Add match rule for Bound/Implicit

Author: purefunctor

compiler-core/checking/src/algorithm/constraint.rs

@@ -584,6 +584,15 @@ fn match_given_type(state: &mut CheckState, wanted: TypeId, given: TypeId) -> Ma
             }
         }
 
+        // Implicit and Bound variables at the same level represent the same
+        // logical variable. Implicit produces them, Bound consumes them.
+        (Type::Variable(Variable::Implicit(w_level)), Type::Variable(Variable::Bound(g_level)))
+        | (Type::Variable(Variable::Bound(w_level)), Type::Variable(Variable::Implicit(g_level)))
+            if w_level == g_level =>
+        {
+            MatchType::Match
+        }
+
         _ => MatchType::Apart,
     }
 }