UseExplicitNilCheckInConditions generates invalid rewrite

[jmccance]

The new formatting rule for UseExplicitNilCheckInConditions generates invalid output when the righthand side returns a constrained generic type.

Reproduction

protocol P {}

struct S: P, Equatable { }

func getValue<T: P>(forKey: String) -> T? {
    nil
}

@main
struct UseExplicitNilCheckInConditionsBug {
  static func main() {
    /*
    // Before formatting
    if let _: S? = getValue(forKey: "fnord") {
        print("Found it")
    }
    */

    // ❌ error: type 'any (~Copyable & ~Escapable).Type' cannot conform to 'P' 
    //    note: only concrete types such as structs, enums and classes can conform to protocols
    if getValue(forKey: "fnord") != nil {
      print("Found it")
    }
  }
}

I think the correct rewrite would be:

    if getValue(forKey: "fnord") != (nil as S?) {
      print("Found it")
    }

Versions

swift --version:

swift-driver version: 1.127.14.1 Apple Swift version 6.2 (swiftlang-6.2.0.19.9 clang-1700.3.19.1)
Target: arm64-apple-macosx15.0

swift format --version:

6.2.0

[allevato]

Having the type coercion on the left hand side would be closer to the original intent:

if getValue(forKey: "fnord") as S? != nil { ... }

But we could also just skip the transformation if there's an explicit type annotation on the conditional binding, to avoid the issue. I don't really feel strongly here. Skipping it is safer, since we don't have to worry about the precedence of the operators in the original expression compared to the newly inserted cast.