Does this work?

Author: tjammer

.github/workflows/main.yml

@@ -1,47 +1,83 @@
-name: build
+name: Windows CI
 
 on:
   - push
   - pull_request
 
+env:
+  OPAMROOT: D:\opamroot
+  OPAMSOLVERTIMEOUT: 120
+  MSYS: winsymlinks:native
 jobs:
-  build-and-test:
-    name: build
-
+  build:
     strategy:
       fail-fast: false
       matrix:
-        include:
-          - { ocaml-compiler: 5.0.0, os: ubuntu-latest }
-          - { ocaml-compiler: 5.0.0, os: macos-latest }
-          - { ocaml-compiler: 4.14.0, os: ubuntu-latest }
-          - { ocaml-compiler: 4.14.0, os: windows-latest }
-          - { ocaml-compiler: 4.14.0, os: macos-latest }
-          - { ocaml-compiler: 4.08.0, os: ubuntu-latest }
-
+        os:
+          - windows-latest
+        windows_env:
+          - cygwin
+          - msys2
     runs-on: ${{ matrix.os }}
-
     steps:
-      - name: Checkout code
-        uses: actions/checkout@v2
+      - name: Checkout tree
+        uses: actions/checkout@v6
         with:
-          submodules: true
+          fetch-depth: 2
 
-      - name: Re-synchronize the package index files on Ubuntu
-        if: matrix.os == 'ubuntu-latest'
-        run: sudo apt-get update
+      - name: Download install.ps1
+        run: |
+          [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
+          (New-Object System.Net.WebClient).DownloadFile("https://raw.githubusercontent.com/ocaml/opam/master/shell/install.ps1", ".\install.ps1")
 
-      - name: Use OCaml ${{ matrix.ocaml-compiler }}
-        uses: ocaml/setup-ocaml@v3
+      - name: Restore opam cache
+        id: cache-opam
+        uses: actions/cache/restore@v4
         with:
-          ocaml-compiler: ${{ matrix.ocaml-compiler }}
+          path: |
+            D:\opam\bin
+            D:\opamroot
+          key: ${{ runner.os }}-${{ matrix.windows_env }}-opam-${{ hashFiles('install.ps1') }}
+
+      - name: Add MSys2 to PATH and install prerequisites
+        if: matrix.windows_env == 'msys2'
+        run: |
+          "C:\msys64" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
+          C:\msys64\usr\bin\pacman.exe --noconfirm -Syuu  # Core update (in case any core packages are outdated)
+          C:\msys64\usr\bin\pacman.exe --noconfirm -Syuu m4 make mingw-w64-i686-gcc mingw-w64-x86_64-gcc rsync unzip
 
-      - run: opam pin -n .
+      - name: Install opam
+        if: steps.cache-opam.outputs.cache-hit != 'true'
+        run: |
+          Invoke-Expression "& ./install.ps1 -OpamBinDir 'D:\opam\bin'"
 
-      - run: opam install . --deps-only
+      - name: Add opam to PATH
+        run: |
+          D:\opam\bin\opam --version
+          "D:\opam\bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
+
+      - name: Init opam
+        if: steps.cache-opam.outputs.cache-hit != 'true'
+        run: opam init --yes --no-setup ${{ matrix.windows_env == 'msys2' && '--cygwin-local-install' || '' }}
+
+      - name: Restrict testing to available compilers
+        if: steps.cache-opam.outputs.cache-hit != 'true'
+        # TODO Amend this lowerbound as older compiler packages are updated
+        run: opam switch set-invariant --formula "`"ocaml`" {>= `"4.13`"}"
+
+      - name: Save opam cache
+        if: steps.cache-opam.outputs.cache-hit != 'true'
+        uses: actions/cache/save@v4
+        with:
+          path: |
+            D:\opam\bin
+            D:\opamroot
+          key: ${{ steps.cache-opam.outputs.cache-primary-key }}
 
-      - run: opam exec -- dune build
-        if: matrix.ocaml-compiler != '4.08.0'
+      - name: Print version information
+        run: |
+          opam --version
+          opam exec -- ocaml -version
+          opam var
 
-      - run: opam install .
-        if: matrix.os != 'windows-latest'
+      - run: opam install . -y