fix: better handling of PYBIND11_CPP_STANDARD

tools/pybind11Tools.cmake

@@ -25,14 +25,23 @@ include(CheckCXXCompilerFlag)
 
 # Warn or error if old variable name used
 if(PYBIND11_CPP_STANDARD)
-  if(NOT CMAKE_CXX_STANDARD)
-    string(REGEX MATCH [=[..^]=] VAL "${PYBIND11_CPP_STANDARD}")
+  string(REGEX MATCH [[..$]] VAL "${PYBIND11_CPP_STANDARD}")
+  if(CMAKE_CXX_STANDARD)
+    if(NOT CMAKE_CXX_STANDARD STREQUAL VAL)
+      message(WARNING "CMAKE_CXX_STANDARD=${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_PYTHON_VERSION")
-      set(CMAKE_CXX_STANDARD ${VAL})
+      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")
+      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()
   endif()
 endif()