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The _Arg_value::_M_set method, initialized the union member, by assigning to reference to that member produced by _M_get(*this). However, per language rules, such assignment has undefined behavior, if alternative was not already active, same as for any object not within its lifetime. To address above, we modify _M_set to use placement new for the class types, and invoke _S_access with two arguments for all other types. The _S_access (rename of _S_get) is modified to assign the value of the second parameter (if provided) to the union member. Such direct assignments are treated specially in the language (see N5032 [class.union.general] p5), and will start lifetime of trivially default constructible alternative. libstdc++-v3/ChangeLog: * include/std/format (_Arg_value::_M_get): Rename to... (_Arg_value::_M_access): Modified to accept optional second parameter that is assigned to value. (_Arg_value::_M_get): Handle rename. (_Arg_value::_M_set): Use construct_at for basic_string_view, handle, and two-argument _S_access for other types. Reviewed-by: Jonathan Wakely <jwakely@redhat.com> Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com> Signed-off-by: Ivan Lazaric <ivan.lazaric1@gmail.com> Co-authored-by: Ivan Lazaric <ivan.lazaric1@gmail.com>