[SR-3914] Invalid "Mixed-type subtraction is deprecated." warning Xcode 8.3 beta 2

[keith]

Previous ID	SR-3914
Radar	rdar://problem/30472072
Original Reporter	@keith
Type	Bug
Status	Resolved
Resolution	Won't Do

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug, 3.1Regression, TypeChecker
Assignee	None
Priority	Medium

md5: 8ff5f9ec83e0c164daad3b038df2038b

Issue Description:

This code produces the "Mixed-type subtraction is deprecated." warning even though both `tv_usec` calls return a `_darwin_suseconds_t` aka `_int32_t` aka `Int32`

import Darwin

var bootTime: timeval = timeval()
var now: timeval = timeval()

_ = now.tv_usec - bootTime.tv_usec // Doesn't warn
_ = max(0, now.tv_usec - bootTime.tv_usec) // Doesn't warn
_ = UInt64(max(0, now.tv_usec - bootTime.tv_usec)) // Warns

[belkadan]

@swift-ci create

[belkadan]

For some reason it's using the Strideable conformance.

func - <T: Strideable>(lhs: T, rhs: T) -> T.Stride

[belkadan]

I guess the 0 is providing enough context to suggest an Int return? :-(

[belkadan]

Keith, you can work around this by using 0 as Int32.

[moiseev]

Yeah... Tricky to understand, but correct behavior. Should be worked around for now, until the deprecated overload is not removed in Swift 4.