The proposed feature set for :extend

On issue #1155, there was community consensus on the usage of extend in a way that offered flexibility for different use cases but kept things simple through the use of keyword flags. I've pulled out the relevant section/syntax that was agreed upon:

:extend( selector [exact|any] [shallow|deep] [all])

• exact [default] - Match complete selectors (:extend(.button exact) matches .button {} but not #main .button a - :extend(.a .b) is considered an exact match to .a { .b { } })
• any - Match partial selector (:extend(.button any) matches .button {} and #main .button a)
• shallow [default] - extend only root properties (.a:extend(.button shallow) will not include hover in .button { &:hover { } })
• deep - extend all nested selectors/properties (.a:extend(.button deep) will include hover in .button { &:hover { } })
• all - set "any" and "deep" to true. Extend all matches, partial and exact, and extend all base and nested properties

Example:

.b:extend(.a);  // because of defaults, exact=true, shallow=true
.b:extend(.a any);  // exact=false, shallow=true
.b:extend(.a deep);  //exact=true, shallow=false
.b:extend(.a all); // exact=false, shallow=false

The reasons, arguments, and use cases for these other uses of :extend are documented in issue #1155. After the discussion and consensus, the first case and last case were implemented because they were easier to implement first, but I think the remaining implementation got lost in the noise. I'd love to see the rest of :extend implemented as I think these other use cases are still valid and there's various issues that come up here that essentially relate to what's still missing. (See: less/less-docs#177).

I'd love to see the work on :extend continue! (And other great ideas, like extending mixins.)

No files or tests are named. Start by reading issue #1155 and less-docs#177, then inspect the existing :extend implementation for the already-supported default and all cases. Done means implementing the remaining exact/any and shallow/deep combinations described here, with coverage for the listed examples.