[SR-1350] REPL doesn't correctly handle guard variable shadowing

[swift-ci]

Previous ID	SR-1350
Radar	None
Original Reporter	jaybuff (JIRA User)
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	LLDB for Swift
Labels	Bug, REPL
Assignee	None
Priority	Medium

md5: 3866fdf30f82bf574e50d9a237eeb02f

Issue Description:

let hex = "DEADBEEF"
let num = Int(hex, radix: 16)
guard let num = num else {
    fatalError("Couldn't parse \(hex) as an integer")
}

pasting the above program into the REPL, then adding print(num) prints Optional(3735928559). When I compile and run it, I get 3735928559.

[belkadan]

Per comments in SR-1353 it's not quite clear what the correct behavior is in the compiler, but the REPL has generally always allowed you to replace a binding.