sass-lint crashes when using !important with an if guarding

[nyon]

What version of Sass Lint are you using?

1.10.2

Please include any relevant parts of your configuration

No configuration used. I used the CLI tool without configuration.

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

I created a test file named test.scss:

@mixin testcase($important: null) {
  @if ($important == true) {$important: !important;}
  width: auto $important;
}

body {
  @include testcase(true);
}

I ran scss test.scss test.css which produced no errors and a valid css file.
But if i run sass-lint test.scss the linter crashes.

What did you expect to happen?

That sass-lint does not crash. I would propose, that sass-lint ignores bangs like these.

What actually happened? Please include any error messages given to you by Sass Lint.

sass-lint crashes with the following error message:

/home/nyon/.nvm/versions/node/v7.7.4/lib/node_modules/sass-lint/lib/rules/space-before-bang.js:16
      if (!previous.is('space')) {
                  ^

TypeError: Cannot read property 'is' of undefined
    at Node.<anonymous> (/home/nyon/.nvm/versions/node/v7.7.4/lib/node_modules/sass-lint/lib/rules/space-before-bang.js:16:20)
    at /home/nyon/.nvm/versions/node/v7.7.4/lib/node_modules/sass-lint/node_modules/gonzales-pe/lib/gonzales.js:305:54
    at Node.traverse (/home/nyon/.nvm/versions/node/v7.7.4/lib/node_modules/sass-lint/node_modules/gonzales-pe/lib/gonzales.js:279:6)
    at Node.traverse (/home/nyon/.nvm/versions/node/v7.7.4/lib/node_modules/sass-lint/node_modules/gonzales-pe/lib/gonzales.js:284:36)
    at Node.traverse (/home/nyon/.nvm/versions/node/v7.7.4/lib/node_modules/sass-lint/node_modules/gonzales-pe/lib/gonzales.js:284:36)
    at Node.traverse (/home/nyon/.nvm/versions/node/v7.7.4/lib/node_modules/sass-lint/node_modules/gonzales-pe/lib/gonzales.js:284:36)
    at Node.traverse (/home/nyon/.nvm/versions/node/v7.7.4/lib/node_modules/sass-lint/node_modules/gonzales-pe/lib/gonzales.js:284:36)
    at Node.traverse (/home/nyon/.nvm/versions/node/v7.7.4/lib/node_modules/sass-lint/node_modules/gonzales-pe/lib/gonzales.js:284:36)
    at Node.traverse (/home/nyon/.nvm/versions/node/v7.7.4/lib/node_modules/sass-lint/node_modules/gonzales-pe/lib/gonzales.js:284:36)
    at Node.traverse (/home/nyon/.nvm/versions/node/v7.7.4/lib/node_modules/sass-lint/node_modules/gonzales-pe/lib/gonzales.js:284:36)

If you're using a IDE plugin have you tried the CLI too?

I've only used the CLI tool.

Possible Fix

I fixed it by inserting guarding code into the space-before-bang linting rule. I'm not sure whether this has side effects. I doubt it would:

// file: lib/rules/space-before-bang.js
    // ...
    ast.traverseByTypes(['important', 'default'], function (block, i, parent) {
      var previous = parent.content[i - 1];
      
      if(previous === undefined) {
        return null;
      }

      if (!previous.is('space')) {
        if (parser.options.include) {
    // ...