sass-site/source/documentation/breaking-changes/strict-unary.md

title	introduction
Breaking Change: Strict Unary Operators	Sass has historically allowed `-` and `+` to be used in ways that make it ambiguous whether the author intended them to be a binary or unary operator. This confusing syntax is being deprecated.

How is this property compiled?

{% codeExample 'strict-unary', false %} $size: 10px;

div { margin: 15px -$size; }

$size: 10px

div margin: 15px -$size {% endcodeExample %}

Some users might say "the - is attached to $size, so it should be margin: 20px -10px". Others might say "the - is between 20px and $size, so it should be margin: 5px." Sass currently agrees with the latter opinion, but the real problem is that it's so confusing in the first place! This is a natural but unfortunate consequence of CSS's space-separated list syntax combined with Sass's arithmetic syntax.

That's why we're moving to make this an error. In the future, if you want to use a binary - or + operator (that is, one that subtracts or adds two numbers), you'll need to put whitespace on both sides or on neither side:

• Valid: 15px - $size
• Valid: (15px)-$size
• Invalid: 15px -$size

If you want to use a unary - or + operator as part of a space-separated list, you'll (still) need to wrap it in parentheses:

• Valid: 15px (-$size)

Transition Period

{% compatibility 'dart: "1.55.0"', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

We'll make this an error in Dart Sass 2.0.0, but until then it'll just emit a deprecation warning.

{% render 'doc_snippets/silence-deprecations' %}

Automatic Migration

You can use the Sass migrator to automatically update your stylesheets to add a space after any - or + operators that need it, which will preserve the existing behavior of these stylesheets.

$ npm install -g sass-migrator
$ sass-migrator strict-unary **/*.scss