Unexpected result when C++ store element into std::vector from return value of function

The behaviour is undefined in all C++ versions before C++17. The simple reason is that the two sides of the assignment operator can be evaluated in any order:

• Assuming A[1] is evaluated first, you get an int& referring to the second element of A at that point.
• Then, the func() is evaluated, which can reallocate the storage for the vector, leaving the previously retrieved int& a dangling reference.
• Finally, the assignment is performed, writing to unallocated storage. Since the standard allocators cache memory, the OS often won’t catch this error.

Only in C++17, the special rule 20 for the assignment was made:

In every simple assignment expression E1=E2 and every compound
assignment expression E1@=E2, every value computation and side-effect
of E2 is sequenced before every value computation and side effect of
E1

With C++17, A[1] must be evaluated after the call to func(), which then provides defined, reliable behaviour.