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Comparison operator negation rules can be incorrect #902

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alangpierce opened this Issue Mar 6, 2017 · 0 comments

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alangpierce commented Mar 6, 2017

decaffeinate is producing the wrong JavaScript based on my CoffeeScript input:

unless NaN > 1
  console.log 'first'
else
  console.log 'second'

(repl)

I get this output:

if (NaN <= 1) {
  console.log('first');
} else {
  console.log('second');
}

Here's what I expect it to be instead:

if (!(NaN > 1)) {
  console.log('first');
} else {
  console.log('second');
}

This is causing at least two problems in atom. I think this negation behavior probably needs to be behind a --loose flag, since it doesn't work for operands like NaN and undefined.

@alangpierce alangpierce self-assigned this Mar 6, 2017

alangpierce added a commit to alangpierce/decaffeinate that referenced this issue Mar 6, 2017

fix: don't unsafely negate operators like `>` by default
Fixes decaffeinate#902

For both normal binary operators and chained comparison operators, we need to be
careful about rewriting `!(a < b)` as `a >= b`, since the rewrite isn't correct
for cases like `NaN`. Now, we just do the simpler negation by default and only
do the nicer-looking one with an opt-in flag.

alangpierce added a commit that referenced this issue Mar 6, 2017

fix: don't unsafely negate operators like `>` by default (#903)
Fixes #902

For both normal binary operators and chained comparison operators, we need to be
careful about rewriting `!(a < b)` as `a >= b`, since the rewrite isn't correct
for cases like `NaN`. Now, we just do the simpler negation by default and only
do the nicer-looking one with an opt-in flag.
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