Post-increment in transpiled code is executed twice (T7432)

[babel-bot]

Issue originally made by @mariusschulz

Bug information

• Babel version: 6.7.7

Input code

function first(...values) {
    let index = 0;
    return values[index++];
}

console.log(first(1, 2));

Description

Here's the code that Babel emits for the above input:

"use strict";

function first() {
    var index = 0;
    return arguments.length <= index++ + 0 ? undefined : arguments[index++ + 0];
}

console.log(first(1, 2));

As you can see, index++ is executed twice, both within the condition of the conditional expression and within its second branch.

• Actual: 2 is logged to the console
• Expected: 1 is logged to the console

Emitting an additional local variable and a sequence expression would solve the problem:

"use strict";

function first() {
    var index = 0, _index;
    return _index = index++, arguments.length <= _index + 0 ? undefined : arguments[_index + 0];
}

console.log(first(1, 2));

[motiz88]

Closed via #4674.