Ensure bindings are created in correct order for e.g. "x, y = y, x".

artiq/compiler/transforms/asttyped_rewriter.py

@@ -107,12 +107,25 @@ class LocalExtractor(algorithm.Visitor):
 
     def visit_Name(self, node):
         if self.in_assign:
-            # code like:
+            # Code like:
             # x = 1
             # def f():
             #   x = 1
             # creates a new binding for x in f's scope
             self._assignable(node.id)
+        else:
+            # This is duplicated here as well as below so that
+            # code like:
+            # x, y = y, x
+            # where y and x were not defined earlier would be invalid.
+            if node.id in self.typing_env:
+                return
+            for outer_env in reversed(self.env_stack):
+                if node.id in outer_env:
+                    return
+            diag = diagnostic.Diagnostic("fatal",
+                "name '{name}' is not bound to anything", {"name":node.id}, node.loc)
+            self.engine.process(diag)
 
     def visit_Attribute(self, node):
         self.visit_in_assign(node.value, in_assign=False)