Invalid path in vg.install installing vg from opam on Windows

[Tchou]

Hi,
I have the following set-up (in a VM):

• Windows 11
• Windows Terminal
• opam installed with winget install OCaml.opam (version 2.4.1)
• either Powershell or Bash (from cygwin installed via opam) as a shell

When doing opam install vg, it fails during installation with the following error message:

[ERROR] At C:\Users\user\AppData\Local\opam\default\.opam-switch\build\vg.0.9.5\vg.install:11:7-11:7::
        illegal escape sequence [skipped]

(this is in powershell, the same error message when using bash, but with Unix paths).
Then opam thinks the package is installed, but the META and library files are not installed. Upon closer investigation, the file vg.install contains:

#vg

doc: [
  "_build/CHANGES.md" {"CHANGES.md"}
  "_build/LICENSE.md" {"LICENSE.md"}
  "_build/README.md" {"README.md"}
  "_build/doc/image_howto.mld" {"odoc-pages/image_howto.mld"}
  "_build/doc/index.mld" {"odoc-pages/index.mld"}
  "_build/doc/semantics.mld" {"odoc-pages/semantics.mld"}
  "_build/doc/tutorial.mld" {"odoc-pages/tutorial.mld"}
  "doc\doc-aeo.png" {"odoc-assets/doc-aeo.png"}
  "doc\doc-anz.png" {"odoc-assets/doc-anz.png"}
  "doc\doc-caps.png" {"odoc-assets/doc-caps.png"}
  "doc\doc-circle-outline.png" {"odoc-assets/doc-circle-outline.png"}
  "doc\doc-dot.png" {"odoc-assets/doc-dot.png"}
  "doc\doc-earcs.png" {"odoc-assets/doc-earcs.png"}
  "doc\doc-gray-circle.png" {"odoc-assets/doc-gray-circle.png"}
  "doc\doc-gray-square.png" {"odoc-assets/doc-gray-square.png"}
  "doc\doc-joins.png" {"odoc-assets/doc-joins.png"}
  "doc\doc-scatter-plot.png" {"odoc-assets/doc-scatter-plot.png"}
  "doc\doc-subpaths.png" {"odoc-assets/doc-subpaths.png"} ]
lib: [
  "_build/opam" {"opam"}
...

Line 11, column 7 corresponds to \d. Now I'm not sure whether this is a bug in opam or vg, but vg is the only package so far for which this happens (e.g. gg and many of your other packages install just fine).

[dbuenzli]

This is likely a bug in topkg. I suspect there's stray use of the Filename module to generate the .install file which results in a mixture of unix and windows paths.

[dbuenzli]

Ah no in fact it's like this part of the package definition that generates platform specific paths.