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• This constructor isconstexpr if and only if the value initialization of the alternative typeT_0 would satisfy the requirements for aconstexpr function.
• This overload participates in overload resolution only ifstd::is_default_constructible_v<T_0> istrue.

• This constructor is defined as deleted unlessstd::is_copy_constructible_v<T_i> istrue for allT_i inTypes....
• It is trivial ifstd::is_trivially_copy_constructible_v<T_i> istrue for allT_i inTypes....

• This overload participates in overload resolution only ifstd::is_move_constructible_v<T_i> istrue for allT_i inTypes....
• It is trivial ifstd::is_trivially_move_constructible_v<T_i> istrue for allT_i inTypes....

Formally:

• An overloadF(T_i) is only considered if the declarationT_i x[]={std::forward<T>(t)}; is valid for some invented variablex.

• This overload participates in overload resolution only if
  • sizeof...(Types)>0,
  • std::decay_t<T>(until C++20)std::remove_cvref_t<T>(since C++20) is neither the same type asvariant, nor a specialization ofstd::in_place_type_t, nor a specialization ofstd::in_place_index_t,
  • std::is_constructible_v<T_j, T> istrue,
  • and the expressionF(std::forward<T>(t)) (with F being the above-mentioned set of imaginary functions) is well formed.
• This constructor is aconstexpr constructor ifT_j's selected constructor is a constexpr constructor.

std::variant<std::string> v("abc");// OKstd::variant<std::string,std::string> w("abc");// ill-formedstd::variant<std::string,constchar*> x("abc");// OK, chooses const char*std::variant<std::string,bool> y("abc");// OK, chooses string; bool is not a candidatestd::variant<float,long,double> z=0;// OK, holds long// float and double are not candidates

• IfT's selected constructor is aconstexpr constructor, this constructor is also aconstexpr constructor.
• This overload participates in overload resolution only if there is exactly one occurrence ofT inTypes... andstd::is_constructible_v<T, Args...> istrue.

• IfT's selected constructor is aconstexpr constructor, this constructor is also aconstexpr constructor.
• This overload participates in overload resolution only if there is exactly one occurrence ofT inTypes... andstd::is_constructible_v<T, initializer_list<U>&, Args...> istrue.

• IfT_i's selected constructor is aconstexpr constructor, this constructor is also aconstexpr constructor.
• This overload participates in overload resolution only ifI< sizeof...(Types) andstd::is_constructible_v<T_i, Args...> istrue.

• IfT_i's selected constructor is aconstexpr constructor, this constructor is also aconstexpr constructor.
• This overload participates in overload resolution only ifI< sizeof...(Types) andstd::is_constructible_v<T_i,std::initializer_list<U>&, Args...> istrue.

#include <cassert>#include <iostream>#include <string>#include <variant>#include <vector> using vector_t=std::vector<int>; auto& operator<<(auto& out,const vector_t& v){    out<<"{ ";for(int e: v)        out<< e<<' ';return out<<'}';} int main(){// value-initializes first alternativestd::variant<int,std::string> var0;assert(std::holds_alternative<int>(var0) and           var0.index()==0 and           std::get<int>(var0)==0); // initializes first alternative with std::string{"STR"};std::variant<std::string,int> var1{"STR"};assert(var1.index()==0);std::cout<<"1) "<< std::get<std::string>(var1)<<'\n'; // initializes second alternative with int == 42;std::variant<std::string,int> var2{42};assert(std::holds_alternative<int>(var2));std::cout<<"2) "<< std::get<int>(var2)<<'\n'; // initializes first alternative with std::string{4, 'A'};std::variant<std::string, vector_t,float> var3{std::in_place_type<std::string>,4,'A'};assert(var3.index()==0);std::cout<<"3) "<< std::get<std::string>(var3)<<'\n'; // initializes second alternative with std::vector{1,2,3,4,5};std::variant<std::string, vector_t,char> var4{std::in_place_type<vector_t>,{1,2,3,4,5}};assert(var4.index()==1);std::cout<<"4) "<< std::get<vector_t>(var4)<<'\n'; // initializes first alternative with std::string{"ABCDE", 3};std::variant<std::string, vector_t,bool> var5{std::in_place_index<0>,"ABCDE",3};assert(var5.index()==0);std::cout<<"5) "<< std::get<std::string>(var5)<<'\n'; // initializes second alternative with std::vector(4, 42);std::variant<std::string, vector_t,char> var6{std::in_place_index<1>,4,42};assert(std::holds_alternative<vector_t>(var6));std::cout<<"6) "<< std::get<vector_t>(var6)<<'\n';}

Output:

1) STR2) 423) AAAA4) { 1 2 3 4 5 }5) ABC6) { 42 42 42 42 }