Wouldn't that have changed the semantics (since you're not calling the copy constructor anymore?)
The student accidentally changed the semantics by adding the extra call to the copy contstructor, and therefore Clang didn't know anymore that it could be removed
Good point, it doesn't seem to be any bigger of a change than the original suggestion. Perhaps they mistakenly thought nobody would call emplace_back in that way by accident.
The warning was not explicit enough to ask you to remove the constructor as well as changing the method name. If it called for "hey, you can remove the constructor and emplace_back would automagically call it for you in place!" and then checked that the constructor is not spelled out there, then there would not be such issue and the tool would be 100% helpful.
It did ask to remove the constructor as well as changing the method (note which parts are covered by tildes).
My question is, after having incorrectly only done one of those things, how could it have known that the user was only halfway though a two-part operation and didn't actually mean for it to be that way? Clang doesn't know how the code was in the past.
The student accidentally changed the semantics by adding the extra call to the copy contstructor, and therefore Clang didn't know anymore that it could be removed