Recursive definition failing to compile with inlining enabled

[Izaakwltn]

I'm working through current benchmarks and ran across one that fails to compile with inlining enabled:

(coalton-toplevel

  (declare fib (Integer -> Integer))
  (define (fib n)
    (when (== n 0)
      (return 0))

    (when (== n 1)
      (return 1))

    (+ (fib (- n 1))
        (fib (- n 2)))))

  error: 
    (during macroexpansion of (COALTON-TOPLEVEL
      (DECLARE FIB ...)
      ...))
    Invalid scc binding group. This should have been detected during typechecking.

[Izaakwltn]

On the other hand, this compiles:

(declare fib-generic (Num :a => :a -> :a))
  (define (fib-generic n)
    (when (== n 0)
      (return 0))

    (when (== n 1)
      (return 1))

    (+ (fib-generic (- n 1)) (fib-generic (- n 2))))

[stylewarning]

CC @amorphedstar