std::variant can hold a value of any one of its alternative types, or no value. To refer to the alternative types, it uses an index i.

§[variant.ctor]¶1 in the standard:

(...) let i be in the range [0, sizeof...(Types)), and T_i be the i^th type in Types.

In our case, T₀ means int, T₁ means double, and T₂ means char.

Now what happens if you define a variant without initializing it with a certain type? The default constructor will pick the type T₀, in our case int, and value-initialize it.

§[variant.ctor]¶3:

constexpr variant() noexcept(see below);

Effects: Constructs a variant holding a value-initialized value of type T₀.

Finally, we call index() and print the result. index() returns the index of the type of the contained value. The contained value is an int, aka T₀, so 0 is returned.

§[variant.status]¶3:

constexpr size_t index() const noexcept;

Effects: If [it doesn't contain a value], returns variant_npos. Otherwise, returns the zero- based index of the alternative of the contained value.