def fails without error (by in variable)

[digama0]

In this (definitely contrived) example, the foo declaration fails, as evidenced by the fact that it is not in the environment, but no error is reported:

variable (A : Nat) (B : by skip)
def foo := A = B -- no error is reported

#print foo -- unknown constant 'foo'

Using def foo : Prop instead causes the declaration to succeed and report that it uses sorry.