foo.cc:2:16: error: expected identifier before ';' token
    2 | using namespace;
      |                ^
foo.cc:3:5: error: cannot declare '::main' to be a global variable
    3 | int main;
      |     ^~~~
foo.cc:4:1: error: expected unqualified-id before '{' token
    4 | {
      | ^

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