test(tests/lean/run): add test showing new coercion module addresses issue #668

tests/lean/run/668.lean

@@ -0,0 +1,20 @@
+set_option pp.coercions true
+
+namespace Nat
+  constant Nat : Type₁
+  constant num2Nat : num → Nat
+  attribute num2Nat [coercion]
+  check (0 : Nat)
+end Nat
+
+constant Int : Type₁
+
+namespace Int
+  open Nat
+  constant Nat2Int : Nat → Int
+  attribute Nat2Int [coercion]
+  check (0 : Int)
+end Int
+
+open Int
+check (0 : Int)