[Bug]: :where(:nth-child(n), :nth-child(n) ~ *) has second :nth-child parentheses removed

[Zr4g0n]

Describe the bug

The following selector fails when using minifySelectors

:where(:nth-child(7),:nth-child(7) ~ *){
is changed to
:where(:nth-child(7),:nth-child~*){

As long as the content of the second :nth-child() is the same as the first, the second :nth-child() gets the parentheses removed.

disabling minifySelectors fixes this.

this also happens with :is(...) and :not(...)
this also happens with :nth-last-child(), :nth-of-type() and :nth-last-of-type()

Expected behaviour

:where(:nth-child(7),:nth-child(7) ~ *){
gets changed to
:where(:nth-child(7),:nth-child(7)~*){

Steps to reproduce

1: clone https://github.com/Zr4g0n/nth-child-css-bug
2: run npx postcss input.css > output.css
3: output.css has broken selector

Version

4.1.11

Preset

default

Environment

System:
  OS: Windows 10 10.0.19042
  CPU: (24) x64 AMD Ryzen 9 3900X 12-Core Processor
  Memory: 36.40 GB / 63.92 GB
Windows PowerShell

Package details

folder@ C:\Dev Personal\folder
+-- cssnano@5.0.8
| +-- cssnano-preset-default@5.1.4
| | +-- css-declaration-sorter@6.1.3
| | | `-- postcss@8.3.11 deduped
| | +-- cssnano-utils@2.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-calc@8.0.0
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-colormin@5.2.0
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-convert-values@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-discard-comments@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-discard-duplicates@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-discard-empty@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-discard-overridden@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-merge-longhand@5.0.2
| | | +-- postcss@8.3.11 deduped
| | | `-- stylehacks@5.0.1
| | |   `-- postcss@8.3.11 deduped
| | +-- postcss-merge-rules@5.0.2
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-minify-font-values@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-minify-gradients@5.0.2
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-minify-params@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-minify-selectors@5.1.0
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-normalize-charset@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-normalize-display-values@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-normalize-positions@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-normalize-repeat-style@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-normalize-string@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-normalize-timing-functions@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-normalize-unicode@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-normalize-url@5.0.2
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-normalize-whitespace@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-ordered-values@5.0.2
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-reduce-initial@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-reduce-transforms@5.0.1
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-svgo@5.0.2
| | | `-- postcss@8.3.11 deduped
| | +-- postcss-unique-selectors@5.0.1
| | | `-- postcss@8.3.11 deduped
| | `-- postcss@8.3.11 deduped
| `-- postcss@8.3.11 deduped
+-- postcss-cli@9.0.1
| +-- postcss-reporter@7.0.4
| | `-- postcss@8.3.11 deduped
| `-- postcss@8.3.11 deduped
`-- postcss@8.3.11

Additional context

The following have no issues:
:where(:nth-child(6),:nth-child(7) ~ *){ -> :where(:nth-child(6),:nth-child(7)~*){
:is(:nth-child(foo),:nth-child(bar) ~ *){ -> :is(:nth-child(foo),:nth-child(bar)~*){

The following all results in the second parenthesis getting removed:
:where(:nth-child(7),:nth-child(7) ~ *){ -> :where(:nth-child(7),:nth-child~*){
:where(:nth-child(2),:nth-child(2) ~ *){ -> :where(:nth-child(2),:nth-child~*){
:where(:nth-child(1),:nth-child(1) ~ *){ -> :where(:first-child),:nth-child~*){
:where(:nth-child(foo),:nth-child(foo) ~ *){ -> :where(:nth-child(foo),:nth-child~*){
:is(:nth-child(7),:nth-child(7) ~ *){ -> :is(:nth-child(7),:nth-child~*){
:not(:nth-child(7),:nth-child(7) ~ *){ -> :not(:nth-child(7),:nth-child~*){
:where(:nth-child(7),:nth-child(7) #id){ -> :where(:nth-child(7),:nth-child #id){

I found the bug initially when using a selector looking for a class's direct children when there's exactly 6 of them.

.class>*:where(:first-child:nth-last-child(6), 
               :first-child:nth-last-child(6) ~ *){

[Zr4g0n]

For reasons beyond me, this actually works:
:where(:nth-child(7),:nth-child(7 /**/ ) ~ *){ -> :where(:nth-child(7),:nth-child(7)~*){

EDIT: further testing showed that the extra space, not the comment in and of itself, was enough. In other words, this also works:
:where(:nth-child(7),:nth-child(7 ) ~ *){ -> :where(:nth-child(7),:nth-child(7)~*){

EDIT2:
This also fails
:where(:nth-child(6), :nth-last-child(6), :nth-of-type(6), :nth-last-of-type(6)){ -> :where(:nth-child(6),:nth-last-child,:nth-of-type,:nth-last-of-type){

[ludofischer]

Confirmed in 5.0.12

[ludofischer]

I think there might be a bug in postcss-selector-parser, because the argument of the second :nth-child pseudo selector is not in the AST when it's identical to the argument of the first.