Dependency Error in Type Checking Pattern Matches

[ggeier]

g x =
     x ~ a U b : a
     otherwise: x;

produces with tslcheck:

"Dependency Error" (line 2, column 9 - 10):
detected circular dependencies between:
  a (defined at line 2, column 9 - 10)
  b (defined at line 2, column 13 - 14)

As it is parsed, the foundation for the pattern match is layed, however it does not type check correctly.