[common.iter.const]

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constexpr common_iterator(I i);

Effects: Initializes v_ as if by v_{in_place_type, std::move(i)}.

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constexpr common_iterator(S s);

Effects: Initializes v_ as if by v_{in_place_type<S>, std::move(s)}.

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template<class I2, class S2>
  requires convertible_to<const I2&, I> && convertible_to<const S2&, S>
    constexpr common_iterator(const common_iterator<I2, S2>& x);

Effects: Initializes v_ as if by v_{in_place_index, get(x.v_)}, where i is x.v_.index().

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template<class I2, class S2>
  requires convertible_to<const I2&, I> && convertible_to<const S2&, S> &&
           assignable_from<I&, const I2&> && assignable_from<S&, const S2&>
    constexpr common_iterator& operator=(const common_iterator<I2, S2>& x);

Effects: Equivalent to:

(6.1)
If v_.index() == x.v_.index(), then get(v_) = get(x.v_).
(6.2)
Otherwise, v_.emplace(get(x.v_).

where i is x.v_.index().

Returns: *this.

24 Iterators library [iterators]

24.5 Iterator adaptors [predef.iterators]

24.5.5 Common iterators [iterators.common]

24.5.5.3 Constructors and conversions [common.iter.const]