error: no member named 'async' in namespace 'std'

I know that std::async is a C++11 thing but I am pretty sure that my compiler has C++11 support.

#include <future>
using namespace::std;

void functionToRun() {
  // some code
}

int main() {
    auto x = 2; // throws warning warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
    std::future<void> metricLoggerFuture = std::async(std::launch::async, functionToRun); // throws error  no member named 'async' in namespace 'std'
}

and if I use

std::future<void> metricLoggerFuture = async(std::launch::async, functionToRun); // error: use of undeclared identifier 'async'

Also g++ --version shows

Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 11.0.3 (clang-1103.0.32.62)
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

I have gone through the following,

• No member named 'size' in namespace 'std'
• No Member named stoi in namespace std

Also,

• OS: macOS Catalina
• This code is part of a bigger project (wrapper over TigerVNC) so there is a makefile, but since auto complies without any issues I don't think C++11 is the issue and hence passing -std=c++11 in the CXXFLAGS and CPPFLAGS also doen't help.