[SR-11127] Swift is unable to type-check a common statistical expression

[swift-ci]

Previous ID	SR-11127
Radar	rdar://problem/72801678
Original Reporter	kam (JIRA User)
Type	Bug

Environment

Xcode 10.2.1 (1E1001)

Apple Swift version 5.0.1 (swiftlang-1001.0.82.4 clang-1001.0.46.5)

macOS Mojave 10.14.5

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 6a2175251c6d14255f04f22de3eb5237

Issue Description:

Unexpected error when coding up some statistical work. Considering that all the variable types are declared, it is unclear what makes this so complicated that the compiler can't handle it.

To reproduce, place the following code into a playground:

import Foundation
public func standardErrorOfEstimateForObservation(standardErrorOfEstimate: Double,
                                                  sampleSize: Int,
                                                  scoreOnThePredictor: Double,
                                                  meanOfThePredictor: Double,
                                                  standardDeviationOfThePredictor: Double) {
    return standardErrorOfEstimate *
        sqrt(1.0 +
            1.0 / sampleSize +
            square(scoreOnThePredictor - meanOfThePredictor) / (sampleSize * square(standardDeviationOfThePredictor)))
}

private func square(_ x: Double) -> Double {
    return x * x
}

standardErrorOfEstimateForObservation(standardErrorOfEstimate: 1.2,
                                      sampleSize: 90,
                                      scoreOnThePredictor: 12,
                                      meanOfThePredictor: 24.2,
                                      standardDeviationOfThePredictor: 11.53)

This gives the error:

error: SwiftBug.playground:9:36: error: the compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions
    return standardErrorOfEstimate *
           ~~~~~~~~~~~~~~~~~~~~~~~~^

While I could break it up into expressions, I'm writing the code to match an equation from a paper. It helps to write the expression the same way, especially if I ask a non-programmer to review my work.

[swift-ci]

Comment by Kevin A. Mitchell (JIRA)

This works:

import Foundation
public func standardErrorOfEstimateForObservation(standardErrorOfEstimate: Double,
                                                  sampleSize: Int,
                                                  scoreOnThePredictor: Double,
                                                  meanOfThePredictor: Double,
                                                  standardDeviationOfThePredictor: Double) -> Double {
    let dSampleSize = Double(sampleSize)
    return standardErrorOfEstimate *
        sqrt(1.0 +
            1.0 / dSampleSize +
            square(scoreOnThePredictor - meanOfThePredictor) / (dSampleSize * square(standardDeviationOfThePredictor)))
}

private func square(_ x: Double) -> Double {
    return x * x
}

standardErrorOfEstimateForObservation(standardErrorOfEstimate: 1.2,
                                      sampleSize: 90,
                                      scoreOnThePredictor: 12,
                                      meanOfThePredictor: 24.2,
                                      standardDeviationOfThePredictor: 11.53) 

My bad: I had mistakes in my code. The error about the complexity hid the errors about the missing return type and incompatibility between Int and Double in math. That in itself may be interesting.

[belkadan]

cc @xedin

[typesanitizer]

@swift-ci create

[slavapestov]

Still slow. I'll add a test case.