overlapping impl candidates are no longer an error

[aliemjay]

The following passes typeck and ICEs in MIR building for test with the message encountered ambiguity selecting <std::option::Option<T> as Overlap> during codegen:

trait Local {}

trait Overlap { fn f(); }
impl<T> Overlap for Option<T> where Self: Clone, { fn f() {} }
impl<T> Overlap for Option<T> where Self: Local, { fn f() {} }

fn test<T>()
where
    Option<T>: Clone + Local,
{
    <Option<T> as Overlap>::f();
}

test is not really callable as coherence disallows adding a impl Local for Option, but we should still return an ambiguity in case of overlapping impl candidates.

https://github.com/rust-lang/rust/blob/fe37f37e4b764bb82fc0eb8c727842746de7c93f/compiler/rustc_trait_selection/src/solve/mod.rs#L197

[compiler-errors]

I kind of want this to not return an ambiguity, but just compile without error. The only time this should ICE is during codegen...

[lcnr]

this will no longer ICE with rust-lang/rust#124374 and is something I'd like to work. Still keeping this open as it should be mentioned when stabilizing the new solver