kernel_ex.lean:7:4: error: kernel failed to type check declaration 't' this is usually due to a buggy tactic or a bug in the builtin elaborator
elaborated type:
  ℕ → ℕ
elaborated value:
  λ (y : ℕ), let x : ℕ := 1 + y in I tt
nested exception message:
type mismatch at application
  I tt
term
  tt
has type
  bool
but is expected to have type
  ℕ