Why are begin(container) and end(container) accessible everywhere but aren't in global namespace? [duplicate]

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• What is “Argument-Dependent Lookup” (aka ADL, or “Koenig Lookup”)? 4 answers

I can do

std::vector<int> v;
begin(v);

yet, when I do

std::vector<int> v;
::begin(v);

I get

error: ‘::begin’ has not been declared ::begin(v);

It works again if I put

using namespace std;

at the top of the file, which I find weird, because I thought doing ::begin would actively prevent it from using std::begin. What is happening here?