/cc @vladima @ahejlsberg @billti

Motivating Examples

var arr = [1, 2, 3];

This should be an error (k is always a string):

for(var k in arr) { if(k === 0) { ... } }

This should not be an error, even under no implicit any:

for(var k in arr) { if(arr[k] === 0) { ... } }

This should somehow? not be an error, even though it involves a coercion we would normally disallow

for(var k in arr) { if (k > 0) { ... } }