Open Radar

 
Summary:
The following code snippet takes a long time to compile:

import Foundation

struct DurationList {
    private var runtimes: [Double] = []

    var mean: Double {
        let total = runtimes.reduce(0) { return $0 + $1 }
        return total / Double(runtimes.count)
    }

    var stddev: Double {
        let mean = self.mean
        
        // This relatively complex expression is the culprit
        // Compiles fine when split up into 3 separate expressions
        let variance = runtimes.map { $0 - mean }
            .map { $0 * $0 }
            .reduce(0, combine: +) / Double(runtimes.count)

        return sqrt(variance)
    }
}

Steps to Reproduce:
1. Download the attached project
2. Open the project & build

Expected Results:
The project should build in reasonable time.

Actual Results:
The project takes > 10s to build (on my machine).


Version:
Xcode 7.2b4 (7C62b)
Apple Swift version 2.1.1 (swiftlang-700.1.101.13 clang-700.1.81)
OS X 10.11.1 (15B42)