Re-enable removed Windows builds

[nlohmann]

Description

In #4871, we had to remove several Windows builds to fix the CI on short notice. One reason for this was the recently deprecated windows-2019 image which was the base for the Windows-based Clang and Clang-CL builds, as well as for certain MSVC versions.

The goal of this issue is to re-enable them where possible.

Reproduction steps

This is the previous workflow file .github/workflows/windows.yml:

name: Windows

on:
  push:
    branches:
      - develop
      - master
      - release/*
  pull_request:
  workflow_dispatch:

permissions:
  contents: read

concurrency:
  group: ${{ github.workflow }}-${{ github.ref || github.run_id }}
  cancel-in-progress: true

jobs:
  mingw:
    runs-on: windows-2019
    strategy:
      matrix:
        architecture: [x64, x86]

    steps:
      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
      - name: Set up MinGW
        uses: egor-tensin/setup-mingw@84c781b557efd538dec66bde06988d81cd3138cf # v2.2.0
        with:
          platform: ${{ matrix.architecture }}
          version: 12.2.0 # https://github.com/egor-tensin/setup-mingw/issues/14
      - name: Run CMake
        run: cmake -S . -B build -G "MinGW Makefiles" -DCMAKE_BUILD_TYPE=Debug -DJSON_BuildTests=On
      - name: Build
        run: cmake --build build --parallel 10
      - name: Test
        run: cd build ; ctest -j 10 -C Debug --output-on-failure

  msvc:
    strategy:
      matrix:
        runs_on: [windows-2019, windows-2022]
        build_type: [Debug, Release]
        architecture: [Win32, x64]
        std_version: [default, latest]

    runs-on: ${{ matrix.runs_on }}

    steps:
      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
      - name: Set generator
        id: generator
        run: |
          if [ "${{ matrix.runs_on }}" = "windows-2019" ]; then
            echo "generator=Visual Studio 16 2019" >> $GITHUB_ENV
          else
            echo "generator=Visual Studio 17 2022" >> $GITHUB_ENV
          fi
        shell: bash
      - name: Set extra CXX_FLAGS for latest std_version
        id: cxxflags
        run: |
          if [ "${{ matrix.std_version }}" = "latest" ]; then
            echo "flags=/permissive- /std:c++latest /utf-8 /W4 /WX" >> $GITHUB_ENV
          else
            echo "flags=/W4 /WX" >> $GITHUB_ENV
          fi
        shell: bash
      - name: Run CMake (Release)
        run: cmake -S . -B build -G "$env:generator" -A ${{ matrix.architecture }} -DJSON_BuildTests=On -DCMAKE_CXX_FLAGS="$env:flags"
        if: matrix.build_type == 'Release'
        shell: pwsh
      - name: Run CMake (Debug)
        run: cmake -S . -B build -G "$env:generator" -A ${{ matrix.architecture }} -DJSON_BuildTests=On -DJSON_FastTests=ON -DCMAKE_CXX_FLAGS="$env:flags"
        if: matrix.build_type == 'Debug'
        shell: pwsh
      - name: Build
        run: cmake --build build --config ${{ matrix.build_type }} --parallel 10
      - name: Test
        run: cd build ; ctest -j 10 -C ${{ matrix.build_type }} --output-on-failure

  clang:
    runs-on: windows-2019
    strategy:
      matrix:
        version: [11, 12, 13, 14, 15]

    steps:
      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
      - name: Install Clang
        run: curl -fsSL -o LLVM${{ matrix.version }}.exe https://github.com/llvm/llvm-project/releases/download/llvmorg-${{ matrix.version }}.0.0/LLVM-${{ matrix.version }}.0.0-win64.exe ; 7z x LLVM${{ matrix.version }}.exe -y -o"C:/Program Files/LLVM"
      - name: Run CMake
        run: cmake -S . -B build -DCMAKE_CXX_COMPILER="C:/Program Files/LLVM/bin/clang++.exe" -G"MinGW Makefiles" -DCMAKE_BUILD_TYPE=Debug -DJSON_BuildTests=On
      - name: Build
        run: cmake --build build --parallel 10
      - name: Test
        run: cd build ; ctest -j 10 -C Debug --exclude-regex "test-unicode" --output-on-failure

  clang-cl-12:
    runs-on: windows-2019
    strategy:
      matrix:
        architecture: [Win32, x64]

    steps:
      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
      - name: Run CMake
        run: cmake -S . -B build -G "Visual Studio 16 2019" -A ${{ matrix.architecture }} -T ClangCL -DJSON_BuildTests=On
      - name: Build
        run: cmake --build build --config Debug --parallel 10
      - name: Test
        run: cd build ; ctest -j 10 -C Debug --exclude-regex "test-unicode" --output-on-failure

Expected vs. actual results

Expected:

• Windows build is green with as many versions of MSVC/Clang/Clang-CL working.

Minimal code example

N/A

Error messages

> Windows Server 2019 has been retired. The Windows Server 2019 image has been removed as of 2025-06-30. For more details, see https://github.com/actions/runner-images/issues/12045

Compiler and operating system

Windows

Library version

develop

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

[CrysisKiller]

I was wondering if llvm-mingw could be used for the Clang job on windows-2022, since the old windows-2019 image is gone.
I tried setting up CMake to use clang.exe / clang++.exe with MinGW Makefiles and passing the --target=x86_64-w64-mingw32 + --sysroot=C:/msys64/mingw64 flags, but the workflow ran into errors.
clang-cl seems to work fine with VS2022, but is it a requirement that the builds must use LLVM’s Clang itself, or could llvm-mingw be an alternative?

[nlohmann]

I'll check.