User-defined literal doesn't work when the definition is written in a different .cpp file

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C++

debugging

C++20

I created a user-defined literal like this in is OWN .cpp file (declared as a friend function in .h file):

<pre lang="C++">
fraction operator"" _frac(const long double val)
{
	return fraction(static_cast<float>(val));
}

But in main it produces this error:

Error (active)E2486 user-defined literal operator not found

But when I write the SAME code (definitely the same as I copy it and also do some words comparison) in main or .h(outside of the class scope) file, it works fine? Is it because it must be written as a global function? I'm using Visual Studio 2019 and C++ 20. GitHub

What I have tried:

I created other classes and use an older version of C++, I still get the same result (definition in the same file as main works fine but if written in another file doesn't work). I can accept to write it in the same file, but then what's the purpose of orientated programming anymore.