
     def __init__(self, engine):
         self.engine = engine
         self.env_stack = [{}]
+        self.function  = None # currently visited function
 
     def _unify(self, typea, typeb, loca, locb, kind):
         try:
@@ -137,21 +138,32 @@ class Inferencer(algorithm.Transformer):
         except types.UnificationError as e:
             printer = types.TypePrinter()
 
-            if kind == "generic":
-                note1 = diagnostic.Diagnostic("note",
-                    "expression of type {typea}",
-                    {"typea": printer.name(typea)},
-                    loca)
-            elif kind == "expects":
+            if kind == "expects":
                 note1 = diagnostic.Diagnostic("note",
                     "expression expecting an operand of type {typea}",
                     {"typea": printer.name(typea)},
                     loca)
+            elif kind == "return_type" or kind == "return_type_none":
+                note1 = diagnostic.Diagnostic("note",
+                    "function with return type {typea}",
+                    {"typea": printer.name(typea)},
+                    loca)
+            else:
+                note1 = diagnostic.Diagnostic("note",
+                    "expression of type {typea}",
+                    {"typea": printer.name(typea)},
+                    loca)
 
-            note2 = diagnostic.Diagnostic("note",
-                "expression of type {typeb}",
-                {"typeb": printer.name(typeb)},
-                locb)
+            if kind == "return_type_none":
+                note2 = diagnostic.Diagnostic("note",
+                    "implied expression of type {typeb}",
+                    {"typeb": printer.name(typeb)},
+                    locb)
+            else:
+                note2 = diagnostic.Diagnostic("note",
+                    "expression of type {typeb}",
+                    {"typeb": printer.name(typeb)},
+                    locb)
 
             if e.typea.find() == typea.find() and e.typeb.find() == typeb.find():
                 diag = diagnostic.Diagnostic("fatal",
@@ -166,23 +178,6 @@ class Inferencer(algorithm.Transformer):
                     loca, [locb], notes=[note1, note2])
             self.engine.process(diag)
 
-    def visit_FunctionDef(self, node):
-        extractor = LocalExtractor(env_stack=self.env_stack, engine=self.engine)
-        extractor.visit(node)
-
-        self.env_stack.append(extractor.typing_env)
-        node = asttyped.FunctionDefT(
-            typing_env=extractor.typing_env, globals_in_scope=extractor.global_,
-            name=node.name, args=self.visit(node.args), returns=self.visit(node.returns),
-            body=[self.visit(x) for x in node.body], decorator_list=node.decorator_list,
-            keyword_loc=node.keyword_loc, name_loc=node.name_loc,
-            arrow_loc=node.arrow_loc, colon_loc=node.colon_loc, at_locs=node.at_locs,
-            loc=node.loc)
-        self.generic_visit(node)
-        self.env_stack.pop()
-
-        return node
-
     def _find_name(self, name, loc):
         for typing_env in reversed(self.env_stack):
             if name in typing_env:
@@ -198,6 +193,39 @@ class Inferencer(algorithm.Transformer):
                              arg=node.arg, annotation=self.visit(node.annotation),
                              arg_loc=node.arg_loc, colon_loc=node.colon_loc, loc=node.loc)
 
+    def visit_FunctionDef(self, node):
+        extractor = LocalExtractor(env_stack=self.env_stack, engine=self.engine)
+        extractor.visit(node)
+
+        self.env_stack.append(extractor.typing_env)
+
+        node = asttyped.FunctionDefT(
+            typing_env=extractor.typing_env, globals_in_scope=extractor.global_,
+            return_type=types.TVar(),
+
+            name=node.name, args=node.args, returns=node.returns,
+            body=node.body, decorator_list=node.decorator_list,
+            keyword_loc=node.keyword_loc, name_loc=node.name_loc,
+            arrow_loc=node.arrow_loc, colon_loc=node.colon_loc, at_locs=node.at_locs,
+            loc=node.loc)
+
+        old_function, self.function = self.function, node
+        self.generic_visit(node)
+        self.function = old_function
+
+        self.env_stack.pop()
+
+        return node
+
+    def visit_Return(self, node):
+        node = self.generic_visit(node)
+        if node.value is None:
+            self._unify(self.function.return_type, types.TNone(),
+                        self.function.name_loc, node.value.loc, kind="return_type_none")
+        else:
+            self._unify(self.function.return_type, node.value.type,
+                        self.function.name_loc, node.value.loc, kind="return_type")
+
     def visit_Num(self, node):
         if isinstance(node.n, int):
             typ = types.TInt()
@@ -307,6 +335,12 @@ class Printer(algorithm.Visitor):
     def rewrite(self):
         return self.rewriter.rewrite()
 
+    def visit_FunctionDefT(self, node):
+        self.rewriter.insert_before(node.colon_loc,
+                                    "->{}".format(self.type_printer.name(node.return_type)))
+
+        super().generic_visit(node)
+
     def generic_visit(self, node):
         if hasattr(node, "type"):
             self.rewriter.insert_after(node.loc,