WG21-N4958 [deque.capacity]/6:

Effects: shrink_to_fit is a non-binding request to reduce memory use but does not change the size of the sequence.
[Note 1: The request is non-binding to allow latitude for implementation-specific optimizations. - end note]
If the size is equal to the old capacity, or if an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable T, then there are no effects."

We unconditionally move, which is a bug:

(#4071 is patching this line in an unrelated way.)

vector provides its strong guarantees correctly, like this (note that it doesn't directly use move_if_noexcept()):