[win] Increase CXX_STANDARD_STRING with VS 2026 (#20398)

* [win] Increase CXX_STANDARD_STRING with VS 2026

Use `/std:c++20` by default with Visual Studio 2026

* Fix compilation error with /std:c++20

* Add missing #include with /std:c++20

Author: bellenot

cmake/modules/CheckCompiler.cmake

@@ -162,7 +162,11 @@ endif()
 # so we check the value of __cplusplus.
 # This default value can be overridden by specifying one at the prompt.
 if (MSVC)
-   set(CXX_STANDARD_STRING "201703L")
+   if(MSVC_VERSION GREATER_EQUAL 1950)
+     set(CXX_STANDARD_STRING "202002L")
+   else()
+     set(CXX_STANDARD_STRING "201703L")
+   endif()
 else()
    execute_process(COMMAND echo __cplusplus
                    COMMAND ${CMAKE_CXX_COMPILER} -E -x c++ -

io/io/test/TFileTests.cxx

@@ -1,6 +1,7 @@
 #include <memory>
 #include <vector>
 #include <string>
+#include <array>
 
 #include "gtest/gtest.h"
 

roottest/root/io/evolution/rules/execSourceTypes.cxx

@@ -8,6 +8,7 @@
 #include <type_traits>
 #include <list>
 #include <string>
+#include <array>
 
 struct A {
   float f = 1.1;