[_; 33] implements Copy, but not Clone

[LunaBorowska]

From https://www.reddit.com/r/rust/comments/616rc8/rcpp_discusses_rust/dfcjk09/

The following code:

fn clone_copy_type<T: Copy>(t: T) -> T {
    t.clone()
}

fn main() {
    // [_; 33] implements Copy, but not Clone, violating trait inheritance
    clone_copy_type([(); 33]);
}

Causes the following crash in playpen, due to [T; 33] (where T: Copy) implementing Copy, but not Clone, but Copy trait requiring Clone implementation.

error: internal compiler error: /checkout/src/librustc_trans/monomorphize.rs:151: Encountered error `Unimplemented` selecting `Binder(<[(); 33] as std::clone::Clone>)` during trans

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:377
note: Run with `RUST_BACKTRACE=1` for a backtrace.

[steveklabnik]

It's well known that this doesn't implement the trait, but it shouldn't ICE.

[jonas-schievink]

The ICE is also well known. Since Clone is a supertrait of Copy, but isn't implemented in this case, this is technically a prime example for unsoundness. The Compiler is basically correct in assuming this will never happen, hence the ICE.

Also see #24000 and #28229

[tbu-]

Basically a duplicate of those.

[steveklabnik]

Okay! Sorry about that @xfix , I didn't see those when I was looking.