[SR-12399] Bad diagnostic when failing to unwrap an optional

[hamishknight]

Previous ID	SR-12399
Radar	None
Original Reporter	@hamishknight
Type	Bug
Status	Closed
Resolution	Done

Environment

Swift version 5.3-dev (LLVM fa328ef, Swift de46690)
Target: x86_64-apple-darwin18.5.0

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, DiagnosticsQoI, TypeChecker
Assignee	@hamishknight
Priority	Medium

md5: 893b2bc146886084679b87f760198979

Issue Description:

For the following:

func makeArray<T>(_ x: T) -> [T] { [x] }

func foo(_ x: Int?) {
  var s1116 = Set(0...10).subtracting(makeArray(x))
}

We emit the diagnostics:

main.swift:979:20: error: missing argument label '_immutableCocoaSet:' in call [missing_argument_labels]
  var s1116 = Set(0...10).subtracting(makeArray(x))
                   ^
                  _immutableCocoaSet:
main.swift:979:20: error: argument type 'ClosedRange<Int>' expected to be an instance of a class or class-constrained type [cannot_convert_argument_value_anyobject]
  var s1116 = Set(0...10).subtracting(makeArray(x))
                   ^

We should instead provide an optional unwrap diagnostic on x.

[hamishknight]

Fixed by #30686