c++ - copy_if() stream_iterator works with C++20 but not C++17

Question

$ cat so.cpp 
#include <iostream>
#include <string> 
#include <iterator> 
int main()
{
        std::istream_iterator<char> cin_it(std::cin),eos;
        std::ostream_iterator<char> cout_it(std::cout);
        std::copy_if(cin_it, eos, cout_it, [](char x){return std::isupper(x);});
        return 0;
    }

Works with C++20 but not C++17. Why does C++20 not require <algorithm>?

$ g++ -std=c++2a so.cpp
$ g++ -std=c++17 so.cpp
so.cpp: In function ‘int main()’:
so.cpp:8:7: error: ‘copy_if’ is not a member of ‘std’; did you mean ‘copy’?
    8 |  std::copy_if(cin_it, eos, cout_it, [](char x){return std::isupper(x);});
      |       ^~~~~~~
      |       copy

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 9.3.0-17ubuntu1~20.04' --with-bugurl=file:///usr/share/doc/gcc-9/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,gm2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-9 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-9-HskZEa/gcc-9-9.3.0/debian/tmp-nvptx/usr,hsa --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04) 

Answer 1

等待大神答复