Fix: no-constant-condition false positives with unary expressions

[mdjermanovic]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[x] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

Tell us about your environment

• ESLint Version: v7.15.0
• Node Version: v12.18.4
• npm Version: v6.14.6

What parser (default, @babel/eslint-parser, @typescript-eslint/parser, etc.) are you using?

default

Please show your full configuration:

Configuration

module.exports = {
  parserOptions: {
    ecmaVersion: 2015
  }
};

What did you do? Please include the actual source code causing the issue.

Online Demo

/* eslint no-constant-condition: error */

if (+(a || true));

What did you expect to happen?

No errors.

For example, with a = "0" the condition is falsy, with a = "1" the condition is truthy.

What actually happened? Please include the actual, raw output from ESLint.

+(a || true) is reported as a constant condition:

  3:5  error  Unexpected constant condition  no-constant-condition

What changes did you make? (Give an overview)

Fixed the no-constant-condition rule to not treat the argument of a unary expression as a "boolean position", unless the operator is !.

Is there anything you'd like reviewers to focus on?

• valid tests were failing before this change.
• invalid tests are regression tests, they were already passing as invalid tests before this change.

[btmills]

LGTM. I appreciate the additional tests to guard against regression. The original bug, values that are simultaneously truthy and falsy depending on what route you use to coerce them, is a bit of a mind-bender.