👷 build ci_module_cpp20 with MSVC

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/check_amalgamation.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf # v2.11.1
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
         with:
           egress-policy: audit
 
@@ -34,7 +34,7 @@ jobs:
 
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf # v2.11.1
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
         with:
           egress-policy: audit
 

.github/workflows/cifuzz.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Harden Runner
-      uses: step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf # v2.11.1
+      uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
       with:
         egress-policy: audit
 

.github/workflows/codeql-analysis.yml

@@ -27,7 +27,7 @@ jobs:
 
     steps:
     - name: Harden Runner
-      uses: step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf # v2.11.1
+      uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
       with:
         egress-policy: audit
 
@@ -36,14 +36,14 @@ jobs:
 
     # Initializes the CodeQL tools for scanning.
     - name: Initialize CodeQL
-      uses: github/codeql-action/init@fc7e4a0fa01c3cca5fd6a1fddec5c0740c977aa2 # v3.28.14
+      uses: github/codeql-action/init@28deaeda66b76a05916b6923827895f2b14ab387 # v3.28.16
       with:
         languages: c-cpp
 
     # Autobuild attempts to build any compiled languages  (C/C++, C#, or Java).
     # If this step fails, then you should remove it and run the build manually (see below)
     - name: Autobuild
-      uses: github/codeql-action/autobuild@fc7e4a0fa01c3cca5fd6a1fddec5c0740c977aa2 # v3.28.14
+      uses: github/codeql-action/autobuild@28deaeda66b76a05916b6923827895f2b14ab387 # v3.28.16
 
     - name: Perform CodeQL Analysis
-      uses: github/codeql-action/analyze@fc7e4a0fa01c3cca5fd6a1fddec5c0740c977aa2 # v3.28.14
+      uses: github/codeql-action/analyze@28deaeda66b76a05916b6923827895f2b14ab387 # v3.28.16

.github/workflows/comment_check_amalgamation.yml

@@ -19,7 +19,7 @@ jobs:
       pull-requests: write
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf # v2.11.1
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
         with:
           egress-policy: audit
 

.github/workflows/dependency-review.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf # v2.11.1
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
         with:
           egress-policy: audit
 

.github/workflows/labeler.yml

@@ -17,7 +17,7 @@ jobs:
 
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf # v2.11.1
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
         with:
           egress-policy: audit
 

.github/workflows/publish_documentation.yml

@@ -27,7 +27,7 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf # v2.11.1
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
         with:
           egress-policy: audit
 

.github/workflows/scorecards.yml

@@ -36,7 +36,7 @@ jobs:
 
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf # v2.11.1
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
         with:
           egress-policy: audit
 
@@ -76,6 +76,6 @@ jobs:
 
       # Upload the results to GitHub's code scanning dashboard.
       - name: "Upload to code-scanning"
-        uses: github/codeql-action/upload-sarif@fc7e4a0fa01c3cca5fd6a1fddec5c0740c977aa2 # v3.28.14
+        uses: github/codeql-action/upload-sarif@28deaeda66b76a05916b6923827895f2b14ab387 # v3.28.16
         with:
           sarif_file: results.sarif

.github/workflows/stale.yml

@@ -16,7 +16,7 @@ jobs:
 
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf # v2.11.1
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
         with:
           egress-policy: audit
 

.github/workflows/ubuntu.yml

@@ -23,7 +23,7 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@28983e0d3955dba2bb0a6810caae0c6cf268ec0c # v4.0.0
+        uses: lukka/get-cmake@57c20a23a6cac5b90f31864439996e5b206df9dc # v4.0.1
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -46,7 +46,7 @@ jobs:
         target: [ci_test_amalgamation, ci_test_single_header, ci_cppcheck, ci_cpplint, ci_reproducible_tests, ci_non_git_tests, ci_offline_testdata, ci_reuse_compliance, ci_test_valgrind]
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf # v2.11.1
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
         with:
           egress-policy: audit
 
@@ -54,7 +54,7 @@ jobs:
         run: sudo apt-get update ; sudo apt-get install -y valgrind
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@28983e0d3955dba2bb0a6810caae0c6cf268ec0c # v4.0.0
+        uses: lukka/get-cmake@57c20a23a6cac5b90f31864439996e5b206df9dc # v4.0.1
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -71,7 +71,7 @@ jobs:
         run: apt-get update ; apt-get install -y git clang-tools iwyu unzip
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@28983e0d3955dba2bb0a6810caae0c6cf268ec0c # v4.0.0
+        uses: lukka/get-cmake@57c20a23a6cac5b90f31864439996e5b206df9dc # v4.0.1
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -88,7 +88,7 @@ jobs:
         run: apt-get update ; apt-get install -y build-essential unzip wget git libssl-dev
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@28983e0d3955dba2bb0a6810caae0c6cf268ec0c # v4.0.0
+        uses: lukka/get-cmake@57c20a23a6cac5b90f31864439996e5b206df9dc # v4.0.1
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -98,7 +98,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf # v2.11.1
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
         with:
           egress-policy: audit
 
@@ -143,12 +143,12 @@ jobs:
     strategy:
       matrix:
         # older GCC docker images (4, 5, 6) fail to check out code
-        compiler: ['7', '8', '9', '10', '11', '12', '13', '14', 'latest']
+        compiler: ['7', '8', '9', '10', '11', '12', '13', '14', '15', 'latest']
     container: gcc:${{ matrix.compiler }}
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@28983e0d3955dba2bb0a6810caae0c6cf268ec0c # v4.0.0
+        uses: lukka/get-cmake@57c20a23a6cac5b90f31864439996e5b206df9dc # v4.0.1
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -165,7 +165,7 @@ jobs:
         run: apt-get update ; apt-get install -y unzip git
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@28983e0d3955dba2bb0a6810caae0c6cf268ec0c # v4.0.0
+        uses: lukka/get-cmake@57c20a23a6cac5b90f31864439996e5b206df9dc # v4.0.1
       - name: Set env FORCE_STDCPPFS_FLAG for clang 7 / 8 / 9 / 10
         run: echo "JSON_FORCED_GLOBAL_COMPILE_OPTIONS=-DJSON_HAS_FILESYSTEM=0;-DJSON_HAS_EXPERIMENTAL_FILESYSTEM=0" >> "$GITHUB_ENV"
         if: ${{ matrix.compiler == '7' || matrix.compiler == '8' || matrix.compiler == '9' || matrix.compiler == '10' }}
@@ -183,7 +183,7 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@28983e0d3955dba2bb0a6810caae0c6cf268ec0c # v4.0.0
+        uses: lukka/get-cmake@57c20a23a6cac5b90f31864439996e5b206df9dc # v4.0.1
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
@@ -201,7 +201,7 @@ jobs:
         run: apt-get update ; apt-get install -y git unzip
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - name: Get latest CMake and ninja
-        uses: lukka/get-cmake@28983e0d3955dba2bb0a6810caae0c6cf268ec0c # v4.0.0
+        uses: lukka/get-cmake@57c20a23a6cac5b90f31864439996e5b206df9dc # v4.0.1
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build with libc++
@@ -221,6 +221,21 @@ jobs:
       - name: Build
         run: cmake --build build --target ci_cuda_example
 
+  ci_module_cpp20:
+    strategy:
+      matrix:
+        container: ['gcc:latest', 'silkeh/clang:latest']
+    runs-on: ubuntu-latest
+    container: ${{ matrix.container }}
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      - name: Get latest CMake and ninja
+        uses: lukka/get-cmake@57c20a23a6cac5b90f31864439996e5b206df9dc # v4.0.1
+      - name: Run CMake
+        run: cmake -S . -B build -DJSON_CI=On
+      - name: Build
+        run: cmake --build build --target ci_module_cpp20
+
   ci_icpc:
     runs-on: ubuntu-latest
     container: ghcr.io/nlohmann/json-ci:v2.2.0
@@ -233,6 +248,24 @@ jobs:
           . /opt/intel/oneapi/setvars.sh
           cmake --build build --target ci_icpc
 
+  ci_emscripten:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
+        with:
+          egress-policy: audit
+
+      - name: Install emscripten
+        uses: mymindstorm/setup-emsdk@v14
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      - name: Get latest CMake and ninja
+        uses: lukka/get-cmake@57c20a23a6cac5b90f31864439996e5b206df9dc # v4.0.1
+      - name: Run CMake
+        run: cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=$EMSDK/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake -GNinja
+      - name: Build
+        run: cmake --build build
+
   ci_test_documentation:
     runs-on: ubuntu-latest
     strategy:
@@ -240,7 +273,7 @@ jobs:
         target: [ci_test_examples, ci_test_build_documentation]
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf # v2.11.1
+        uses: step-security/harden-runner@0634a2670c59f64b4a01f0f96f84700a4088b9f0 # v2.12.0
         with:
           egress-policy: audit
 

.github/workflows/windows.yml

@@ -37,69 +37,48 @@ jobs:
       - name: Test
         run: cd build ; ctest -j 10 -C Debug --output-on-failure
 
-  msvc2019:
-    runs-on: windows-2019
+  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: Run CMake
-      run: cmake -S . -B build -G "Visual Studio 16 2019" -A ${{ matrix.architecture }} -DJSON_BuildTests=On -DCMAKE_CXX_FLAGS="/W4 /WX"
-      if: matrix.build_type == 'Release'
-    - name: Run CMake
-      run: cmake -S . -B build -G "Visual Studio 16 2019" -A ${{ matrix.architecture }} -DJSON_BuildTests=On -DJSON_FastTests=ON -DCMAKE_CXX_FLAGS="/W4 /WX"
-      if: matrix.build_type == 'Debug'
-    - 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
-
-  msvc2019_latest:
-    runs-on: windows-2019
-
-    steps:
-    - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
-    - name: Run CMake
-      run: cmake -S . -B build -G "Visual Studio 16 2019" -DJSON_BuildTests=On -DCMAKE_CXX_FLAGS="/permissive- /std:c++latest /utf-8 /W4 /WX"
-    - name: Build
-      run: cmake --build build --config Release --parallel 10
-    - name: Test
-      run: cd build ; ctest -j 10 -C Release --output-on-failure
-
-  msvc2022:
-    runs-on: windows-2022
-    strategy:
-      matrix:
-        build_type: [Debug, Release]
-        architecture: [Win32, x64]
-
-    steps:
-    - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
-    - name: Run CMake
-      run: cmake -S . -B build -G "Visual Studio 17 2022" -A ${{ matrix.architecture }} -DJSON_BuildTests=On -DCMAKE_CXX_FLAGS="/W4 /WX"
-      if: matrix.build_type == 'Release'
-    - name: Run CMake
-      run: cmake -S . -B build -G "Visual Studio 17 2022" -A ${{ matrix.architecture }} -DJSON_BuildTests=On -DJSON_FastTests=ON -DCMAKE_CXX_FLAGS="/W4 /WX"
-      if: matrix.build_type == 'Debug'
-    - name: Build
-      run: cmake --build build --config ${{ matrix.build_type }} --parallel 10
-    - name: Test
-      run: cd build ; ctest -j 10 -C ${{ matrix.build_type }} --output-on-failure
-
-  msvc2022_latest:
-    runs-on: windows-2022
-
-    steps:
-    - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
-    - name: Run CMake
-      run: cmake -S . -B build -G "Visual Studio 17 2022" -DJSON_BuildTests=On -DCMAKE_CXX_FLAGS="/permissive- /std:c++latest /utf-8 /W4 /WX"
-    - name: Build
-      run: cmake --build build --config Release --parallel 10
-    - name: Test
-      run: cd build ; ctest -j 10 -C Release --output-on-failure
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      - name: Set generator
+        id: generator
+        run: |
+          if [ "${{ 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
@@ -118,7 +97,7 @@ jobs:
       - name: Test
         run: cd build ; ctest -j 10 -C Debug --exclude-regex "test-unicode" --output-on-failure
 
-  clang-cl-11:
+  clang-cl-12:
     runs-on: windows-2019
     strategy:
       matrix:
@@ -132,3 +111,12 @@ jobs:
         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
+
+  ci_module_cpp20:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      - name: Run CMake (Debug)
+        run: cmake -S . -B build -G "Visual Studio 17 2022" -DJSON_CI=ON -DCMAKE_CXX_FLAGS="/permissive- /std:c++latest /utf-8 /W4 /WX"
+      - name: Build
+        run: cmake --build build --config Debug --target ci_module_cpp20

cmake/ci.cmake

@@ -659,6 +659,17 @@ add_custom_target(ci_cuda_example
     COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/build_cuda_example
 )
 
+###############################################################################
+# C++ 20 modules
+###############################################################################
+
+add_custom_target(ci_module_cpp20
+    COMMAND ${CMAKE_COMMAND}
+        -DCMAKE_BUILD_TYPE=Debug -GNinja
+        -S${PROJECT_SOURCE_DIR}/tests/module_cpp20 -B${PROJECT_BINARY_DIR}/ci_module_cpp20
+    COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/ci_module_cpp20
+)
+
 ###############################################################################
 # Intel C++ Compiler
 ###############################################################################

cmake/requirements/requirements-cpplint.txt

@@ -1 +1 @@
-cpplint==2.0.1
+cpplint==2.0.2

docs/mkdocs/docs/community/quality_assurance.md

@@ -57,6 +57,7 @@ violations will result in a failed build.
         | Clang 19.1.7                                 | x86_64       | Ubuntu 22.04.1 LTS       | GitHub    |
         | Clang 20.1.1                                 | x86_64       | Ubuntu 22.04.1 LTS       | GitHub    |
         | Clang 21.0.0                                 | x86_64       | Ubuntu 22.04.1 LTS       | GitHub    |
+        | Emscripten 4.0.6                             | x86_64       | Ubuntu 22.04.1 LTS       | GitHub    |
         | GNU 4.8.5                                    | x86_64       | Ubuntu 22.04.1 LTS       | GitHub    |
         | GNU 4.9.3                                    | x86_64       | Ubuntu 22.04.1 LTS       | GitHub    |
         | GNU 5.5.0                                    | x86_64       | Ubuntu 22.04.1 LTS       | GitHub    |
@@ -75,6 +76,8 @@ violations will result in a failed build.
         | GNU 13.3.0                                   | x86_64       | Ubuntu 22.04.1 LTS       | GitHub    |
         | GNU 14.2.0                                   | x86_64       | Ubuntu 22.04.1 LTS       | GitHub    |
         | GNU 14.2.0                                   | arm64        | Linux 6.1.100            | Cirrus CI |
+        | GNU 15.1.0                                   | x86_64       | Ubuntu 22.04.1 LTS       | GitHub    |
+        | icpc (ICC) 2021.5.0 20211109                 | x86_64       | Ubuntu 20.04.3 LTS       | GitHub    |
         | MSVC 19.0.24241.7                            | x86          | Windows 8.1              | AppVeyor  |
         | MSVC 19.16.27035.0                           | x86          | Windows-10 (Build 14393) | AppVeyor  |
         | MSVC 19.29.30157.0                           | x86          | Windows 10 (Build 17763) | GitHub    |

docs/mkdocs/requirements.txt

@@ -2,7 +2,7 @@ wheel==0.45.1
 
 mkdocs==1.6.1                                       # documentation framework
 mkdocs-git-revision-date-localized-plugin==1.4.5    # plugin "git-revision-date-localized"
-mkdocs-material==9.6.11                             # theme for mkdocs
+mkdocs-material==9.6.12                             # theme for mkdocs
 mkdocs-material-extensions==1.3.1                   # extensions
 mkdocs-minify-plugin==0.8.0                         # plugin "minify"
 mkdocs-redirects==1.2.2                             # plugin "redirects"

include/nlohmann/detail/conversions/from_json.hpp

@@ -13,9 +13,6 @@
 #include <forward_list> // forward_list
 #include <iterator> // inserter, front_inserter, end
 #include <map> // map
-#ifdef JSON_HAS_CPP_17
-    #include <optional> // optional
-#endif
 #include <string> // string
 #include <tuple> // tuple, make_tuple
 #include <type_traits> // is_arithmetic, is_same, is_enum, underlying_type, is_convertible
@@ -32,6 +29,15 @@
 #include <nlohmann/detail/string_concat.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
+// include after macro_scope.hpp
+#ifdef JSON_HAS_CPP_17
+    #include <optional> // optional
+#endif
+
+#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
+    #include <string_view> // u8string_view
+#endif
+
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
@@ -47,7 +53,6 @@ inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
 }
 
 #ifdef JSON_HAS_CPP_17
-#ifndef JSON_USE_IMPLICIT_CONVERSIONS
 template<typename BasicJsonType, typename T>
 void from_json(const BasicJsonType& j, std::optional<T>& opt)
 {
@@ -60,8 +65,6 @@ void from_json(const BasicJsonType& j, std::optional<T>& opt)
         opt.emplace(j.template get<T>());
     }
 }
-
-#endif // JSON_USE_IMPLICIT_CONVERSIONS
 #endif // JSON_HAS_CPP_17
 
 // overloads for basic_json template parameters
@@ -540,7 +543,10 @@ inline void from_json(const BasicJsonType& j, std_fs::path& p)
         JSON_THROW(type_error::create(302, concat("type must be string, but is ", j.type_name()), &j));
     }
     const auto& s = *j.template get_ptr<const typename BasicJsonType::string_t*>();
-#ifdef JSON_HAS_CPP_20
+    // Checking for C++20 standard or later can be insufficient in case the
+    // library support for char8_t is either incomplete or was disabled
+    // altogether. Use the __cpp_lib_char8_t feature test instead.
+#if defined(__cpp_lib_char8_t) && (__cpp_lib_char8_t >= 201907L)
     p = std_fs::path(std::u8string_view(reinterpret_cast<const char8_t*>(s.data()), s.size()));
 #else
     p = std_fs::u8path(s); // accepts UTF-8 encoded std::string in C++17, deprecated in C++20

include/nlohmann/detail/conversions/to_json.hpp

@@ -15,7 +15,8 @@
 
 #include <algorithm> // copy
 #include <iterator> // begin, end
-#include <string> // string
+#include <memory> // allocator_traits
+#include <string> //  basic_string, char_traits
 #include <tuple> // tuple, get
 #include <type_traits> // is_same, is_constructible, is_floating_point, is_enum, underlying_type
 #include <utility> // move, forward, declval, pair
@@ -267,7 +268,7 @@ struct external_constructor<value_t::object>
 #ifdef JSON_HAS_CPP_17
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_constructible<BasicJsonType, T>::value, int> = 0>
-void to_json(BasicJsonType& j, const std::optional<T>& opt)
+void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept
 {
     if (opt.has_value())
     {
@@ -440,15 +441,21 @@ inline void to_json(BasicJsonType& j, const T& t)
 }
 
 #if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
+#if defined(__cpp_lib_char8_t)
+template<typename BasicJsonType, typename Tr, typename Allocator>
+inline void to_json(BasicJsonType& j, const std::basic_string<char8_t, Tr, Allocator>& s)
+{
+    using OtherAllocator = typename std::allocator_traits<Allocator>::template rebind_alloc<char>;
+    j = std::basic_string<char, std::char_traits<char>, OtherAllocator>(s.begin(), s.end(), s.get_allocator());
+}
+#endif
+
 template<typename BasicJsonType>
 inline void to_json(BasicJsonType& j, const std_fs::path& p)
 {
-#ifdef JSON_HAS_CPP_20
-    const std::u8string s = p.u8string();
-    j = std::string(s.begin(), s.end());
-#else
-    j = p.u8string(); // returns std::string in C++17
-#endif
+    // Returns either a std::string or a std::u8string depending whether library
+    // support for char8_t is enabled.
+    j = p.u8string();
 }
 #endif
 

include/nlohmann/detail/input/binary_reader.hpp

@@ -53,7 +53,7 @@ enum class cbor_tag_handler_t
 
 @note from https://stackoverflow.com/a/1001328/266378
 */
-static inline bool little_endianness(int num = 1) noexcept
+inline bool little_endianness(int num = 1) noexcept
 {
     return *reinterpret_cast<char*>(&num) == 1;
 }
@@ -2826,17 +2826,22 @@ class binary_reader
         {
             return;
         }
-        if constexpr(std::is_integral_v<NumberType>)
+        else if constexpr(std::is_integral_v<NumberType>)
         {
             number = std::byteswap(number);
             return;
         }
-#endif
-        auto* ptr = reinterpret_cast<std::uint8_t*>(&number);
-        for (std::size_t i = 0; i < sz / 2; ++i)
+        else
         {
-            std::swap(ptr[i], ptr[sz - i - 1]);
+#endif
+            auto* ptr = reinterpret_cast<std::uint8_t*>(&number);
+            for (std::size_t i = 0; i < sz / 2; ++i)
+            {
+                std::swap(ptr[i], ptr[sz - i - 1]);
+            }
+#ifdef __cpp_lib_byteswap
         }
+#endif
     }
 
     /*

include/nlohmann/detail/meta/type_traits.hpp

@@ -13,7 +13,9 @@
 #include <tuple> // tuple
 #include <type_traits> // false_type, is_constructible, is_integral, is_same, true_type
 #include <utility> // declval
-
+#if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
+    #include <cstddef> // byte
+#endif
 #include <nlohmann/detail/iterators/iterator_traits.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/meta/call_std/begin.hpp>
@@ -239,6 +241,30 @@ struct char_traits<signed char> : std::char_traits<char>
     }
 };
 
+#if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
+template<>
+struct char_traits<std::byte> : std::char_traits<char>
+{
+    using char_type = std::byte;
+    using int_type = uint64_t;
+
+    static int_type to_int_type(char_type c) noexcept
+    {
+        return static_cast<int_type>(std::to_integer<unsigned char>(c));
+    }
+
+    static char_type to_char_type(int_type i) noexcept
+    {
+        return std::byte(static_cast<unsigned char>(i));
+    }
+
+    static constexpr int_type eof() noexcept
+    {
+        return static_cast<int_type>(std::char_traits<char>::eof());
+    }
+};
+#endif
+
 ///////////////////
 // is_ functions //
 ///////////////////

include/nlohmann/detail/output/binary_writer.hpp

@@ -973,9 +973,9 @@ class binary_writer
         if (JSON_HEDLEY_UNLIKELY(it != BasicJsonType::string_t::npos))
         {
             JSON_THROW(out_of_range::create(409, concat("BSON key cannot contain code point U+0000 (at byte ", std::to_string(it), ")"), &j));
-            static_cast<void>(j);
         }
 
+        static_cast<void>(j);
         return /*id*/ 1ul + name.size() + /*zero-terminator*/1u;
     }
 

include/nlohmann/detail/output/serializer.hpp

@@ -762,7 +762,7 @@ class serializer
 
         // jump to the end to generate the string from backward,
         // so we later avoid reversing the result
-        buffer_ptr += n_chars;
+        buffer_ptr += static_cast<typename decltype(number_buffer)::difference_type>(n_chars);
 
         // Fast int2ascii implementation inspired by "Fastware" talk by Andrei Alexandrescu
         // See: https://www.youtube.com/watch?v=o4-CwDo2zpg

include/nlohmann/detail/string_escape.hpp

@@ -62,7 +62,7 @@ inline StringType escape(StringType s)
  * Note the order of escaping "~1" to "/" and "~0" to "~" is important.
  */
 template<typename StringType>
-static void unescape(StringType& s)
+inline void unescape(StringType& s)
 {
     replace_substring(s, StringType{"~1"}, StringType{"/"});
     replace_substring(s, StringType{"~0"}, StringType{"~"});

include/nlohmann/json.hpp

@@ -39,6 +39,7 @@
 #include <nlohmann/detail/hash.hpp>
 #include <nlohmann/detail/input/binary_reader.hpp>
 #include <nlohmann/detail/input/input_adapters.hpp>
+#include <nlohmann/detail/input/json_sax.hpp>
 #include <nlohmann/detail/input/lexer.hpp>
 #include <nlohmann/detail/input/parser.hpp>
 #include <nlohmann/detail/iterators/internal_iterator.hpp>

single_include/nlohmann/json.hpp

@@ -173,9 +173,6 @@
 #include <forward_list> // forward_list
 #include <iterator> // inserter, front_inserter, end
 #include <map> // map
-#ifdef JSON_HAS_CPP_17
-    #include <optional> // optional
-#endif
 #include <string> // string
 #include <tuple> // tuple, make_tuple
 #include <type_traits> // is_arithmetic, is_same, is_enum, underlying_type, is_convertible
@@ -3129,7 +3126,7 @@ inline StringType escape(StringType s)
  * Note the order of escaping "~1" to "/" and "~0" to "~" is important.
  */
 template<typename StringType>
-static void unescape(StringType& s)
+inline void unescape(StringType& s)
 {
     replace_substring(s, StringType{"~1"}, StringType{"/"});
     replace_substring(s, StringType{"~0"}, StringType{"~"});
@@ -3370,7 +3367,9 @@ NLOHMANN_JSON_NAMESPACE_END
 #include <tuple> // tuple
 #include <type_traits> // false_type, is_constructible, is_integral, is_same, true_type
 #include <utility> // declval
-
+#if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
+    #include <cstddef> // byte
+#endif
 // #include <nlohmann/detail/iterators/iterator_traits.hpp>
 //     __ _____ _____ _____
 //  __|  |   __|     |   | |  JSON for Modern C++
@@ -3779,6 +3778,30 @@ struct char_traits<signed char> : std::char_traits<char>
     }
 };
 
+#if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
+template<>
+struct char_traits<std::byte> : std::char_traits<char>
+{
+    using char_type = std::byte;
+    using int_type = uint64_t;
+
+    static int_type to_int_type(char_type c) noexcept
+    {
+        return static_cast<int_type>(std::to_integer<unsigned char>(c));
+    }
+
+    static char_type to_char_type(int_type i) noexcept
+    {
+        return std::byte(static_cast<unsigned char>(i));
+    }
+
+    static constexpr int_type eof() noexcept
+    {
+        return static_cast<int_type>(std::char_traits<char>::eof());
+    }
+};
+#endif
+
 ///////////////////
 // is_ functions //
 ///////////////////
@@ -4817,6 +4840,15 @@ NLOHMANN_JSON_NAMESPACE_END
 // #include <nlohmann/detail/value_t.hpp>
 
 
+// include after macro_scope.hpp
+#ifdef JSON_HAS_CPP_17
+    #include <optional> // optional
+#endif
+
+#if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
+    #include <string_view> // u8string_view
+#endif
+
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
@@ -4832,7 +4864,6 @@ inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
 }
 
 #ifdef JSON_HAS_CPP_17
-#ifndef JSON_USE_IMPLICIT_CONVERSIONS
 template<typename BasicJsonType, typename T>
 void from_json(const BasicJsonType& j, std::optional<T>& opt)
 {
@@ -4845,8 +4876,6 @@ void from_json(const BasicJsonType& j, std::optional<T>& opt)
         opt.emplace(j.template get<T>());
     }
 }
-
-#endif // JSON_USE_IMPLICIT_CONVERSIONS
 #endif // JSON_HAS_CPP_17
 
 // overloads for basic_json template parameters
@@ -5325,7 +5354,10 @@ inline void from_json(const BasicJsonType& j, std_fs::path& p)
         JSON_THROW(type_error::create(302, concat("type must be string, but is ", j.type_name()), &j));
     }
     const auto& s = *j.template get_ptr<const typename BasicJsonType::string_t*>();
-#ifdef JSON_HAS_CPP_20
+    // Checking for C++20 standard or later can be insufficient in case the
+    // library support for char8_t is either incomplete or was disabled
+    // altogether. Use the __cpp_lib_char8_t feature test instead.
+#if defined(__cpp_lib_char8_t) && (__cpp_lib_char8_t >= 201907L)
     p = std_fs::path(std::u8string_view(reinterpret_cast<const char8_t*>(s.data()), s.size()));
 #else
     p = std_fs::u8path(s); // accepts UTF-8 encoded std::string in C++17, deprecated in C++20
@@ -5380,7 +5412,8 @@ NLOHMANN_JSON_NAMESPACE_END
 
 #include <algorithm> // copy
 #include <iterator> // begin, end
-#include <string> // string
+#include <memory> // allocator_traits
+#include <string> //  basic_string, char_traits
 #include <tuple> // tuple, get
 #include <type_traits> // is_same, is_constructible, is_floating_point, is_enum, underlying_type
 #include <utility> // move, forward, declval, pair
@@ -5914,7 +5947,7 @@ struct external_constructor<value_t::object>
 #ifdef JSON_HAS_CPP_17
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_constructible<BasicJsonType, T>::value, int> = 0>
-void to_json(BasicJsonType& j, const std::optional<T>& opt)
+void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept
 {
     if (opt.has_value())
     {
@@ -6087,15 +6120,21 @@ inline void to_json(BasicJsonType& j, const T& t)
 }
 
 #if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
+#if defined(__cpp_lib_char8_t)
+template<typename BasicJsonType, typename Tr, typename Allocator>
+inline void to_json(BasicJsonType& j, const std::basic_string<char8_t, Tr, Allocator>& s)
+{
+    using OtherAllocator = typename std::allocator_traits<Allocator>::template rebind_alloc<char>;
+    j = std::basic_string<char, std::char_traits<char>, OtherAllocator>(s.begin(), s.end(), s.get_allocator());
+}
+#endif
+
 template<typename BasicJsonType>
 inline void to_json(BasicJsonType& j, const std_fs::path& p)
 {
-#ifdef JSON_HAS_CPP_20
-    const std::u8string s = p.u8string();
-    j = std::string(s.begin(), s.end());
-#else
-    j = p.u8string(); // returns std::string in C++17
-#endif
+    // Returns either a std::string or a std::u8string depending whether library
+    // support for char8_t is enabled.
+    j = p.u8string();
 }
 #endif
 
@@ -9829,7 +9868,7 @@ enum class cbor_tag_handler_t
 
 @note from https://stackoverflow.com/a/1001328/266378
 */
-static inline bool little_endianness(int num = 1) noexcept
+inline bool little_endianness(int num = 1) noexcept
 {
     return *reinterpret_cast<char*>(&num) == 1;
 }
@@ -12602,17 +12641,22 @@ class binary_reader
         {
             return;
         }
-        if constexpr(std::is_integral_v<NumberType>)
+        else if constexpr(std::is_integral_v<NumberType>)
         {
             number = std::byteswap(number);
             return;
         }
-#endif
-        auto* ptr = reinterpret_cast<std::uint8_t*>(&number);
-        for (std::size_t i = 0; i < sz / 2; ++i)
+        else
         {
-            std::swap(ptr[i], ptr[sz - i - 1]);
+#endif
+            auto* ptr = reinterpret_cast<std::uint8_t*>(&number);
+            for (std::size_t i = 0; i < sz / 2; ++i)
+            {
+                std::swap(ptr[i], ptr[sz - i - 1]);
+            }
+#ifdef __cpp_lib_byteswap
         }
+#endif
     }
 
     /*
@@ -12845,6 +12889,8 @@ NLOHMANN_JSON_NAMESPACE_END
 
 // #include <nlohmann/detail/input/input_adapters.hpp>
 
+// #include <nlohmann/detail/input/json_sax.hpp>
+
 // #include <nlohmann/detail/input/lexer.hpp>
 
 // #include <nlohmann/detail/input/parser.hpp>
@@ -16713,9 +16759,9 @@ class binary_writer
         if (JSON_HEDLEY_UNLIKELY(it != BasicJsonType::string_t::npos))
         {
             JSON_THROW(out_of_range::create(409, concat("BSON key cannot contain code point U+0000 (at byte ", std::to_string(it), ")"), &j));
-            static_cast<void>(j);
         }
 
+        static_cast<void>(j);
         return /*id*/ 1ul + name.size() + /*zero-terminator*/1u;
     }
 
@@ -19483,7 +19529,7 @@ class serializer
 
         // jump to the end to generate the string from backward,
         // so we later avoid reversing the result
-        buffer_ptr += n_chars;
+        buffer_ptr += static_cast<typename decltype(number_buffer)::difference_type>(n_chars);
 
         // Fast int2ascii implementation inspired by "Fastware" talk by Andrei Alexandrescu
         // See: https://www.youtube.com/watch?v=o4-CwDo2zpg

tests/module_cpp20/CMakeLists.txt

@@ -0,0 +1,12 @@
+cmake_minimum_required(VERSION 3.28)
+
+project(json_test CXX)
+
+add_executable(json_test)
+
+target_sources(json_test
+    PRIVATE main.cpp
+    PUBLIC FILE_SET cxx_modules TYPE CXX_MODULES FILES json.cpp)
+
+target_compile_features(json_test PUBLIC cxx_std_20)
+target_include_directories(json_test PRIVATE ../../include)

tests/module_cpp20/json.cpp

@@ -0,0 +1,17 @@
+module;
+#include <nlohmann/json.hpp>
+export module json;
+
+export namespace nlohmann
+{
+using ::nlohmann::adl_serializer;
+
+using ::nlohmann::basic_json;
+using ::nlohmann::json_pointer;
+
+using ::nlohmann::json;
+using ::nlohmann::ordered_json;
+using ::nlohmann::ordered_map;
+
+using ::nlohmann::json_pointer;
+}  // namespace nlohmann

tests/module_cpp20/main.cpp

@@ -0,0 +1,6 @@
+import json;
+
+int main()
+{
+    nlohmann::json j;
+}

tests/src/unit-deserialization.cpp

@@ -1134,9 +1134,10 @@ TEST_CASE("deserialization")
     }
 }
 
-// select the types to test - char8_t is only available in C++20
+// select the types to test - char8_t is only available since C++20 if and only
+// if __cpp_char8_t is defined.
 #define TYPE_LIST(...) __VA_ARGS__
-#ifdef JSON_HAS_CPP_20
+#if defined(__cpp_char8_t) && (__cpp_char8_t >= 201811L)
     #define ASCII_TYPES TYPE_LIST(char, wchar_t, char16_t, char32_t, char8_t)
 #else
     #define ASCII_TYPES TYPE_LIST(char, wchar_t, char16_t, char32_t)

tests/src/unit-iterators2.cpp

@@ -955,7 +955,8 @@ TEST_CASE("iterators 2")
                 };
                 json j_expected{"a_key", "b_key", "c_key"};
 
-                auto transformed = j.items() | std::views::transform([](const auto & item)
+                // NOLINTNEXTLINE(fuchsia-trailing-return)
+                auto transformed = j.items() | std::views::transform([](const auto & item) -> std::string_view
                 {
                     return item.key();
                 });

tests/src/unit-msgpack.cpp

@@ -1880,3 +1880,82 @@ TEST_CASE("MessagePack roundtrips" * doctest::skip())
         }
     }
 }
+
+#ifdef JSON_HAS_CPP_17
+// Test suite for verifying MessagePack handling with std::byte input
+TEST_CASE("MessagePack with std::byte")
+{
+
+    SECTION("std::byte compatibility")
+    {
+        SECTION("vector roundtrip")
+        {
+            json original =
+            {
+                {"name", "test"},
+                {"value", 42},
+                {"array", {1, 2, 3}}
+            };
+
+            std::vector<uint8_t> temp = json::to_msgpack(original);
+            // Convert the uint8_t vector to std::byte vector
+            std::vector<std::byte> msgpack_data(temp.size());
+            for (size_t i = 0; i < temp.size(); ++i)
+            {
+                msgpack_data[i] = std::byte(temp[i]);
+            }
+            // Deserialize from std::byte vector back to JSON
+            json from_bytes;
+            CHECK_NOTHROW(from_bytes = json::from_msgpack(msgpack_data));
+
+            CHECK(from_bytes == original);
+        }
+
+        SECTION("empty vector")
+        {
+            const std::vector<std::byte> empty_data;
+            CHECK_THROWS_WITH_AS([&]()
+            {
+                [[maybe_unused]] auto result = json::from_msgpack(empty_data);
+                return true;
+            }
+            (),
+            "[json.exception.parse_error.110] parse error at byte 1: syntax error while parsing MessagePack value: unexpected end of input",
+            json::parse_error&);
+        }
+
+        SECTION("comparison with workaround")
+        {
+            json original =
+            {
+                {"string", "hello"},
+                {"integer", 42},
+                {"float", 3.14},
+                {"boolean", true},
+                {"null", nullptr},
+                {"array", {1, 2, 3}},
+                {"object", {{"key", "value"}}}
+            };
+
+            std::vector<uint8_t> temp = json::to_msgpack(original);
+
+            std::vector<std::byte> msgpack_data(temp.size());
+            for (size_t i = 0; i < temp.size(); ++i)
+            {
+                msgpack_data[i] = std::byte(temp[i]);
+            }
+            // Attempt direct deserialization using std::byte input
+            json direct_result = json::from_msgpack(msgpack_data);
+
+            // Test the workaround approach: reinterpret as unsigned char* and use iterator range
+            const auto* const char_start = reinterpret_cast<unsigned char const*>(msgpack_data.data());
+            const auto* const char_end = char_start + msgpack_data.size();
+            json workaround_result = json::from_msgpack(char_start, char_end);
+
+            // Verify that the final deserialized JSON matches the original JSON
+            CHECK(direct_result == workaround_result);
+            CHECK(direct_result == original);
+        }
+    }
+}
+#endif

tests/src/unit-regression2.cpp

@@ -36,6 +36,14 @@ using ordered_json = nlohmann::ordered_json;
     #include <variant>
 #endif
 
+#ifdef JSON_HAS_CPP_17
+    #if __has_include(<optional>)
+        #include <optional>
+    #elif __has_include(<experimental/optional>)
+        #include <experimental/optional>
+    #endif
+#endif
+
 #ifdef JSON_HAS_CPP_20
     #if __has_include(<span>)
         #include <span>
@@ -388,7 +396,20 @@ struct Example_3810
     Example_3810() = default;
 };
 
-NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(Example_3810, bla); // NOLINT(misc-use-internal-linkage)
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(Example_3810, bla) // NOLINT(misc-use-internal-linkage)
+
+/////////////////////////////////////////////////////////////////////
+// for #4740
+/////////////////////////////////////////////////////////////////////
+
+#ifdef JSON_HAS_CPP_17
+struct Example_4740
+{
+    std::optional<std::string> host = std::nullopt;
+    std::optional<int> port = std::nullopt;
+    NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(Example_4740, host, port)
+};
+#endif
 
 TEST_CASE("regression tests 2")
 {
@@ -545,7 +566,7 @@ TEST_CASE("regression tests 2")
             const auto length = 300;
 
             json dump_test;
-            dump_test["1"] = std::string(length, -1);
+            dump_test["1"] = std::string(length, static_cast<std::string::value_type>(-1));
 
             std::string expected = R"({"1":")";
             for (int i = 0; i < length; ++i)
@@ -562,7 +583,7 @@ TEST_CASE("regression tests 2")
             const auto length = 500;
 
             json dump_test;
-            dump_test["1"] = std::string(length, -2);
+            dump_test["1"] = std::string(length, static_cast<std::string::value_type>(-2));
 
             std::string expected = R"({"1":")";
             for (int i = 0; i < length; ++i)
@@ -1037,6 +1058,28 @@ TEST_CASE("regression tests 2")
         oj["test"] = states;
         CHECK(oj["test"].dump() == expected);
     }
+
+#ifdef JSON_HAS_CPP_17
+    SECTION("issue #4740 - build issue with std::optional")
+    {
+        const auto t1 = Example_4740();
+        const auto j1 = nlohmann::json(t1);
+        CHECK(j1.dump() == "{\"host\":null,\"port\":null}");
+        const auto t2 = j1.get<Example_4740>();
+        CHECK(!t2.host.has_value());
+        CHECK(!t2.port.has_value());
+
+        // improve coverage
+        auto t3 = Example_4740();
+        t3.port = 80;
+        t3.host = "example.com";
+        const auto j2 = nlohmann::json(t3);
+        CHECK(j2.dump() == "{\"host\":\"example.com\",\"port\":80}");
+        const auto t4 = j2.get<Example_4740>();
+        CHECK(t4.host.has_value());
+        CHECK(t4.port.has_value());
+    }
+#endif
 }
 
 DOCTEST_CLANG_SUPPRESS_WARNING_POP

tests/src/unit-udt.cpp

@@ -841,7 +841,7 @@ class Evil
   public:
     Evil() = default;
     template <typename T>
-    Evil(T t) : m_i(sizeof(t))
+    Evil(const T& t) : m_i(sizeof(t))
     {
         static_cast<void>(t); // fix MSVC's C4100 warning
     }
@@ -863,6 +863,10 @@ TEST_CASE("Issue #924")
     // silence Wunused-template warnings
     Evil e(1);
     CHECK(e.m_i >= 0);
+
+    // suppress warning: function "<unnamed>::Evil::Evil(T) [with T=std::string]" was declared but never referenced [declared_but_not_referenced]
+    Evil e2(std::string("foo"));
+    CHECK(e2.m_i >= 0);
 }
 
 TEST_CASE("Issue #1237")

tests/src/unit-user_defined_input.cpp

@@ -60,12 +60,12 @@ TEST_CASE("Custom container member begin/end")
     {
         const char* data;
 
-        const char* begin() const
+        const char* begin() const noexcept
         {
             return data;
         }
 
-        const char* end() const
+        const char* end() const noexcept
         {
             return data + strlen(data); // NOLINT(cppcoreguidelines-pro-bounds-pointer-arithmetic)
         }