Set libstdc++ implementation version using cmake - c++

Question

Consider the following CMakeLists.txt file:
cmake_minimum_required(VERSION 3.22)
project(demo CXX)
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)
add_executable(demo main.cpp)
target_compile_features(demo PUBLIC cxx_std_14)
And now the following main.cpp c++ code:
#include <iostream>
#include <vector>
int main(int argc, char *argv[])
{
std::cout << "__cplusplus=" << __cplusplus << std::endl;
std::vector<int> v;
// junk code to prevent g++ from removing code (CMAKE_BUILD_TYPE=Release)
v.reserve(10);
v.push_back(42);
v.shrink_to_fit();
return v[argc-1];
}
If I run the above on my Debian bullseye machine (with backports). Here is what I find out after configuration && build:
% cmake -DCMAKE_BUILD_TYPE:STRING=Release . && make
Leads to:
% nm demo | grep cxx17
0000000000001370 W _ZNSt6vectorIiSaIiEE17_M_realloc_insertIJiEEEvN9__gnu_cxx17__normal_iteratorIPiS1_EEDpOT_
However:
% ./demo
__cplusplus=201402
How come I see cxx17 related symbol when I make sure to compile with c++14 only flags ?
For reference:
% g++ --version
g++ (Debian 10.2.1-6) 10.2.1 20210110
and
% cmake --version
cmake version 3.22.0