fix(Inference): Account for self-referencial constraints (#551)

Resolves #549

src/extension/infer.rs

@@ -658,7 +658,17 @@ impl UnificationContext {
  pub fn instantiate_variables(&mut self) {
  for m in self.variables.clone().into_iter() {
  if !self.solved.contains_key(&m) {
- self.add_solution(m, ExtensionSet::new());
+ // Handle the case where the constraints for `m` contain a self
+ // reference, i.e. "m = Plus(E, m)", in which case the variable
+ // should be instantiated to E rather than the empty set.
+ let solution =
+ ExtensionSet::from_iter(self.get_constraints(&m).unwrap().iter().filter_map(
+ |c| match c {
+ Constraint::Plus(x, other_m) if &m == other_m => Some(x.clone()),
+ _ => None,
+ },
+ ));
+ self.add_solution(m, solution);
  }
  }
  self.variables = HashSet::new();