A program that declares
—(3.1) a variable main that belongs to the global scope, or
—(3.2) a function main that belongs to the global scope and is attached to a named module, or
—(3.3) a function template main that belongs to the global scope, or
—(3.4) an entity named main with C language linkage (in any namespace)
is ill-formed. The name main is not otherwise reserved

main.cpp:3:5: error: cannot attach '::main' to a named module
    3 | int main() {}