Class is undefined if build with empty string.

[Mighty683]

If you build class with empty string for example:
@variable: ~''; .a@{variable}-b { a-b: 1 }
Output will be: .a-b { a-b: 1 } and this class can be used.
But if you try use .a-b class in another class as mixin for example:
.c { .a-b; }
Error 'a-b' is undefined will be present.
.c { .a-b; } will work only if you declare a-b class this way .a-b { a-b: 1 }.

[matthew-dean]

That's neat but what's the real use case here? Why is an empty string getting injected into a class name like that and why would you ever need it?

[Mighty683]

I was generating class for different media queries in loop. For example:
className-xs
className-lg
className-md
And wanted to generate non-media overall class named className. It's problem when you want generate classes in loop and one of variants is ~''.
I had to workaround this by adding this class outside the loop.

[seven-phases-max]

This is a duplicate of #2831.

[seven-phases-max]

Closing as a duplicate of #2831.

[seven-phases-max]

As for the use-case itself (regardless of the bug) - it's the same story as in #2831: Never reuse CSS classes as mixins, use real mixins instead (for a detailed description on why this suggestion is so bold see #2702).