https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32667
--- Comment #49 from post+gcc at ralfj dot de --- Even glibc itself seems to use `restrict`: https://codebrowser.dev/glibc/glibc/string/string.h.html#43 So the compiler building glibc might inadvertently rely on the memory written through dst and the memory read through src being disjoint, making even the perfectly-overlapping case UB (unless the implementation has a guard somewhere that skips the copy when src==dst, but I was not able to find such a guard). (The implementation at https://codebrowser.dev/glibc/glibc/string/memcpy.c.html does not have the `restrict`, but it includes the string.h header and I think the compiler is allowed to apply attributes from the declaration to the definition. Or, alternatively, it might even be UB to have `restrict` in one place and not the other: "All declarations that refer to the same object or function shall have compatible type; otherwise, the behavior is undefined" [C23 §6.2.7.2] and "For two qualified types to be compatible, both shall have the identically qualified version of a compatible type; the order of type qualifiers within a list of specifiers or qualifiers does not affect the specified type" [C23 §6.7.3.11].)
