https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68350
Barry Revzin <barry.revzin at gmail dot com> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |barry.revzin at gmail dot com --- Comment #8 from Barry Revzin <barry.revzin at gmail dot com> --- Here's an example: struct A { int i; }; struct B { int i{}; }; Both are trivially copyable, A is additionally trivially default constructible while B is not. This distinction is irrelevant in this case - we're copying, and that does not involve default constructing an A or B (trivial or otherwise). But this means that __is_trivial(B) is false, so we don't go into std::copy, so these do different things: #include <memory> struct A { int i; }; struct B { int i{}; }; void copy_a(A* f, A* l, A* out) { std::uninitialized_copy(f, l, out); } void copy_b(B* f, B* l, B* out) { std::uninitialized_copy(f, l, out); } emits (with g++ 11.2, -std=c++20 -O3, https://godbolt.org/z/eG9hsbcTE): copy_a(A*, A*, A*): mov r8, rdi mov rdi, rdx cmp r8, rsi je .L1 mov rdx, rsi mov rsi, r8 sub rdx, r8 jmp memmove .L1: ret copy_b(B*, B*, B*): cmp rdi, rsi je .L4 sub rsi, rdi xor eax, eax .L6: mov ecx, DWORD PTR [rdi+rax] mov DWORD PTR [rdx+rax], ecx add rax, 4 cmp rax, rsi jne .L6 .L4: ret Whereas the copy_b case could also use memmove.