Minimal reproduction of rustc ICE issue #48638

main.rs

use std::any::Any;
use std::fmt::Debug;
use std::marker::PhantomData;
// use std::ops::Mul;

pub trait Scalar: Copy + PartialEq + Debug + Any {}
impl Scalar for f64 {}

pub trait Dim: Any + Debug + Copy + PartialEq + Send {}

// pub trait DimName : Dim {
//     type Value: NamedDim<Name = Self>;
// }
// pub trait NamedDim: Sized + Any/* + Unsigned*/ {
//     type Name: DimName<Value = Self>;
// }

#[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
pub struct U1;
impl Dim for U1 {}
// impl DimName for U1 {}

pub unsafe trait ContiguousStorageMut<N: Scalar, R: Dim, C: Dim = U1> { }

pub trait Allocator<N: Scalar, R: Dim, C: Dim = U1>: Any + Sized {
    type Buffer: ContiguousStorageMut<N, R, C> + Clone;
}

// #[repr(C)]
// pub struct MatrixArray<N, R, C>
// where
//     R: DimName,
//     C: DimName,
//     R::Value: Mul<C::Value>,
//     Prod<R::Value, C::Value>: ArrayLength<N>,
// {
//     data: GenericArray<N, Prod<R::Value, C::Value>>,
// }

pub struct DefaultAllocator;
// impl<N, R, C> Allocator<N, R, C> for DefaultAllocator
// where
//     N: Scalar,
//     R: DimName,
//     C: DimName,
// {
//     type Buffer = MatrixArray<N, R, C>;
// }

pub type Owned<N, R, C = U1> = <DefaultAllocator as Allocator<N, R, C>>::Buffer;

#[repr(C)]
#[derive(Hash, Clone, Copy)]
pub struct Matrix<N: Scalar, R: Dim, C: Dim, S> {
    pub data: S,
    _phantoms: PhantomData<(N, R, C)>,
}

#[repr(packed)]
struct S {
    x:Matrix<f64, U1, U1, Owned<f64, U1, U1>>,
}

fn main() {
}

error: internal compiler error: librustc_typeck/check/wfcheck.rs:243: inference variables in Matrix<f64, U1, U1, _>
  --> src/main.rs:60:1
   |
60 | / struct S {
61 | |     x:Matrix<f64, U1, U1, Owned<f64, U1, U1>>,
62 | | }
   | |_^