std, which finds the declaration of namespace std in the header
cout, which finds a variable declaration in the namespace
endl, which finds a function template declaration in the namespace
operator <<which finds multiple function template declarations in the namespace std and qualified name lookup for the name
std::ostream::operator<<which finds multiple member function declarations in class
For function and function template names, name lookup can associate multiple declarations with the same name, and may obtain additional declarations from argument-dependent lookup. Template argument deduction may also apply, and the set of declarations is passed to overload resolution, which selects the declaration that will be used. Member access rules, if applicable, are considered only after name lookup and overload resolution.
For all other names (variables, namespaces, classes, etc), name lookup must produce a single declaration in order for the program to compile. Lookup for a name in a scope finds all declarations of that name, with one exception, known as the "struct hack" or "type/non-type hiding": Within the same scope, some occurrences of a name may refer to a declaration of a class/struct/union/enum that is not a typedef, while all other occurrences of the same name either all refer to the same variable, non-static data member (since C++14), or enumerator, or they all refer to possibly overloaded function or function template names. In this case, there is no error, but the type name is hidden from lookup (the code must use elaborated type specifier to access it).
If the name appears immediately to the right of the scope resolution operator
:: or possibly after
:: followed by the disambiguating keyword
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