ntuple: ensure eltype is always Int

[barucden]

Fixes #55790

[nsajko]

I think it'd be slightly better to construct an Int, with an ::Int type assertion immediately afterwards, right at the beginning of the ntuple method.

• I think this would be faster for BigInt input, as comparing Int with Int should be faster than with BigInt.
• This would lead to more consistent error messages on the off chance someone passes in an Integer value whose Int constructor doesn't return Int.

[barucden]

Makes sense, thank you.

[timholy]

If there's any risk this could take a while to compile (e.g., f is big and @inline), then the pattern

function slow_to_compile(f::F, n::Int) where F
    # body
end
slow_to_compile(f::F, n::Integer) where F = slow_to_compile(f, convert(Int, n)::Int)

might be better: it ensures the "real work" function only gets compiled for Int.

[barucden]

Got it! Thank you.