Cast from decimal to uint8, uint16, uint32, uint, uint64, int, etc. don't follow .NET behaviour

[MangelMaxime]

Running this code in .NET will throw exceptions:

uint8 -1m |> printfn "%A"
uint16 -1m |> printfn "%A"
uint32 -1m |> printfn "%A"
uint -1m |> printfn "%A"
uint64 -1m |> printfn "%A"
int -18446744073709551615m |> printfn "%A"

Exceptions looks like Value was either too large or too small for a UInt16.

On Fable, the code succeeds and output:

255
65535
4294967295
4294967295
18446744073709551615
0

I am not sure how to approach fixing this.

My first idea was to apply a fix in toLong function here:

Fable/src/Fable.Transforms/Replacements.fs

Lines 299 to 303 in 204d30d

	| Number(fromKind, _), Number(toKind, _) ->
	let fromMeth = "from" + fromKind.ToString()
	Helper.LibCall(com, "BigInt", fromMeth, BigInt.Number, args, ?loc = r)
	|> wrapLong com ctx r targetType

But I think this would be to wide because in some scenario .NET allows to do that:

let value = 4294967295UL
let c = uint8 value
printfn "%A" c
// No exception thrown it actually prints: 255uy

I believe we should only fix portion of the code when we call Decimal.op_explicit at:

Fable/src/Fable.Transforms/Replacements.fs

Lines 2445 to 2454 in 204d30d

	| "op_Explicit", _ ->
	match t with
	| Number(kind, _) ->
	match kind with
	| BigIntegers _ -> toLong com ctx r t args |> Some
	| Integers _ -> toInt com ctx r t args |> Some
	| Floats _ -> toFloat com ctx r t args |> Some
	| Decimal -> toDecimal com ctx r t args |> Some
	| _ -> None
	| _ -> None

Should we add function like that to Decimal.ts ?

export function op_Explicit_ToChar(d: Decimal) {
  const n = toNumber(d);
  if (n < 0 || n > 65535 || isNaN(n)) {
    throw new Error("Value was either too large or too small for a character.");
  }

  return String.fromCharCode(n);
}

export function op_Explicit_ToUint64(d: Decimal) {
  const n = toUInt64(fromDecimal(d));
  if (n < 0 || n > 18446744073709551615n) {
    throw new Error("Value was either too large or too small for a UInt64.");
  }

  return n;
}

Is it the right approach / place to store them? @ncave @alfonsogarciacaro

[ncave]

@MangelMaxime Thanks, I'll take a look.