To allow std::string construction from std::string_viewthere is a template constructor
template<class T>
explicit basic_string(const T& t, const Allocator& alloc = Allocator());
which is enabled only if const T& is convertible to std::basic_string_view<CharT, Traits> (link).
In the meantime there is a special deduction guide to deduce basic_string from basic_string_view (link). A comment to the guide says:
Guides (2-3) are needed because the std::basic_string constructors for std::basic_string_views are made templates to avoid causing ambiguities in existing code, and those templates do not support class template argument deduction.
So I'm curious, what is that ambiguity that requires to have that deduction guide and template constructor instead of simply a constructor that takes std::basic_string_view, e.g. something like
explicit basic_string(basic_string_view<CharT, Traits> sv, const Allocator& alloc = Allocator());
Note that I'm not asking why the constructor is marked explicit.
The ambiguity is that std::string and std::string_view are both constructible from const char *. That makes things like
std::string{}.assign("ABCDE", 0, 1)
ambiguous if the first parameter can be either a string or a string_view.
There are several defect reports trying to sort this out, starting here.
https://cplusplus.github.io/LWG/issue2758
The first thing was to make members taking string_view into templates, which lowers their priority in overload resolution. Apparently, that was a bit too effective, so other adjustments were added later.