From Jason Turner

[polymorphic.ctor]

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+ #### Constructors <a id="polymorphic.ctor">[[polymorphic.ctor]]</a>
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+
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+ The following element applies to all functions in  [[polymorphic.ctor]]:
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+
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+ *Throws:* Nothing unless `allocator_traits<Allocator>::allocate` or
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+ `allocator_traits<Allocator>::construct` throws.
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+
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+ ``` cpp
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+ constexpr explicit polymorphic();
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+ ```
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+
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+ *Constraints:* `is_default_constructible_v<Allocator>` is `true`.
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+
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+ *Mandates:*
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+
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+ - `is_default_constructible_v<T>` is `true`, and
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+ - `is_copy_constructible_v<T>` is `true`.
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+
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+ *Effects:* Constructs an owned object of type `T` with an empty argument
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+ list using the allocator *alloc*.
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+
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+ ``` cpp
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+ constexpr explicit polymorphic(allocator_arg_t, const Allocator& a);
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+ ```
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+
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+ *Mandates:*
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+
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+ - `is_default_constructible_v<T>` is `true`, and
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+ - `is_copy_constructible_v<T>` is `true`.
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+
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+ *Effects:* *alloc* is direct-non-list-initialized with `a`. Constructs
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+ an owned object of type `T` with an empty argument list using the
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+ allocator *alloc*.
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+
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+ ``` cpp
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+ constexpr polymorphic(const polymorphic& other);
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+ ```
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+
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+ *Effects:* *alloc* is direct-non-list-initialized with
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+ `allocator_traits<Allocator>::select_on_container_copy_construction(other.`*`alloc`*`)`.
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+ If `other` is valueless, `*this` is valueless. Otherwise, constructs an
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+ owned object of type `U`, where `U` is the type of the owned object in
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+ `other`, with the owned object in `other` using the allocator *alloc*.
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+
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+ ``` cpp
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+ constexpr polymorphic(allocator_arg_t, const Allocator& a, const polymorphic& other);
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+ ```
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+
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+ *Effects:* *alloc* is direct-non-list-initialized with `a`. If `other`
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+ is valueless, `*this` is valueless. Otherwise, constructs an owned
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+ object of type `U`, where `U` is the type of the owned object in
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+ `other`, with the owned object in `other` using the allocator *alloc*.
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+
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+ ``` cpp
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+ constexpr polymorphic(polymorphic&& other) noexcept;
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+ ```
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+
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+ *Effects:* *alloc* is direct-non-list-initialized with
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+ `std::move(other.`*`alloc`*`)`. If `other` is valueless, `*this` is
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+ valueless. Otherwise, either `*this` takes ownership of the owned object
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+ of `other` or, owns an object of the same type constructed from the
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+ owned object of `other` considering that owned object as an rvalue,
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+ using the allocator *alloc*.
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+
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+ ``` cpp
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+ constexpr polymorphic(allocator_arg_t, const Allocator& a, polymorphic&& other)
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+ noexcept(allocator_traits<Allocator>::is_always_equal::value);
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+ ```
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+
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+ *Effects:* *alloc* is direct-non-list-initialized with `a`. If `other`
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+ is valueless, `*this` is valueless. Otherwise, if
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+ *`alloc`*` == other.`*`alloc`* is `true`, either constructs an object of
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+ type `polymorphic` that owns the owned object of `other`, making `other`
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+ valueless; or, owns an object of the same type constructed from the
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+ owned object of `other` considering that owned object as an rvalue.
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+ Otherwise, if *`alloc`*` != other.`*`alloc`* is `true`, constructs an
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+ object of type `polymorphic`, considering the owned object in `other` as
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+ an rvalue, using the allocator *alloc*.
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+
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+ ``` cpp
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+ template<class U = T>
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+ constexpr explicit polymorphic(U&& u);
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+ ```
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+
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+ *Constraints:* Where `UU` is `remove_cvref_t<U>`,
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+
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+ - `is_same_v<UU, polymorphic>` is `false`,
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+ - `derived_from<UU, T>` is `true`,
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+ - `is_constructible_v<UU, U>` is `true`,
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+ - `is_copy_constructible_v<UU>` is `true`,
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+ - `UU` is not a specialization of `in_place_type_t`, and
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+ - `is_default_constructible_v<Allocator>` is `true`.
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+
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+ *Effects:* Constructs an owned object of type `UU` with
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+ `std::forward<U>(u)` using the allocator *alloc*.
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+
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+ ``` cpp
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+ template<class U = T>
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+ constexpr explicit polymorphic(allocator_arg_t, const Allocator& a, U&& u);
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+ ```
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+
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+ *Constraints:* Where `UU` is `remove_cvref_t<U>`,
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+
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+ - `is_same_v<UU, polymorphic>` is `false`,
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+ - `derived_from<UU, T>` is `true`,
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+ - `is_constructible_v<UU, U>` is `true`,
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+ - `is_copy_constructible_v<UU>` is `true`, and
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+ - `UU` is not a specialization of `in_place_type_t`.
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+
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+ *Effects:* *alloc* is direct-non-list-initialized with `a`. Constructs
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+ an owned object of type `UU` with `std::forward<U>(u)` using the
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+ allocator *alloc*.
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+
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+ ``` cpp
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+ template<class U, class... Ts>
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+ constexpr explicit polymorphic(in_place_type_t<U>, Ts&&... ts);
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+ ```
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+
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+ *Constraints:*
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+
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+ - `is_same_v<remove_cvref_t<U>, U>` is `true`,
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+ - `derived_from<U, T>` is `true`,
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+ - `is_constructible_v<U, Ts...>` is `true`,
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+ - `is_copy_constructible_v<U>` is `true`, and
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+ - `is_default_constructible_v<Allocator>` is `true`.
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+
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+ *Effects:* Constructs an owned object of type `U` with
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+ `std::forward<Ts>(ts)...` using the allocator *alloc*.
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+
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+ ``` cpp
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+ template<class U, class... Ts>
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+ constexpr explicit polymorphic(allocator_arg_t, const Allocator& a,
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+ in_place_type_t<U>, Ts&&... ts);
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+ ```
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+
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+ *Constraints:*
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+
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+ - `is_same_v<remove_cvref_t<U>, U>` is `true`,
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+ - `derived_from<U, T>` is `true`,
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+ - `is_constructible_v<U, Ts...>` is `true`, and
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+ - `is_copy_constructible_v<U>` is `true`.
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+
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+ *Effects:* *alloc* is direct-non-list-initialized with `a`. Constructs
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+ an owned object of type `U` with `std::forward<Ts>(ts)...` using the
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+ allocator *alloc*.
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+
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+ ``` cpp
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+ template<class U, class I, class... Us>
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+ constexpr explicit polymorphic(in_place_type_t<U>, initializer_list<I> ilist, Us&&... us);
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+ ```
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+
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+ *Constraints:*
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+
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+ - `is_same_v<remove_cvref_t<U>, U>` is `true`,
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+ - `derived_from<U, T>` is `true`,
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+ - `is_constructible_v<U, initializer_list<I>&, Us...>` is `true`,
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+ - `is_copy_constructible_v<U>` is `true`, and
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+ - `is_default_constructible_v<Allocator>` is `true`.
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+
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+ *Effects:* Constructs an owned object of type `U` with the arguments
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+ `ilist, std::forward<Us>(us)...` using the allocator *alloc*.
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+
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+ ``` cpp
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+ template<class U, class I, class... Us>
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+ constexpr explicit polymorphic(allocator_arg_t, const Allocator& a,
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+ in_place_type_t<U>, initializer_list<I> ilist, Us&&... us);
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+ ```
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+
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+ *Constraints:*
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+
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+ - `is_same_v<remove_cvref_t<U>, U>` is `true`,
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+ - `derived_from<U, T>` is `true`,
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+ - `is_constructible_v<U, initializer_list<I>&, Us...>` is `true`, and
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+ - `is_copy_constructible_v<U>` is `true`.
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+
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+ *Effects:* *alloc* is direct-non-list-initialized with `a`. Constructs
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+ an owned object of type `U` with the arguments
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+ `ilist, std::forward<Us>(us)...` using the allocator *alloc*.
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+