I want to use flat_hash_map with a key type that's very expensive to copy.
To avoid accidental copies I want to make the copy ctor inaccessible,
and instead provide an explicit ExpensiveCopy() function that will make copies.
Something along the following lines:

class X {
  X(const X&) = default;
 public:
  X(...);
  X(X&&) = default;
  X ExpensiveCopy() const { return *this; }
};

And then use it as follows:

flat_hash_map<X, bool> m;

m[x.ExpensiveCopy()] = ...;

Currently it does not compile, since hash map code still tries
to call the copy ctor on a non-executed path.
Use "if constexpr" to avoid calling the ctor.

PiperOrigin-RevId: 797989104