Does the behavior of guaranteed copy elision depend on existence of user-defined copy constructor?

Question

The following code behaves differently with or without user-defined copy constructor under GCC 8.0.1:

#include 

struct S {
    int i;
    int         


        

Answer 1

Quoting from C++17 Working Draft §15.2 Temporary Objects Paragraph 3 (https://timsong-cpp.github.io/cppwp/class.temporary#3):

When an object of class type X is passed to or returned from a function, if each copy constructor, move constructor, and destructor of X is either trivial or deleted, and X has at least one non-deleted copy or move constructor, implementations are permitted to create a temporary object to hold the function parameter or result object. ... [ Note: This latitude is granted to allow objects of class type to be passed to or returned from functions in registers. — end note]

In your case, when I made both copy and move constructors defaulted:

S(const S &) = default;
S(S &&) = default;

assertion failed as well with GCC and Clang. Note that implicitly-defined constructors are trivial.