Issue Description:

Redeclaring a variable which is already bound by a guard statement does not cause a compiler error when done on the file level. The following compiles and runs without errors or warnings:

// main.swift
import Swift

guard let number = Int("1234") else { fatalError() }
print(number) // Output: 1234
let number = 5678
print(number) // Output: 1234

It looks as if the statement `let number = 5678` is completely ignored.

The problem only occurs with code on the file level. Doing the same inside a function causes a compiler error, as expected.

func foo() {
    guard let number = Int("1234") else { fatalError() }
    print(number)
    let number = 5678 //  error: definition conflicts with previous value
}

Tested with

• Xcode 7.3.1, "Default" and "Snapshot 2016-06-06 (a)" toolchain

• Xcode 8 beta.