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P0966R1 allows reserve() and reserve(0) to do different things, so they need to be overloads rather than use a default argument of 0, and the libc++ commit does split the function into two overloaded functions. libc++'s string::reserve(size_type) function still lowers the capacity, though. Its shrink_to_fit() still calls reserve(), which calls reserve(0).
constexpr void reserve(size_type res_arg);
Effects: A directive that informs a basic_string of a planned change in size, so that the storage allocation can be managed accordingly. After reserve(), capacity() is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of capacity() otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument of reserve().
The libc++ commit added a test that looks like it verifies that reserve doesn't shrink, but it doesn't really do that. In string.capacity/reserve.pass.cpp:
We have a string, s with size() == 50 and capacity() >= 100. Calling reserve(5) should leave the capacity() >= 100 as of P0966R1, but libc++ shrinks the string to a little above 50. However, main() passes the string by-value to test(), and the copy constructor makes a new string that's shrunk-to-fit. i.e. AFAICT, the sections in main() that use erase() aren't testing the intended situation.
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