[SR-12486] Value.self is Any.Protocol broke in 5.1, got bad warning in 5.2

[lilyball]

Previous ID	SR-12486
Radar	rdar://problem/62201613
Original Reporter	@lilyball
Type	Bug
Status	Resolved
Resolution	Done

Environment

Swift 5.1.5, Swift 5.2

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug
Assignee	@tbkka
Priority	Medium

md5: 868c763c40543f8ea25aec14dbda84d3

Issue Description:

I have some generic code that, in an attempt to check if the generic type is literally Any, evaluates Value.self is Any.Protocol. This used to work correctly up through Swift 5.0.3. Testing in Swift 5.1.5, this code now always evaluates to true, which is a surprise. Testing in Swift 5.2, this code not only evaluates to true, but triggers a compiler warning that erroneously claims it will always evaluate to false.

func isAny<Value>(_ type: Value.Type) -> Bool {
  return Value.self is Any.Protocol
}

print(isAny(Int.self))
print(isAny(Any.self))
print(isAny(Any?.self))

false
true
false

true
true
true

unnamed.swift:2:21: warning: cast from 'Value.Type' to unrelated type 'Any.Protocol' always fails
  return Value.self is Any.Protocol
         ~~~~~~~~~~ ^  ~~~~~~~~~~~~
true
true
true

[lilyball]

Bizarrely, in my source project where I ran into this, my tests all pass with Swift 5.1.5 and as best as I can tell, it's because the Value.self is Any.Protocol expression is returning false where Value is Any? even though my sample code shows that as returning true.

[lilyball]

Oh dear. Behavior changes under optimization with both Swift 5.1.5 and Swift 5.2. When I turn on -O both start returning the correct values (the ones that 5.0.3 returned always).

[lilyball]

Swift 5.1.5 also changes behavior based on deployment target. With Swift 5.1.5, -target x86_64-apple-macos10.9 always produces the correct answer (regardless of optimization) but -target x86_64-apple-macos10.15 (or no target) produces the wrong answer without optimization. Swift 5.2 produces the wrong answer without optimization regardless of target.

[slavapestov]

There are two bugs here. The false positive warning is actually a different code path than the runtime cast implementation...

[beccadax]

@swift-ci create

[tbkka]

Lily,

Thanks for the report. I'm trying to dig through and figure out what changed and why.

As a workaround can you change the original code to `return Value.self == Any.self`? If that works for you, it should actually be a lot faster.

[tbkka]

I believe this is fixed by #31662

[LucianoPAlmeida]

@tbkka@lilyball The false positive warning, in this case, is fixed on master by #32234 🙂

[tbkka]

Nominated for inclusion in Swift 5.3: #32725

[tbkka]

I believe this is fixed in Swift 5.3. Please let me know if you see any further problems in this area.