...
The copy constructor for an object thrown as an exception must be declared noexcept(true), including any implicitly-defined copy constructors. Note, any function declared noexcept(true) that terminates by throwing an exception violates ERR55-CPP. Honor exception specifications.
...
In this noncompliant code example, an exception of type S is thrown from f(). However, because S has a std::string data member, and the copy constructor for std::string is not declared noexcept(true), the implicitly defined copy constructor for S is also not declared to be noexcept(true). In low-memory situations, the copy constructor for std::string may be unable to allocate sufficient memory to complete the copy operation, resulting in a std::bad_alloc exception being thrown.
...