Functions with optional arguments do not type-check their mandatory parameters

[bennn]

This code compiles and runs, even though the type annotation on the mandatory parameter x is wrong:

def g(x:int, y=0)->int:
  return x[y]

g([3])

If you remove the optional argument y, retic raises a static type error (good!):

def g(x:int)->int:
  y = 0
  return x[y]

# Cannot index into a value of type Int

Tested on the master branch, 7adef84