I tried this code:

trait Mirror { type Assoc: ?Sized; } impl<T: ?Sized> Mirror for T { type Assoc = T; } trait Q: 'static { fn q(&self); } impl Q for i32 { fn q(&self) { println!("i32"); } } impl Q for <dyn Q as Mirror>::Assoc where Self: 'static { fn q(&self) { println!("dyn Q"); } } fn foo<T: Q + ?Sized>(t: &T) { t.q(); } fn main() { foo(&1 as &dyn Q); }

I expected it to work. But instead:

Exited with signal 11 (SIGSEGV): segmentation violation 

This is a manifestation of two bugs:

• Overlap between built-in and user-written impls for dyn Trait: Trait
• Ambiguity was being treated as a hard error in the impossible_predicates method

Since the new trait solver treats some dyn Trait: Trait overlap as ambiguity (specifically when the responses from the two candidates differ), and we strike methods from the vtable if impossible_predicates returns true, a false positive will lead to a segfault.

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rustc --version --verbose:

rustc 1.89.0-nightly (16d2276fa 2025-05-16) binary: rustc commit-hash: 16d2276fa6fccb0cc239a542d4c3f0eb46f660ec commit-date: 2025-05-16 host: x86_64-unknown-linux-gnu release: 1.89.0-nightly LLVM version: 20.1.4