[SR-897] Misleading diagnostic when testing for nil in `assertionFailure`

[swift-ci]

Previous ID	SR-897
Radar	None
Original Reporter	elia (JIRA User)
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, DiagnosticsQoI
Assignee	None
Priority	Medium

md5: d23aee39609546cf931f8061ece93852

Issue Description:

While refactoring a piece of code, I transformed an unconditional `assertionFailure` in an `assert` but I forgot to change the function name. I got this piece of code, which produced a confusing diagnostic:

class A {}
let a: A? = nil

assertionFailure(a != nil, "error”)

Compiling with `swiftc` leads to:

DiagnosticBug.swift:5:20: error: value of type 'A?' can never be nil, comparison isn't allowed
assertionFailure(a != nil, "error")
                 ~ ^

A simple boolean comparison on the other hand produces the diagnostic you’d expect:

let b = true
assertionFailure(b == false, "error”)

DiagnosticBug.swift:9:20: error: cannot convert value of type 'Bool' to expected argument type 'String'
assertionFailure(b == false, "error")
                 ~~^~~~~~~~

[belkadan]

In Swift 2.2 I get a different error, but still bad:

<stdin>:4:17: error: missing argument label 'file:' in call
assertionFailure(a != nil, "error")
                ^
                           file: