-- Mathlib/Data/Num/Lemmas.lean
theorem castNum_eq_bitwise {f : Num → Num → Num} {g : Bool → Bool → Bool}
    (p : PosNum → PosNum → Num)
    (gff : g false false = false) (f00 : f 0 0 = 0)
    (f0n : ∀ n, f 0 (pos n) = cond (g false true) (pos n) 0)
    (fn0 : ∀ n, f (pos n) 0 = cond (g true false) (pos n) 0)
    (fnn : ∀ m n, f (pos m) (pos n) = p m n) (p11 : p 1 1 = cond (g true true) 1 0)
    (p1b : ∀ b n, p 1 (PosNum.bit b n) = bit (g true b) (cond (g false true) (pos n) 0))
    (pb1 : ∀ a m, p (PosNum.bit a m) 1 = bit (g a true) (cond (g true false) (pos m) 0))
    (pbb : ∀ a b m n, p (PosNum.bit a m) (PosNum.bit b n) = bit (g a b) (p m n)) :
    ∀ m n : Num, (f m n : ℕ) = Nat.bitwise g m n

Proof state:
case pos.pos.bit1.bit1
f : Num → Num → Num
g : Bool → Bool → Bool
p : PosNum → PosNum → Num
gff : g false false = false
f00 : f 0 0 = 0
f0n : ∀ (n : PosNum), f 0 (pos n) = bif g false true then pos n else 0
fn0 : ∀ (n : PosNum), f (pos n) 0 = bif g true false then pos n else 0
fnn : ∀ (m n : PosNum), f (pos m) (pos n) = p m n
p11 : p 1 1 = bif g true true then 1 else 0
p1b : ∀ (b : Bool) (n : PosNum), p 1 (PosNum.bit b n) = bit (g true b) (bif g false true then pos n else 0)
pb1 : ∀ (a : Bool) (m : PosNum), p (PosNum.bit a m) 1 = bit (g a true) (bif g true false then pos m else 0)
pbb : ∀ (a b : Bool) (m n : PosNum), p (PosNum.bit a m) (PosNum.bit b n) = bit (g a b) (p m n)
this : ∀ (b : Bool) (n : PosNum), (bif b then ↑n else 0) = ↑(bif b then pos n else 0)
this' : ∀ (b : Bool) (n : PosNum), ↑(pos (PosNum.bit b n)) = Nat.bit b ↑n
m : PosNum
IH : ∀ (n : PosNum), ↑(p m n) = Nat.bitwise g ↑(pos m) ↑(pos n)
n : PosNum
⊢ ↑(p (PosNum.bit true m) (PosNum.bit true n)) = Nat.bit (g true true) (Nat.bitwise g ↑m ↑n)

case pos.pos.bit1.bit0
f : Num → Num → Num
g : Bool → Bool → Bool
p : PosNum → PosNum → Num
gff : g false false = false
f00 : f 0 0 = 0
f0n : ∀ (n : PosNum), f 0 (pos n) = bif g false true then pos n else 0
fn0 : ∀ (n : PosNum), f (pos n) 0 = bif g true false then pos n else 0
fnn : ∀ (m n : PosNum), f (pos m) (pos n) = p m n
p11 : p 1 1 = bif g true true then 1 else 0
p1b : ∀ (b : Bool) (n : PosNum), p 1 (PosNum.bit b n) = bit (g true b) (bif g false true then pos n else 0)
pb1 : ∀ (a : Bool) (m : PosNum), p (PosNum.bit a m) 1 = bit (g a true) (bif g true false then pos m else 0)
pbb : ∀ (a b : Bool) (m n : PosNum), p (PosNum.bit a m) (PosNum.bit b n) = bit (g a b) (p m n)
this : ∀ (b : Bool) (n : PosNum), (bif b then ↑n else 0) = ↑(bif b then pos n else 0)
this' : ∀ (b : Bool) (n : PosNum), ↑(pos (PosNum.bit b n)) = Nat.bit b ↑n
m : PosNum
IH : ∀ (n : PosNum), ↑(p m n) = Nat.bitwise g ↑(pos m) ↑(pos n)
n : PosNum
⊢ ↑(p (PosNum.bit true m) (PosNum.bit false n)) = Nat.bit (g true false) (Nat.bitwise g ↑m ↑n)

case pos.pos.bit0.bit1
f : Num → Num → Num
g : Bool → Bool → Bool
p : PosNum → PosNum → Num
gff : g false false = false
f00 : f 0 0 = 0
f0n : ∀ (n : PosNum), f 0 (pos n) = bif g false true then pos n else 0
fn0 : ∀ (n : PosNum), f (pos n) 0 = bif g true false then pos n else 0
fnn : ∀ (m n : PosNum), f (pos m) (pos n) = p m n
p11 : p 1 1 = bif g true true then 1 else 0
p1b : ∀ (b : Bool) (n : PosNum), p 1 (PosNum.bit b n) = bit (g true b) (bif g false true then pos n else 0)
pb1 : ∀ (a : Bool) (m : PosNum), p (PosNum.bit a m) 1 = bit (g a true) (bif g true false then pos m else 0)
pbb : ∀ (a b : Bool) (m n : PosNum), p (PosNum.bit a m) (PosNum.bit b n) = bit (g a b) (p m n)
this : ∀ (b : Bool) (n : PosNum), (bif b then ↑n else 0) = ↑(bif b then pos n else 0)
this' : ∀ (b : Bool) (n : PosNum), ↑(pos (PosNum.bit b n)) = Nat.bit b ↑n
m : PosNum
IH : ∀ (n : PosNum), ↑(p m n) = Nat.bitwise g ↑(pos m) ↑(pos n)
n : PosNum
⊢ ↑(p (PosNum.bit false m) (PosNum.bit true n)) = Nat.bit (g false true) (Nat.bitwise g ↑m ↑n)

case pos.pos.bit0.bit0
f : Num → Num → Num
g : Bool → Bool → Bool
p : PosNum → PosNum → Num
gff : g false false = false
f00 : f 0 0 = 0
f0n : ∀ (n : PosNum), f 0 (pos n) = bif g false true then pos n else 0
fn0 : ∀ (n : PosNum), f (pos n) 0 = bif g true false then pos n else 0
fnn : ∀ (m n : PosNum), f (pos m) (pos n) = p m n
p11 : p 1 1 = bif g true true then 1 else 0
p1b : ∀ (b : Bool) (n : PosNum), p 1 (PosNum.bit b n) = bit (g true b) (bif g false true then pos n else 0)
pb1 : ∀ (a : Bool) (m : PosNum), p (PosNum.bit a m) 1 = bit (g a true) (bif g true false then pos m else 0)
pbb : ∀ (a b : Bool) (m n : PosNum), p (PosNum.bit a m) (PosNum.bit b n) = bit (g a b) (p m n)
this : ∀ (b : Bool) (n : PosNum), (bif b then ↑n else 0) = ↑(bif b then pos n else 0)
this' : ∀ (b : Bool) (n : PosNum), ↑(pos (PosNum.bit b n)) = Nat.bit b ↑n
m : PosNum
IH : ∀ (n : PosNum), ↑(p m n) = Nat.bitwise g ↑(pos m) ↑(pos n)
n : PosNum
⊢ ↑(p (PosNum.bit false m) (PosNum.bit false n)) = Nat.bit (g false false) (Nat.bitwise g ↑m ↑n)

Tactic:
all_goals
  rw [← show ∀ n : PosNum, ↑(p m n) = Nat.bitwise g ↑m ↑n from IH]
  rw [← bit_to_nat, pbb]

Result:
no goals