diff --git a/.appveyor.yml b/.appveyor.yml
index 8429d5ec57..2faaa24e45 100644
--- a/.appveyor.yml
+++ b/.appveyor.yml
@@ -39,7 +39,7 @@ build_script:
 - cmake -G "%CMAKE_GENERATOR%" -A "%CMAKE_ARCH%"
     -DPython_ROOT_DIR="%PYTHON_ROOT%"
     -DCMAKE_MAP_IMPORTED_CONFIG_DEBUG=Release
-    -DCMAKE_CXX_STANDARD=14
+    -DCMAKE_CXX_STANDARD=17
     -DPYBIND11_WERROR=ON
     -DDOWNLOAD_CATCH=ON
     -DCMAKE_SUPPRESS_REGENERATION=1
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index eb2fcf2b67..d6ac1454eb 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -40,7 +40,7 @@ jobs:
         include:
           - runs-on: ubuntu-22.04
             python-version: '3.9'
-            cmake-args: -DPYBIND11_FINDPYTHON=OFF -DPYBIND11_NUMPY_1_ONLY=ON
+            cmake-args: -DPYBIND11_FINDPYTHON=OFF
           - runs-on: ubuntu-latest
             python-version: '3.13'
             cmake-args: -DCMAKE_CXX_STANDARD=23 -DPYBIND11_SIMPLE_GIL_MANAGEMENT=ON
@@ -58,7 +58,7 @@ jobs:
             cmake-args: -DCMAKE_CXX_STANDARD=20
           - runs-on: macos-latest
             python-version: '3.14'
-            cmake-args: -DCMAKE_CXX_STANDARD=14
+            cmake-args: -DCMAKE_CXX_STANDARD=17
           - runs-on: windows-2022
             python-version: '3.9'
             cmake-args: -DPYBIND11_FINDPYTHON=OFF
@@ -88,7 +88,7 @@ jobs:
             cmake-args: -DPYBIND11_TEST_SMART_HOLDER=ON -DPYBIND11_SIMPLE_GIL_MANAGEMENT=ON
           - runs-on: ubuntu-latest
             python-version: '3.14'
-            cmake-args: -DCMAKE_CXX_STANDARD=14
+            cmake-args: -DCMAKE_CXX_STANDARD=17
           - runs-on: ubuntu-latest
             python-version: '3.15t'
             cmake-args: -DCMAKE_CXX_STANDARD=17 -DPYBIND11_TEST_SMART_HOLDER=ON
@@ -96,10 +96,10 @@ jobs:
           # No SciPy for macOS ARM
           - runs-on: macos-15-intel
             python-version: '3.9'
-            cmake-args: -DCMAKE_CXX_STANDARD=14
+            cmake-args: -DCMAKE_CXX_STANDARD=17
           - runs-on: macos-15-intel
             python-version: '3.13'
-            cmake-args: -DCMAKE_CXX_STANDARD=11
+            cmake-args: -DCMAKE_CXX_STANDARD=17
           - runs-on: macos-latest
             python-version: '3.12'
             cmake-args: -DCMAKE_CXX_STANDARD=17 -DPYBIND11_DISABLE_HANDLE_TYPE_NAME_DEFAULT_IMPLEMENTATION=ON
@@ -119,7 +119,7 @@ jobs:
             cmake-args: -DPYBIND11_TEST_SMART_HOLDER=ON
           - runs-on: windows-2022
             python-version: '3.9'
-            cmake-args: -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded -DPYBIND11_NUMPY_1_ONLY=ON
+            cmake-args: -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded
           # This needs a python built with MTd
           # - runs-on: windows-2022
           #   python-version: '3.11'
@@ -152,7 +152,7 @@ jobs:
       python-version: ${{ matrix.python-version }}
       cmake-args: ${{ matrix.cmake-args }}
 
-  # This checks inplace builds with C++11
+  # This checks inplace builds with C++17.
   inplace:
     if: github.event.pull_request.draft == false
     strategy:
@@ -166,7 +166,7 @@ jobs:
           - runs-on: windows-latest
             python-version: '3.11'
 
-    name: "🐍 ${{ matrix.python-version }} • ${{ matrix.runs-on }} • x64 inplace C++14"
+    name: "🐍 ${{ matrix.python-version }} • ${{ matrix.runs-on }} • x64 inplace C++17"
     runs-on: ${{ matrix.runs-on }}
     timeout-minutes: 90
 
@@ -202,7 +202,7 @@ jobs:
         -DPYBIND11_PYTEST_ARGS=-v
         -DDOWNLOAD_CATCH=ON
         -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=14
+        -DCMAKE_CXX_STANDARD=17
         -DPYBIND11_INTERNALS_VERSION=10000000
 
     # Checks to makes sure defining `_` is allowed
@@ -254,12 +254,15 @@ jobs:
         run: uv tool install ninja
 
       - name: Configure via preset
-        run: cmake --preset venv -DPYBIND11_CREATE_WITH_UV="${{ matrix.python-version }}"
+        run: >
+          cmake --preset venv
+          -DPYBIND11_CREATE_WITH_UV="${{ matrix.python-version }}"
+          -DCMAKE_CXX_STANDARD=17
 
-      - name: Build C++11
+      - name: Build C++17
         run: cmake --build --preset venv
 
-      - name: Python tests C++11
+      - name: Python tests C++17
         run: cmake --build --preset testsvenv -t pytest
 
   # Testing on clang using the excellent silkeh clang docker images
@@ -270,10 +273,11 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - clang: 5
-            std: 14
+          # Clang 11 is the oldest LLVM toolchain available on a Debian base
+          # that meets pybind11's current Python and CMake requirements.
           - clang: 11
-            std: 20
+            std: 17
+            container_suffix: "-bullseye"
           - clang: 16
             std: 20
             container_suffix: "-bullseye"
@@ -333,7 +337,12 @@ jobs:
       run: apt-get update && DEBIAN_FRONTEND="noninteractive" apt-get install -y cmake git python3-dev python3-pytest python3-numpy
 
     - name: Configure
-      run: cmake -S . -B build -DPYBIND11_CUDA_TESTS=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
+      run: >
+        cmake -S . -B build
+        -DPYBIND11_CUDA_TESTS=ON
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DCMAKE_CXX_STANDARD=17
 
     - name: Build
       run: cmake --build build -j2 --verbose
@@ -371,7 +380,7 @@ jobs:
 #      run: |
 #        source /etc/profile.d/modules.sh
 #        module load /opt/nvidia/hpc_sdk/modulefiles/nvhpc/20.11
-#        cmake -S . -B build -DDOWNLOAD_CATCH=ON -DCMAKE_CXX_STANDARD=14 -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+#        cmake -S . -B build -DDOWNLOAD_CATCH=ON -DCMAKE_CXX_STANDARD=17 -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
 #
 #    - name: Build
 #      run: cmake --build build -j 2 --verbose
@@ -536,44 +545,6 @@ jobs:
         set +e; source /opt/intel/oneapi/setvars.sh; set -e
         python3 -m pip install -r tests/requirements.txt
 
-    - name: Configure C++11
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake -S . -B build-11     \
-        -DPYBIND11_WERROR=ON    \
-        -DDOWNLOAD_CATCH=ON     \
-        -DDOWNLOAD_EIGEN=OFF    \
-        -DCMAKE_CXX_STANDARD=11             \
-        -DCMAKE_CXX_COMPILER=$(which icpc)  \
-        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
-
-    - name: Build C++11
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake --build build-11 -j 2 -v
-
-    - name: Python tests C++11
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        sudo service apport stop
-        cmake --build build-11 --target check
-
-    - name: C++ tests C++11
-      timeout-minutes: 3
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake --build build-11 --target cpptest
-
-    - name: Interface test C++11
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake --build build-11 --target test_cmake_build
-
-    - name: Visibility test
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake --build build-11 --target test_cross_module_rtti
-
     - name: Configure C++17
       run: |
         set +e; source /opt/intel/oneapi/setvars.sh; set -e
@@ -659,7 +630,7 @@ jobs:
         -DPYBIND11_WERROR=ON
         -DDOWNLOAD_CATCH=ON
         -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=11
+        -DCMAKE_CXX_STANDARD=17
         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
 
     - name: Build
@@ -701,6 +672,7 @@ jobs:
       run: >
         cmake .
         -DPYBIND11_INSTALL=1 -DPYBIND11_TEST=0
+        -DCMAKE_CXX_STANDARD=17
         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
 
     - name: Make and install
@@ -717,6 +689,7 @@ jobs:
         cmake ../pybind11-tests
         -DDOWNLOAD_CATCH=ON
         -DPYBIND11_WERROR=ON
+        -DCMAKE_CXX_STANDARD=17
         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
       working-directory: /build-tests
 
@@ -772,6 +745,7 @@ jobs:
           - python: '3.9'
             args: -DCMAKE_CXX_STANDARD=17
           - python: '3.13'
+            args: -DCMAKE_CXX_STANDARD=17
 
 
     name: "🐍 ${{ matrix.python }} • MSVC 2022 • x86 ${{ matrix.args }}"
@@ -810,7 +784,7 @@ jobs:
         -DDOWNLOAD_CATCH=ON
         -DDOWNLOAD_EIGEN=ON
         ${{ matrix.args }}
-    - name: Build C++11
+    - name: Build C++17
       run: cmake --build build -j 2
 
     - name: Python tests
@@ -859,7 +833,7 @@ jobs:
         -DDOWNLOAD_CATCH=ON
         -DDOWNLOAD_EIGEN=ON
         ${{ matrix.args }}
-    - name: Build C++11
+    - name: Build C++20
       run: cmake --build build --config Debug -j 2
 
     - name: Python tests
@@ -969,80 +943,30 @@ jobs:
 
     - uses: actions/checkout@v7
 
-    - name: Configure C++11
+    - name: Configure C++17
       # LTO leads to many undefined reference like
       # `pybind11::detail::function_call::function_call(pybind11::detail::function_call&&)
       run: >-
-        cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=11 -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
+        cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=17 -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
         -DPYTHON_EXECUTABLE=$(python -c "import sys; print(sys.executable)")
         -S . -B build
 
-    - name: Build C++11
+    - name: Build C++17
       run: cmake --build build -j 2
 
-    - name: Python tests C++11
+    - name: Python tests C++17
       run: cmake --build build --target pytest -j 2
 
-    - name: C++11 tests
+    - name: C++17 tests
       timeout-minutes: 3
       run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build --target cpptest -j 2
 
-    - name: Interface test C++11
+    - name: Interface test C++17
       run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build --target test_cmake_build
 
     - name: Visibility test
       run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build --target test_cross_module_rtti
 
-    - name: Clean directory
-      run: git clean -fdx
-
-    - name: Configure C++14
-      run: >-
-        cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=14 -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
-        -DPYTHON_EXECUTABLE=$(python -c "import sys; print(sys.executable)")
-        -S . -B build2
-
-    - name: Build C++14
-      run: cmake --build build2 -j 2
-
-    - name: Python tests C++14
-      run: cmake --build build2 --target pytest -j 2
-
-    - name: C++14 tests
-      timeout-minutes: 3
-      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build2 --target cpptest -j 2
-
-    - name: Interface test C++14
-      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build2 --target test_cmake_build
-
-    - name: Visibility test
-      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build2 --target test_cross_module_rtti
-
-    - name: Clean directory
-      run: git clean -fdx
-
-    - name: Configure C++17
-      run: >-
-        cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=17 -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
-        -DPYTHON_EXECUTABLE=$(python -c "import sys; print(sys.executable)")
-        -S . -B build3
-
-    - name: Build C++17
-      run: cmake --build build3 -j 2
-
-    - name: Python tests C++17
-      run: cmake --build build3 --target pytest -j 2
-
-    - name: C++17 tests
-      timeout-minutes: 3
-      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build3 --target cpptest -j 2
-
-    - name: Interface test C++17
-      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build3 --target test_cmake_build
-
-    - name: Visibility test
-      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build3 --target test_cross_module_rtti
-
   windows_clang:
     if: github.event.pull_request.draft == false
 
diff --git a/.github/workflows/configure.yml b/.github/workflows/configure.yml
index b9c09566bd..19f77aae3a 100644
--- a/.github/workflows/configure.yml
+++ b/.github/workflows/configure.yml
@@ -71,7 +71,11 @@ jobs:
     # These steps use a directory with a space in it intentionally
     - name: Configure
       shell: bash
-      run: cmake -S. -B"build dir" -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
+      run: >
+        cmake -S. -B"build dir"
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DCMAKE_CXX_STANDARD=17
 
     # Only build and test if this was manually triggered in the GitHub UI
     - name: Build
diff --git a/.github/workflows/reusable-standard.yml b/.github/workflows/reusable-standard.yml
index 326c20170b..49b13950e6 100644
--- a/.github/workflows/reusable-standard.yml
+++ b/.github/workflows/reusable-standard.yml
@@ -78,6 +78,7 @@ jobs:
           -DPYBIND11_PYTEST_ARGS=-v
           -DDOWNLOAD_CATCH=ON
           -DDOWNLOAD_EIGEN=ON
+          -DCMAKE_CXX_STANDARD=17
           -DPYTHON_EXECUTABLE=${{ steps.python.outputs.python-path }}
           ${{ inputs.cmake-args }}
 
diff --git a/.github/workflows/tests-cibw.yml b/.github/workflows/tests-cibw.yml
index 83107b23da..dea67eb5ea 100644
--- a/.github/workflows/tests-cibw.yml
+++ b/.github/workflows/tests-cibw.yml
@@ -15,6 +15,9 @@ concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true
 
+env:
+  CIBW_CONFIG_SETTINGS: "cmake.define.CMAKE_CXX_STANDARD=17"
+
 jobs:
   build-wasm-emscripten:
     name: Pyodide wheel
diff --git a/.github/workflows/upstream.yml b/.github/workflows/upstream.yml
index 7ad8c99014..76775516c0 100644
--- a/.github/workflows/upstream.yml
+++ b/.github/workflows/upstream.yml
@@ -49,47 +49,48 @@ jobs:
         cmake --version
         pip list
 
-    # First build - C++11 mode and inplace
-    - name: Configure C++11
+    # First build - C++17 Debug mode
+    - name: Configure C++17 Debug
       run: >
-        cmake -S . -B build11
+        cmake -S . -B build-debug
         -DPYBIND11_WERROR=ON
         -DDOWNLOAD_CATCH=ON
         -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=11
+        -DCMAKE_CXX_STANDARD=17
         -DCMAKE_BUILD_TYPE=Debug
 
-    - name: Build C++11
-      run: cmake --build build11 -j 2
+    - name: Build C++17 Debug
+      run: cmake --build build-debug -j 2
 
-    - name: Python tests C++11
-      run: cmake --build build11 --target pytest -j 2
+    - name: Python tests C++17 Debug
+      run: cmake --build build-debug --target pytest -j 2
 
-    - name: C++11 tests
+    - name: C++17 Debug tests
       timeout-minutes: 3
-      run: cmake --build build11  --target cpptest -j 2
+      run: cmake --build build-debug --target cpptest -j 2
 
-    - name: Interface test C++11
-      run: cmake --build build11 --target test_cmake_build
+    - name: Interface test C++17 Debug
+      run: cmake --build build-debug --target test_cmake_build
 
-    # Second build - C++17 mode and in a build directory
-    - name: Configure C++17
+    # Second build - C++17 MinSizeRel mode
+    - name: Configure C++17 MinSizeRel
       run: >
-        cmake -S . -B build17
+        cmake -S . -B build-minsizerel
         -DPYBIND11_WERROR=ON
         -DDOWNLOAD_CATCH=ON
         -DDOWNLOAD_EIGEN=ON
         -DCMAKE_CXX_STANDARD=17
+        -DCMAKE_BUILD_TYPE=MinSizeRel
 
-    - name: Build C++17
-      run: cmake --build build17 -j 2
+    - name: Build C++17 MinSizeRel
+      run: cmake --build build-minsizerel -j 2
 
-    - name: Python tests C++17
-      run: cmake --build build17 --target pytest
+    - name: Python tests C++17 MinSizeRel
+      run: cmake --build build-minsizerel --target pytest
 
-    - name: C++17 tests
+    - name: C++17 MinSizeRel tests
       timeout-minutes: 3
-      run: cmake --build build17 --target cpptest
+      run: cmake --build build-minsizerel --target cpptest
 
     # Third build - C++17 mode with unstable ABI
     - name: Configure (unstable ABI)
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6f3f8beb2b..d4249ba839 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -301,8 +301,7 @@ if(NOT TARGET pybind11_headers)
     pybind11_headers ${pybind11_system} INTERFACE $<BUILD_INTERFACE:${pybind11_INCLUDE_DIR}>
                                                   $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
 
-  target_compile_features(pybind11_headers INTERFACE cxx_inheriting_constructors cxx_user_literals
-                                                     cxx_right_angle_brackets)
+  target_compile_features(pybind11_headers INTERFACE cxx_std_17)
   if(NOT "${PYBIND11_INTERNALS_VERSION}" STREQUAL "")
     target_compile_definitions(
       pybind11_headers INTERFACE "PYBIND11_INTERNALS_VERSION=${PYBIND11_INTERNALS_VERSION}")
diff --git a/CMakePresets.json b/CMakePresets.json
index 42bf3ade9d..1ae47043fb 100644
--- a/CMakePresets.json
+++ b/CMakePresets.json
@@ -12,6 +12,7 @@
       },
       "cacheVariables": {
         "CMAKE_BUILD_TYPE": "Debug",
+        "CMAKE_CXX_STANDARD": "17",
         "CMAKE_EXPORT_COMPILE_COMMANDS": true,
         "DOWNLOAD_CATCH": true,
         "DOWNLOAD_EIGEN": true,
@@ -35,8 +36,7 @@
       "inherits": "default",
       "binaryDir": "build-tidy",
       "cacheVariables": {
-        "CMAKE_CXX_CLANG_TIDY": "clang-tidy;--use-color;--warnings-as-errors=*",
-        "CMAKE_CXX_STANDARD": "17"
+        "CMAKE_CXX_CLANG_TIDY": "clang-tidy;--use-color;--warnings-as-errors=*"
       }
     }
   ],
diff --git a/include/pybind11/complex.h b/include/pybind11/complex.h
index a6ec11a462..dbc3573814 100644
--- a/include/pybind11/complex.h
+++ b/include/pybind11/complex.h
@@ -27,15 +27,6 @@ struct format_descriptor<std::complex<T>, detail::enable_if_t<std::is_floating_p
     static std::string format() { return std::string(value); }
 };
 
-#ifndef PYBIND11_CPP17
-
-template <typename T>
-constexpr const char
-    format_descriptor<std::complex<T>,
-                      detail::enable_if_t<std::is_floating_point<T>::value>>::value[3];
-
-#endif
-
 PYBIND11_NAMESPACE_BEGIN(detail)
 
 template <typename T>
diff --git a/include/pybind11/detail/common.h b/include/pybind11/detail/common.h
index 7f182afb2c..e4b33162f0 100644
--- a/include/pybind11/detail/common.h
+++ b/include/pybind11/detail/common.h
@@ -72,6 +72,10 @@
 #    endif
 #endif
 
+#if !defined(PYBIND11_CPP17)
+#    error "pybind11 requires C++17 or newer"
+#endif
+
 // These PYBIND11_HAS_... macros are consolidated in pybind11/detail/common.h
 // to simplify backward compatibility handling for users (e.g., via #ifdef checks):
 #define PYBIND11_HAS_TYPE_CASTER_STD_FUNCTION_SPECIALIZATIONS 1
@@ -79,7 +83,7 @@
 #define PYBIND11_HAS_CPP_CONDUIT 1
 #define PYBIND11_HAS_NATIVE_ENUM 1
 
-#if defined(PYBIND11_CPP17) && defined(__has_include)
+#if defined(__has_include)
 #    if __has_include(<filesystem>)
 #        define PYBIND11_HAS_FILESYSTEM 1
 #    elif __has_include(<experimental/filesystem>)
@@ -109,10 +113,8 @@
 
 // Compiler version assertions
 #if defined(__INTEL_COMPILER)
-#    if __INTEL_COMPILER < 1800
-#        error pybind11 requires Intel C++ compiler v18 or newer
-#    elif __INTEL_COMPILER < 1900 && defined(PYBIND11_CPP14)
-#        error pybind11 supports only C++11 with Intel C++ compiler v18. Use v19 or newer for C++14.
+#    if __INTEL_COMPILER < 1900
+#        error pybind11 requires Intel C++ compiler v19 or newer
 #    endif
 /* The following pragma cannot be pop'ed:
    https://community.intel.com/t5/Intel-C-Compiler/Inline-and-no-inline-warning/td-p/1216764 */
@@ -179,19 +181,11 @@
 // For unknown reasons all PYBIND11_DEPRECATED member trigger a warning when declared
 // whether it is used or not
 #    define PYBIND11_DEPRECATED(reason)
-#elif defined(PYBIND11_CPP14)
-#    define PYBIND11_DEPRECATED(reason) [[deprecated(reason)]]
 #else
-#    define PYBIND11_DEPRECATED(reason) __attribute__((deprecated(reason)))
+#    define PYBIND11_DEPRECATED(reason) [[deprecated(reason)]]
 #endif
 
-#if defined(PYBIND11_CPP17)
-#    define PYBIND11_MAYBE_UNUSED [[maybe_unused]]
-#elif defined(_MSC_VER) && !defined(__clang__)
-#    define PYBIND11_MAYBE_UNUSED
-#else
-#    define PYBIND11_MAYBE_UNUSED __attribute__((__unused__))
-#endif
+#define PYBIND11_MAYBE_UNUSED [[maybe_unused]]
 
 // https://en.cppreference.com/w/c/chrono/localtime
 #if defined(__STDC_LIB_EXT1__) && !defined(__STDC_WANT_LIB_EXT1__)
@@ -199,26 +193,24 @@
 #endif
 
 #ifdef __has_include
-// std::optional (but including it in c++14 mode isn't allowed)
-#    if defined(PYBIND11_CPP17) && __has_include(<optional>)
+// std::optional
+#    if __has_include(<optional>)
 #        define PYBIND11_HAS_OPTIONAL 1
 #    endif
-// std::experimental::optional (but not allowed in c++11 mode)
-#    if defined(PYBIND11_CPP14) && (__has_include(<experimental/optional>) && \
-                                 !__has_include(<optional>))
+// std::experimental::optional
+#    if __has_include(<experimental/optional>) && !__has_include(<optional>)
 #        define PYBIND11_HAS_EXP_OPTIONAL 1
 #    endif
 // std::variant
-#    if defined(PYBIND11_CPP17) && __has_include(<variant>)
+#    if __has_include(<variant>)
 #        define PYBIND11_HAS_VARIANT 1
 #    endif
-#elif defined(_MSC_VER) && defined(PYBIND11_CPP17)
+#elif defined(_MSC_VER)
 #    define PYBIND11_HAS_OPTIONAL 1
 #    define PYBIND11_HAS_VARIANT 1
 #endif
 
-#if defined(PYBIND11_CPP17)                                                                       \
-    && ((defined(__has_include) && __has_include(<string_view>)) || defined(_MSC_VER))
+#if (defined(__has_include) && __has_include(<string_view>)) || defined(_MSC_VER)
 #    define PYBIND11_HAS_STRING_VIEW 1
 #endif
 
@@ -706,22 +698,11 @@ static_assert(std::is_standard_layout<instance>::value,
 template <typename>
 struct always_false : std::false_type {};
 
-/// from __cpp_future__ import (convenient aliases from C++14/17)
-#if defined(PYBIND11_CPP14)
+/// Convenient aliases from newer C++ versions
 using std::conditional_t;
 using std::enable_if_t;
 using std::remove_cv_t;
 using std::remove_reference_t;
-#else
-template <bool B, typename T = void>
-using enable_if_t = typename std::enable_if<B, T>::type;
-template <bool B, typename T, typename F>
-using conditional_t = typename std::conditional<B, T, F>::type;
-template <typename T>
-using remove_cv_t = typename std::remove_cv<T>::type;
-template <typename T>
-using remove_reference_t = typename std::remove_reference<T>::type;
-#endif
 
 #if defined(PYBIND11_CPP20) && defined(__cpp_lib_remove_cvref)
 using std::remove_cvref;
@@ -740,56 +721,8 @@ template <typename T, typename U>
 using is_same_ignoring_cvref = std::is_same<detail::remove_cvref_t<T>, U>;
 
 /// Index sequences
-#if defined(PYBIND11_CPP14)
 using std::index_sequence;
 using std::make_index_sequence;
-#else
-template <size_t...>
-struct index_sequence {};
-// Comments about the algorithm below.
-//
-// Credit: This is based on an algorithm by taocpp here:
-//    https://github.com/taocpp/sequences/blob/main/include/tao/seq/make_integer_sequence.hpp
-// but significantly simplified.
-//
-// We build up a sequence S by repeatedly doubling its length and sometimes adding 1 to the end.
-// E.g. if the current S is 0...3, then we either go to 0...7 or 0...8 on the next pass.
-// The goal is to end with S = 0...N-1.
-// The key insight is that the times we need to add an additional digit to S correspond
-// exactly to the 1's in the binary representation of the number N.
-//
-// Invariants:
-// - digit is a power of 2
-// - N_digit_is_1 is whether N's binary representation has a 1 in that digit's position.
-// - end <= N
-// - S is 0...end-1.
-// - if digit > 0, end * digit * 2 <= N < (end+1) * digit * 2
-//
-// The process starts with digit > N, end = 0, and S is empty.
-// The process concludes with digit=0, in which case, end == N and S is 0...N-1.
-
-template <size_t digit, bool N_digit_is_1, size_t N, size_t end, size_t... S> // N_digit_is_1=false
-struct make_index_sequence_impl
-    : make_index_sequence_impl<digit / 2, (N & (digit / 2)) != 0, N, 2 * end, S..., (S + end)...> {
-};
-template <size_t digit, size_t N, size_t end, size_t... S>
-struct make_index_sequence_impl<digit, true, N, end, S...>
-    : make_index_sequence_impl<digit / 2,
-                               (N & (digit / 2)) != 0,
-                               N,
-                               2 * end + 1,
-                               S...,
-                               (S + end)...,
-                               2 * end> {};
-template <size_t N, size_t end, size_t... S>
-struct make_index_sequence_impl<0, false, N, end, S...> {
-    using type = index_sequence<S...>;
-};
-constexpr size_t next_power_of_2(size_t N) { return N == 0 ? 1 : next_power_of_2(N >> 1) << 1; }
-template <size_t N>
-using make_index_sequence =
-    typename make_index_sequence_impl<next_power_of_2(N), false, N, 0>::type;
-#endif
 
 /// Make an index sequence of the indices of true arguments
 template <typename ISeq, size_t, bool...>
@@ -1202,14 +1135,6 @@ struct format_descriptor<T, detail::enable_if_t<std::is_arithmetic<T>::value>> {
     static std::string format() { return std::string(1, c); }
 };
 
-#if !defined(PYBIND11_CPP17)
-
-template <typename T>
-constexpr const char
-    format_descriptor<T, detail::enable_if_t<std::is_arithmetic<T>::value>>::value[2];
-
-#endif
-
 /// RAII wrapper that temporarily clears any Python error state
 struct error_scope {
     PyObject *type, *value, *trace;
@@ -1277,29 +1202,18 @@ struct overload_cast_impl {
 };
 PYBIND11_NAMESPACE_END(detail)
 
-// overload_cast requires variable templates: C++14
-#if defined(PYBIND11_CPP14)
-#    define PYBIND11_OVERLOAD_CAST 1
+#define PYBIND11_OVERLOAD_CAST 1
 /// Syntax sugar for resolving overloaded function pointers:
 ///  - regular: static_cast<Return (Class::*)(Arg0, Arg1, Arg2)>(&Class::func)
 ///  - sweet:   overload_cast<Arg0, Arg1, Arg2>(&Class::func)
 template <typename... Args>
 static constexpr detail::overload_cast_impl<Args...> overload_cast{};
-#endif
 
 /// Const member function selector for overload_cast
 ///  - regular: static_cast<Return (Class::*)(Arg) const>(&Class::func)
 ///  - sweet:   overload_cast<Arg>(&Class::func, const_)
 static constexpr auto const_ = std::true_type{};
 
-#if !defined(PYBIND11_CPP14) // no overload_cast: providing something that static_assert-fails:
-template <typename... Args>
-struct overload_cast {
-    static_assert(detail::deferred_t<std::false_type, Args...>::value,
-                  "pybind11::overload_cast<...> requires compiling in C++14 mode");
-};
-#endif // overload_cast
-
 PYBIND11_NAMESPACE_BEGIN(detail)
 
 // Adaptor for converting arbitrary container arguments into a vector; implicitly convertible from
diff --git a/include/pybind11/detail/init.h b/include/pybind11/detail/init.h
index a1083f8457..ae2f66bd62 100644
--- a/include/pybind11/detail/init.h
+++ b/include/pybind11/detail/init.h
@@ -388,17 +388,9 @@ struct factory<Func, void_type (*)(), Return(Args...), void_type()> {
     // instance, or the alias needs to be constructible from a `Class &&` argument.
     template <typename Class, typename... Extra>
     void execute(Class &cl, const Extra &...extra) && {
-#if defined(PYBIND11_CPP14)
         cl.def(
             "__init__",
-            [func = std::move(class_factory)]
-#else
-        auto &func = class_factory;
-        cl.def(
-            "__init__",
-            [func]
-#endif
-            (value_and_holder &v_h, Args... args) {
+            [func = std::move(class_factory)](value_and_holder &v_h, Args... args) {
                 construct<Class>(
                     v_h, func(std::forward<Args>(args)...), Py_TYPE(v_h.inst) != v_h.type->type);
             },
@@ -435,18 +427,10 @@ struct factory<CFunc, AFunc, CReturn(CArgs...), AReturn(AArgs...)> {
         static_assert(Class::has_alias,
                       "The two-argument version of `py::init()` can "
                       "only be used if the class has an alias");
-#if defined(PYBIND11_CPP14)
-        cl.def(
-            "__init__",
-            [class_func = std::move(class_factory), alias_func = std::move(alias_factory)]
-#else
-        auto &class_func = class_factory;
-        auto &alias_func = alias_factory;
         cl.def(
             "__init__",
-            [class_func, alias_func]
-#endif
-            (value_and_holder &v_h, CArgs... args) {
+            [class_func = std::move(class_factory),
+             alias_func = std::move(alias_factory)](value_and_holder &v_h, CArgs... args) {
                 if (Py_TYPE(v_h.inst) == v_h.type->type) {
                     // If the instance type equals the registered type we don't have inheritance,
                     // so don't need the alias and can construct using the class function:
@@ -524,17 +508,9 @@ struct pickle_factory<Get, Set, RetState(Self), NewInstance(ArgState)> {
     void execute(Class &cl, const Extra &...extra) && {
         cl.def("__getstate__", std::move(get), pos_only());
 
-#if defined(PYBIND11_CPP14)
-        cl.def(
-            "__setstate__",
-            [func = std::move(set)]
-#else
-        auto &func = set;
         cl.def(
             "__setstate__",
-            [func]
-#endif
-            (value_and_holder &v_h, ArgState state) {
+            [func = std::move(set)](value_and_holder &v_h, ArgState state) {
                 setstate<Class>(
                     v_h, func(std::forward<ArgState>(state)), Py_TYPE(v_h.inst) != v_h.type->type);
             },
diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index eebb130694..064a508438 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -1,5 +1,5 @@
 /*
-    pybind11/pybind11.h: Main header file of the C++11 python
+    pybind11/pybind11.h: Main header file of the C++17 python
     binding generator library
 
     Copyright (c) 2016 Wenzel Jakob <wenzel.jakob@epfl.ch>
@@ -44,12 +44,6 @@ PYBIND11_WARNING_DISABLE_CLANG("-Wgnu-zero-variadic-macro-arguments")
 #    include <cxxabi.h>
 #endif
 
-#if defined(__cpp_if_constexpr) && __cpp_if_constexpr >= 201606
-#    define PYBIND11_MAYBE_CONSTEXPR constexpr
-#else
-#    define PYBIND11_MAYBE_CONSTEXPR
-#endif
-
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 
 /* https://stackoverflow.com/questions/46798456/handling-gccs-noexcept-type-warning
@@ -274,10 +268,7 @@ class ReadableFunctionSignature {
     using sig_type = decltype(PYBIND11_READABLE_FUNCTION_SIGNATURE_EXPR);
 
 private:
-    // We have to repeat PYBIND11_READABLE_FUNCTION_SIGNATURE_EXPR in decltype()
-    // because C++11 doesn't allow functions to return `auto`. (We don't
-    // know the type because it's some variant of detail::descr<N> with
-    // unknown N.)
+    // The type is some variant of detail::descr<N>, with an N dependent on the casters.
     static constexpr sig_type sig() { return PYBIND11_READABLE_FUNCTION_SIGNATURE_EXPR; }
 
 public:
@@ -292,19 +283,6 @@ class ReadableFunctionSignature {
 };
 #undef PYBIND11_READABLE_FUNCTION_SIGNATURE_EXPR
 
-// Prior to C++17, we don't have inline variables, so we have to
-// provide an out-of-line definition of the class member.
-#if !defined(PYBIND11_CPP17)
-template <typename cast_in, typename cast_out>
-constexpr typename ReadableFunctionSignature<cast_in, cast_out>::sig_type
-    ReadableFunctionSignature<cast_in, cast_out>::kSig;
-#    if !defined(_MSC_VER)
-template <typename cast_in, typename cast_out>
-constexpr typename ReadableFunctionSignature<cast_in, cast_out>::types_type
-    ReadableFunctionSignature<cast_in, cast_out>::kTypes;
-#    endif
-#endif
-
 PYBIND11_NAMESPACE_END(detail)
 
 /// Wraps an arbitrary C++ function/method/lambda function/.. into a callable Python object
@@ -2436,7 +2414,7 @@ class class_ : public detail::generic_type {
         // so the reinterpret_cast shortcut in load_impl Case 2a is invalid.
         // Force the MI path (implicit_casts) for correct pointer adjustment.
         // Detection: static_cast<Derived*>(Base*) is ill-formed for virtual bases.
-        if PYBIND11_MAYBE_CONSTEXPR (!detail::is_static_downcastable<Base, type>::value) {
+        if constexpr (!detail::is_static_downcastable<Base, type>::value) {
             rec.multiple_inheritance = true;
         }
     }
@@ -3920,9 +3898,9 @@ function get_override(const T *this_ptr, const char *name) {
             auto o = override(__VA_ARGS__);                                                       \
             PYBIND11_WARNING_PUSH                                                                 \
             PYBIND11_WARNING_DISABLE_MSVC(4127)                                                   \
-            if PYBIND11_MAYBE_CONSTEXPR (                                                         \
-                pybind11::detail::cast_is_temporary_value_reference<ret_type>::value              \
-                && !pybind11::detail::is_same_ignoring_cvref<ret_type, PyObject *>::value) {      \
+            if constexpr (pybind11::detail::cast_is_temporary_value_reference<ret_type>::value    \
+                          && !pybind11::detail::is_same_ignoring_cvref<ret_type,                  \
+                                                                       PyObject *>::value) {      \
                 static pybind11::detail::override_caster_t<ret_type> caster;                      \
                 return pybind11::detail::cast_ref<ret_type>(std::move(o), caster);                \
             } else {                                                                              \
diff --git a/pybind11/setup_helpers.py b/pybind11/setup_helpers.py
index 8f42605245..b93002ec73 100644
--- a/pybind11/setup_helpers.py
+++ b/pybind11/setup_helpers.py
@@ -1,5 +1,5 @@
 """
-This module provides helpers for C++11+ projects using pybind11.
+This module provides helpers for C++17+ projects using pybind11.
 
 LICENSE:
 
@@ -85,7 +85,7 @@
 
 class Pybind11Extension(_Extension):
     """
-    Build a C++11+ Extension module with pybind11. This automatically adds the
+    Build a C++17+ Extension module with pybind11. This automatically adds the
     recommended flags when you init the extension and assumes C++ sources - you
     can further modify the options yourself.
 
@@ -98,8 +98,8 @@ class Pybind11Extension(_Extension):
     Finally, you can set ``cxx_std`` via constructor or afterwards to enable
     flags for C++ std, and a few extra helper flags related to the C++ standard
     level. It is _highly_ recommended you either set this, or use the provided
-    ``build_ext``, which will search for the highest supported extension for
-    you if the ``cxx_std`` property is not set. Do not set the ``cxx_std``
+    ``build_ext``, which will check for C++17 support if the ``cxx_std``
+    property is not set. Do not set the ``cxx_std``
     property more than once, as flags are added when you set it. Set the
     property to None to disable the addition of C++ standard flags.
 
@@ -117,7 +117,7 @@ def _add_ldflags(self, flags: list[str]) -> None:
         self.extra_link_args[:0] = flags
 
     def __init__(self, *args: Any, **kwargs: Any) -> None:
-        self._cxx_level = 0
+        self._cxx_level: str | int | None = 0
         cxx_std = kwargs.pop("cxx_std", 0)
 
         if "language" not in kwargs:
@@ -155,27 +155,26 @@ def __init__(self, *args: Any, **kwargs: Any) -> None:
         self._add_cflags(cflags)
 
     @property
-    def cxx_std(self) -> int:
+    def cxx_std(self) -> str | int | None:
         """
         The CXX standard level. If set, will add the required flags. If left at
         0, it will trigger an automatic search when pybind11's build_ext is
         used. If None, will have no effect.  Besides just the flags, this may
-        add a macos-min 10.9 or 10.14 flag if MACOSX_DEPLOYMENT_TARGET is
-        unset.
+        add a macos-min 10.14 flag if MACOSX_DEPLOYMENT_TARGET is unset.
         """
         return self._cxx_level
 
     @cxx_std.setter
-    def cxx_std(self, level: int) -> None:
+    def cxx_std(self, level: str | int | None) -> None:
+        if isinstance(level, int) and level != 0 and level < 17:
+            msg = "pybind11 requires C++17 or newer"
+            raise ValueError(msg)
+
         if self._cxx_level:
             warnings.warn(
                 "You cannot safely change the cxx_level after setting it!", stacklevel=2
             )
 
-        # MSVC only has 14 and later modes, so force a valid flag here.
-        if WIN and level == 11:
-            level = 14
-
         self._cxx_level = level
 
         if not level:
@@ -191,7 +190,7 @@ def cxx_std(self, level: int) -> None:
             # setting for general use. However, never set higher than the
             # current macOS version!
             current_macos = tuple(int(x) for x in platform.mac_ver()[0].split(".")[:2])
-            desired_macos = (10, 9) if level < 17 else (10, 14)
+            desired_macos = (10, 14)
             macos_string = ".".join(str(x) for x in min(current_macos, desired_macos))
             macosx_min = f"-mmacosx-version-min={macos_string}"
             cflags += [macosx_min]
@@ -244,27 +243,24 @@ def has_flag(compiler: Any, flag: str) -> bool:
 @lru_cache
 def auto_cpp_level(compiler: Any) -> str | int:
     """
-    Return the max supported C++ std level (17, 14, or 11). Returns latest on Windows.
+    Return the minimum supported C++ std level (17). Returns latest on Windows.
     """
 
     if WIN:
         return "latest"
 
-    levels = [17, 14, 11]
-
-    for level in levels:
-        if has_flag(compiler, STD_TMPL.format(level)):
-            return level
+    if has_flag(compiler, STD_TMPL.format(17)):
+        return 17
 
-    msg = "Unsupported compiler -- at least C++11 support is needed!"
+    msg = "Unsupported compiler -- at least C++17 support is needed!"
     raise RuntimeError(msg)
 
 
 class build_ext(_build_ext):  # noqa: N801
     """
-    Customized build_ext that allows an auto-search for the highest supported
-    C++ level for Pybind11Extension. This is only needed for the auto-search
-    for now, and is completely optional otherwise.
+    Customized build_ext that checks for C++17 support for Pybind11Extension.
+    This is only needed for the automatic check and is completely optional
+    otherwise.
     """
 
     def build_extensions(self) -> None:
diff --git a/tests/extra_setuptools/test_setuphelper.py b/tests/extra_setuptools/test_setuphelper.py
index 22c0c76f5d..9040b0af10 100644
--- a/tests/extra_setuptools/test_setuphelper.py
+++ b/tests/extra_setuptools/test_setuphelper.py
@@ -13,7 +13,7 @@
 
 
 @pytest.mark.parametrize("parallel", [False, True])
-@pytest.mark.parametrize("std", [11, 0])
+@pytest.mark.parametrize("std", [17, 0])
 def test_simple_setup_py(monkeypatch, tmpdir, parallel, std):
     monkeypatch.chdir(tmpdir)
     monkeypatch.syspath_prepend(MAIN_DIR)
@@ -110,6 +110,15 @@ def test_simple_setup_py(monkeypatch, tmpdir, parallel, std):
     )
 
 
+@pytest.mark.parametrize("std", [11, 14])
+def test_cxx_std_too_old(monkeypatch, std):
+    monkeypatch.syspath_prepend(MAIN_DIR)
+    from pybind11.setup_helpers import Pybind11Extension
+
+    with pytest.raises(ValueError, match=r"pybind11 requires C\+\+17 or newer"):
+        Pybind11Extension("example", ["example.cpp"], cxx_std=std)
+
+
 def test_intree_extensions(monkeypatch, tmpdir):
     monkeypatch.syspath_prepend(MAIN_DIR)
 
diff --git a/tests/test_cmake_build/installed_target/CMakeLists.txt b/tests/test_cmake_build/installed_target/CMakeLists.txt
index 6ee01693df..0659f1a808 100644
--- a/tests/test_cmake_build/installed_target/CMakeLists.txt
+++ b/tests/test_cmake_build/installed_target/CMakeLists.txt
@@ -7,6 +7,8 @@ message(STATUS "Found pybind11 v${pybind11_VERSION}: ${pybind11_INCLUDE_DIRS}")
 
 add_library(test_installed_target MODULE ../main.cpp)
 
+# Verify that the installed interface promotes a consumer that initially requests C++14.
+set_target_properties(test_installed_target PROPERTIES CXX_STANDARD 14 CXX_STANDARD_REQUIRED ON)
 target_link_libraries(test_installed_target PRIVATE pybind11::module)
 set_target_properties(test_installed_target PROPERTIES OUTPUT_NAME test_cmake_build)
 
diff --git a/tests/test_cmake_build/subdirectory_target/CMakeLists.txt b/tests/test_cmake_build/subdirectory_target/CMakeLists.txt
index 88d73f6045..ac5296b2d1 100644
--- a/tests/test_cmake_build/subdirectory_target/CMakeLists.txt
+++ b/tests/test_cmake_build/subdirectory_target/CMakeLists.txt
@@ -13,6 +13,8 @@ add_subdirectory("${pybind11_SOURCE_DIR}" pybind11)
 add_library(test_subdirectory_target MODULE ../main.cpp)
 set_target_properties(test_subdirectory_target PROPERTIES OUTPUT_NAME test_cmake_build)
 
+# Verify that pybind11's interface requirement promotes a consumer that initially requests C++14.
+set_target_properties(test_subdirectory_target PROPERTIES CXX_STANDARD 14 CXX_STANDARD_REQUIRED ON)
 target_link_libraries(test_subdirectory_target PRIVATE pybind11::module)
 
 # Make sure result is, for example, test_installed_target.so, not libtest_installed_target.dylib
diff --git a/tests/test_cross_module_rtti/CMakeLists.txt b/tests/test_cross_module_rtti/CMakeLists.txt
index c9b95bfba1..2b9f0b0549 100644
--- a/tests/test_cross_module_rtti/CMakeLists.txt
+++ b/tests/test_cross_module_rtti/CMakeLists.txt
@@ -36,7 +36,7 @@ add_library(test_cross_module_rtti_lib::test_cross_module_rtti_lib ALIAS
             test_cross_module_rtti_lib)
 target_include_directories(test_cross_module_rtti_lib PUBLIC ${CMAKE_CURRENT_BINARY_DIR})
 target_include_directories(test_cross_module_rtti_lib PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
-target_compile_features(test_cross_module_rtti_lib PUBLIC cxx_std_11)
+target_compile_features(test_cross_module_rtti_lib PUBLIC cxx_std_17)
 
 generate_export_header(test_cross_module_rtti_lib)
 
diff --git a/tools/pybind11Common.cmake b/tools/pybind11Common.cmake
index d75fb67520..227e737923 100644
--- a/tools/pybind11Common.cmake
+++ b/tools/pybind11Common.cmake
@@ -169,23 +169,21 @@ set_property(
 # Warn or error if old variable name used
 if(PYBIND11_CPP_STANDARD)
   string(REGEX MATCH [[..$]] VAL "${PYBIND11_CPP_STANDARD}")
-  if(CMAKE_CXX_STANDARD)
+  set(supported_standards 17 20)
+  if(NOT "${VAL}" IN_LIST supported_standards)
+    message(FATAL_ERROR "PYBIND11_CPP_STANDARD should be replaced with CMAKE_CXX_STANDARD "
+                        "(${VAL} is below pybind11's C++17 minimum or is not supported)")
+  elseif(CMAKE_CXX_STANDARD)
     if(NOT CMAKE_CXX_STANDARD STREQUAL VAL)
       message(WARNING "CMAKE_CXX_STANDARD=${CMAKE_CXX_STANDARD} does not match "
                       "PYBIND11_CPP_STANDARD=${PYBIND11_CPP_STANDARD}, "
                       "please remove PYBIND11_CPP_STANDARD from your cache")
     endif()
   else()
-    set(supported_standards 11 14 17 20)
-    if("${VAL}" IN_LIST supported_standards)
-      message(WARNING "USE -DCMAKE_CXX_STANDARD=${VAL} instead of PYBIND11_CPP_STANDARD")
-      set(CMAKE_CXX_STANDARD
-          ${VAL}
-          CACHE STRING "From PYBIND11_CPP_STANDARD")
-    else()
-      message(FATAL_ERROR "PYBIND11_CPP_STANDARD should be replaced with CMAKE_CXX_STANDARD "
-                          "(last two chars: ${VAL} not understood as a valid CXX std)")
-    endif()
+    message(WARNING "USE -DCMAKE_CXX_STANDARD=${VAL} instead of PYBIND11_CPP_STANDARD")
+    set(CMAKE_CXX_STANDARD
+        ${VAL}
+        CACHE STRING "From PYBIND11_CPP_STANDARD")
   endif()
 endif()