Create highlight group that modifies an existing one

Question

I would like to create a new highlighting group that is exactly like an existing one except for one (or more) properties. For example, the new group could be an underlined version of the existing one.

Context: I'm using coc.nvim and its semantic highlighting feature. To support the language server protocol's token modifiers, coc.nvim creates combined highlighting groups. For example, CocSemVariable for immutable variables, and CocSemMutableVariable for mutable ones. What I would like to do is something like this:

highlight! link CocSemMutableVariable CocSemVariable
highlight! CocSemMutableVariable cterm=underline gui=underline

This doesn't work however since any highlight command will break links. Is there a way to achieve what I want?

Answer 1

There's no way to do this directly with highlight! since links and groups are fundamentally different. You can have a highlight that is a link or a highlight with properties, you can't have both.

Instead you can try to use hlget and hlset as follows:

let resolve = v:true   " set to v:false if not wanting to resolve links

let hl = hlget('CocSemVariable', resolve)[0]
let hl.name = 'CocSemMutableVariable'
let hl.cterm = {'underline': v:true}
let hl.gui = {'underline': v:true}
call hlset([hl])

Of course, this is not a real link and vim is not blending anything. If you modify CocSemVariable, these changes will not appear in CocSemMutableVariable. You can mitigate this to some extent via

function! Tweaks()
   ... code from above ...
endfunction
au ColorScheme * call Tweaks()

So that when groups are changed through changing colorschemes (the usual situation), the dependent groups are automatically updated.