# Automatic Requirement Satisfaction in plain Swift

**URL:** <https://forums.swift.org/t/automatic-requirement-satisfaction-in-plain-swift/37158>\
**Category:** Pitches\
**Created:** [June 2, 2020, 7:06pm UTC](https://forums.swift.org/t/automatic-requirement-satisfaction-in-plain-swift/37158 "2020-06-02T19:06:11Z")\
**Posts on this page:** 1\
**Showing post:** 32

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**Author:** ![Karl](https://avatars.discourse-cdn.com/v4/letter/k/34f0e0/32.png) [@Karl](https://forums.swift.org/u/Karl)\
**Post date:** [June 5, 2020, 8:21am UTC](https://forums.swift.org/t/automatic-requirement-satisfaction-in-plain-swift/37158/32 "2020-06-05T08:21:17Z")

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> [@Joe\_Groff](#):
>
> I feel that compile-time evaluation will also be important to having a satisfying model for working with variadics, since it is natural to want to work with them as collections rather than use otherwise unidiomatic cons-walking techniques.

I'm not sure compile-time evaluation matters to constrained extensions (such as for protocol default implementations), unless you're talking about making compiler-evaluable code part of the type system.

I mean - it would be really cool...

```swift
extension FixedSizeArray<Element, let Size: Int> where Size.isMultiple(of: 2) {
  // Dreaming that we had generic value parameters...
}

```

But I'm doubtful that that kind of thing is actually feasible, though. I remember asking if it would be possible to use it in conditional compilation:

> [@Future of Conditional Compilation](https://forums.swift.org/t/future-of-conditional-compilation/33139/3):
>
> IMO constant-evaluation-based conditional compilation is not easily supported by any traditional model. The current constant evaluation infrastructure runs as a mandatory SIL pass, which is unlikely to change, whereas conditional compilation has to occur before Sema to remain useful across different platforms. This makes any #if directive body that can't be independently lowered to SIL problematic, including the existing support for conditionally compiled case blocks.

So if I understand that right, the compile-time evaluation happens after type-checking, meaning everything before that (parsing, type-checking itself including constrained extensions) can't make use of the results.

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