Version

3.2.31

Reproduction link

codesandbox.io

Steps to reproduce

1. lang="less"
2. ::v-deep() { xxxx many children }
3. compiler error

I know it can be used by ::v-deep(.child1) ::v-deep(.child2) ..., but it's necessary ::v-deep() { xxxx many children } when have many children.

What is expected?

Emit css content same as lang="scss"

What is actually happening?

compiler error

looks like ::v-deep() recognise by less compiler.

Well, ::v-deep() doesn't work like you want it to. it needs a selector as an argument. That's not a bug with less, it just doesn't support what you want it to.