In some situations, the C++ standard requires a copy constructor to be accessible...

In some situations, the C++ standard requires a copy constructor to be accessible even if it is not used. In particular, this holds when one creates a temporary object and binds it to a const reference. In this case, the compiler will usually bind the temporary directly to the reference, but a copy constructor will still have to be accessible. An example is f(FE_Q<dim>(2)). To allow this to compile, we need to make copy constructors accessible, but if they are actually called, we simply throw an exception.

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