[SR-177] Swift does not report a type error at compile-time -- program crashes at runtime instead

[swift-ci]

Previous ID	SR-177
Radar	None
Original Reporter	davidrichards (JIRA User)
Type	Bug
Status	Closed
Resolution	Invalid

Environment

The swift binary ("swift build") from Xcode Swift 2.2 Snapshot, December 1, 2015.
OS X 10.11.2.

Additional Detail from JIRA

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

md5: 66f3b25f0cbbb2f3a680f50012948c63

Issue Description:

Problem:
This code (all in main.swift) crashes at runtime at the line indicated:

protocol DoubleProtocol {}
extension Double : DoubleProtocol {}
func -(d1: DoubleProtocol, d2: DoubleProtocol) -> Double {
    return d1 - d2 //Crash here
}

extension ClosedInterval where Bound : DoubleProtocol {
    func isCloseTo(interval: ClosedInterval) -> Bool {
        if abs(self.start - interval.start) < 1e-3 && abs(self.end - interval.end) < 1e-3 {
            return true
        } else {
            return false
        }
    }
}

let close = ClosedInterval<Double>(0, 1).isCloseTo(ClosedInterval<Double>(0, 1))
print("they're close: \(close)")

Solution:
As expected, if I turn the crashing line into the following line, things work properly again:

return (d1 as! Double) - (d2 as! Double) //No crash anymore.

Expected behavior:
The compiler should have shown a compile-time error as "d1 - d2" is meaningless because there is no "-" operator implemented for the DoubleProtocol.

[swift-ci]

Comment by Tadeas Kriz (JIRA)

But there is a "-" operator implemented for DoubleProtocol. What you see is probably a stack overflow, because you are calling the same operator function over and over again.

Casting it back to double is unsafe and might fail if in future someone would do `extension Float: DoubleProtocol`, or something similar.

EDIT: It would be cool if the compiler told you that it is a recursive call and possibly that is an infinite recursive call as well.

[swift-ci]

Comment by David Richards (JIRA)

TadeasKriz (JIRA User) My bad, of course "-" for DoubleProtocol was my own creation.You're exactly right. The run-time crash I was having was indeed an infinite recursion. I agree. It would be cool if the compiler detected the infinite recursion.

[swift-ci]

Comment by Tadeas Kriz (JIRA)

I think that closing this issue would be the right way to continue now.

[ahoppen]

As mentioned in the comments, this is not a bug