generalization incorrectly handled higher-ranked ali... | rust-lang/rust

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I've found an unsoundess bug in rustc with some LLM-driven analysis. As discussed on zulip (t-types/trait-system-refactor thread), @theemathas produced the minimization below; filing here at @lcnr's request since the underlying issue also affects stable.

I tried this code:

#![forbid(unsafe_code)]

struct Wrap<T>(T);

trait Id {
    type Out;
}
impl<T> Id for T {
    type Out = u16;
}

trait Probe {}
// impl Probe for &'_ fn(u16) {}
impl<'c> Probe for &'c Wrap<for<'a> fn(<&'a &'c u64 as Id>::Out)> {}

trait Indirect {}
// impl Indirect for &'_ fn(u16) {}
impl<P: Probe> Indirect for P {}

trait Mark {}
impl Mark for fn(u16) {}

trait Select {}
impl<X> Select for X
// where X = fn(u16)
where
    for<'d> &'d Wrap<X>: Indirect,
{
}
// where Z = fn(u16)
impl<Z: Mark> Select for Z {}

I expected to see this happen: rejected with E0119

Instead, this happened: it compiles, on stable, nightly, and nightly with -Znext-solver.

Meta

stable: rustc 1.97.1 (8bab26f4f 2026-07-14)
nightly: rustc 1.100.0-nightly (8fa1c96cf 2026-08-17)

ICE variant (by @theemathas; stable + nightly + -Znext-solver)

#![forbid(unsafe_code)]

struct Wrap<T>(T);

trait Id {
    type Out;
}
impl<T> Id for T {
    type Out = u16;
}

trait Probe {}
impl<'c> Probe for &'c Wrap<for<'a> fn(<&'a &'c u64 as Id>::Out)> {}

trait Indirect {}
impl<P: Probe> Indirect for P {}

trait Mark {}
impl Mark for fn(u16) {}

trait Select {
    type Assoc;
}
impl<X> Select for X
where
    for<'d> &'d Wrap<X>: Indirect,
{
    type Assoc = usize;
}
impl<Z: Mark> Select for Z {
    type Assoc = &'static i32;
}

trait Same<U> {
    fn convert(x: <Self as Select>::Assoc) -> <U as Select>::Assoc
    where
        Self: Select,
        U: Select;
}

impl<T> Same<T> for T {
    fn convert(x: <T as Select>::Assoc) -> <T as Select>::Assoc
    where
        Self: Select,
    {
        x
    }
}

fn put<T>(x: usize) -> <T as Select>::Assoc
where
    for<'a> &'a Wrap<T>: Indirect,
{
    x
}

fn get<T>(x: <T as Select>::Assoc) -> &'static i32
where
    T: Mark,
{
    x
}

fn put_get<T, U>(x: usize) -> &'static i32
where
    for<'a> &'a Wrap<T>: Indirect,
    U: Mark,
    T: Same<U>,
{
    get::<U>(<T as Same<U>>::convert(put::<T>(x)))
}

fn foo<T, U>(x: usize) -> &'static i32
where
    for<'a> &'a Wrap<T>: Indirect,
    T: Mark,
{
    put_get::<T, T>(x)
}

fn main() {
    foo::<fn(u16), fn(u16)>(1_usize);
}

error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:201:13:
Failed to normalize Alias(AliasTy { args: [Binder { value: fn(u16), bound_vars: [] }],
kind: Projection { def_id: DefId(... Select::Assoc) }, .. }) in typing_env=... PostAnalysis ...

Original -Znext-solver soundness reproducer (rejected E0284 by the old solver; accepted by -Znext-solver; compiled program will segfault)

#![forbid(unsafe_code)]

trait Overlap {
    type T;
}
impl<S, V> Overlap for S
where
    S: Select<Pick = V>,
    for<'d> &'d [V]: Indirect,
{
    type T = usize;
}
impl Overlap for fn(u16) {
    type T = &'static u8;
}

trait Id {
    type Out;
}
impl<T> Id for T {
    type Out = u16;
}

trait Probe {}
impl<'c> Probe for &'c [for<'a> fn(<&'a &'c u8 as Id>::Out)] {}

trait Indirect {}
impl<P: Probe> Indirect for P {}

trait Select {
    type Pick;
}
impl<X> Select for X
where
    for<'d> &'d [X]: Indirect,
{
    type Pick = X;
}
impl<Z: Copy> Select for Z {
    type Pick = u8;
}

fn g<X>() -> <X as Overlap>::T
where
    for<'d> &'d [X]: Indirect,
{
    1usize
}

fn main() {
    println!("{}", *g::<fn(u16)>());
}

Start by compiling the minimized reproducer on stable, nightly, and nightly with -Znext-solver, then compare the reported acceptance with the expected E0119 rejection. Use the trait and higher-ranked alias interactions in the reproducer as the entry point; done means the unsound example is rejected without the reported ICE or accepted compilation.

Written by the indexing model from the issue text.

generalization incorrectly handled higher-ranked aliases

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generalization incorrectly handled higher-ranked aliases

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