# Integer Generics: Negative literals seem broken

**URL:** <https://forums.swift.org/t/integer-generics-negative-literals-seem-broken/82481>\
**Category:** Using Swift\
**Tags:** generics, integer-generics\
**Created:** [October 2, 2025, 2:26pm UTC](https://forums.swift.org/t/integer-generics-negative-literals-seem-broken/82481 "2025-10-02T14:26:29Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![jeremyp](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/jeremyp/32/3552_2.png) [@jeremyp](https://forums.swift.org/u/jeremyp)\
**Post date:** [October 2, 2025, 2:26pm UTC](https://forums.swift.org/t/integer-generics-negative-literals-seem-broken/82481/1 "2025-10-02T14:26:29Z")

</div>

I was trying to create a type similar to Pascal array types where you can specify the upper bound and lower bound of the index e.g. in Pascal you can write `var a: array [1 .. 8] of integer`and the elements will be indexed from 1 to 8 (as opposed to 0 to 7). I am using the new integer generic parameters.

Consider the following pared down example

```swift
struct MyCollection<let lowerBound: Int, let upperBound: Int, T>
{
	init()
	{
		print("lb = \(lowerBound), ub = \(upperBound)")
	}
    // Implementation would go here but not needed to demo the problem
}

let a = MyCollection< 1, 8, Int>()
let b = MyCollection< 0, 8, Int >()
let c = MyCollection< -1, 8, Int >()
let d = MyCollection< -2, 8, Int >()
let e = MyCollection< -3, 8, Int >()
let f = MyCollection< -4, 8, Int >()
let g = MyCollection< -5, 8, Int >()

```

It is supposed to create successive collections with 8, 9, 10, 11… elements, each one with a more negative lower bound than the previous. However, the above code prints:

```swift
lb = 1, ub = 8
lb = 0, ub = 8
lb = 1, ub = 8
lb = 2, ub = 8
lb = 5, ub = 8
lb = 4, ub = 8
lb = 11, ub = 8

```

The lower bounds specified without the minus sign are fine, but the subsequent ones are, to my eye, completely wrong. The grammar from the [Swift Evolution proposal](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0452-integer-generic-parameters.md)suggests I am allowed to use negative numbers

```swift
generic-argument --> '-'? integer-literal
same-type-requirement --> type-identifier '==' '-'? integer-literal

```

I think this is a compiler bug and I need to raise it as such. Any comments?

* * *

The above is reproducible in a Playground in Xcode 26.0.1 with Swift 6.2 on macOS 26.0

---

<div class="post-metadata">

**Author:** ![Nevin](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/nevin/32/617_2.png) [@Nevin](https://forums.swift.org/u/Nevin)\
**Post date:** [October 2, 2025, 4:44pm UTC](https://forums.swift.org/t/integer-generics-negative-literals-seem-broken/82481/2 "2025-10-02T16:44:36Z")

</div>

It appears that the bit-pattern of the observed values is the same as that of the input values with all-but-one of the leading 1-bits masked off.

Without knowing anything about the implementation, I’m guessing there’s some bit-twiddling mistake in the handling of negative values here.

---

<div class="post-metadata">

**Author:** ![Slava\_Pestov](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/slava_pestov/32/30204_2.png) [@Slava\_Pestov](https://forums.swift.org/u/Slava_Pestov)\
**Post date:** [October 2, 2025, 5:29pm UTC](https://forums.swift.org/t/integer-generics-negative-literals-seem-broken/82481/3 "2025-10-02T17:29:31Z")

</div>

@Alejandro has a fix up already: [[IRGen] Correctly sext instead of zext for negative integer types by Azoy · Pull Request #84655 · swiftlang/swift · GitHub](https://github.com/swiftlang/swift/pull/84655)