error: internal compiler error: /checkout/src/librustc_typeck/check/mod.rs:4419: impossible case reached

[mwchase]

Encountered doing ill-advised type manipulations in the playground. By (mis)using some combination of universal function call syntax, the Fn trait, and other language features, I got an internal compiler error. I then simplified it to the point where it wouldn't make sense even if it didn't hit the error.

Code to reproduce the error:

trait CallSingle<A, B> {
    fn call(&self, a: A) -> B where Self: Fn(A) -> B;
}

impl<A, B, F: Fn(A) -> B> CallSingle<A, B> for F {
    fn call(&self, a: A) -> B {
        <Self as Fn(A) -> B>::call
    }
}

I'm not sure there should be any error it emits beyond what it already does, but this does panic the compiler: error: internal compiler error: /checkout/src/librustc_typeck/check/mod.rs:4419: impossible case reached

[estebank]

This match arm is reached.

[petrochenkov]

I have a fix in progress (accidentally), will submit this weekend.

[petrochenkov]

#43532 fixes the ICE, but the code still doesn't compile.
For some reason associated type bindings are prohibited in traits in UFCS-style paths

<Type as Trait<T = u8 /* <-- can't do this */>>::AssocItem

and B in Fn(A) -> B is exactly an associated type binding

<Self as Fn(A) -> B>::call
// Desugars to
<Self as Fn<(A,), Output = B>>::call

[petrochenkov]

Current output:

#![feature(fn_traits)]

trait CallSingle<A, B> {
    fn call(&self, a: A) -> B where Self: Fn(A) -> B;
}

impl<A, B, F: Fn(A) -> B> CallSingle<A, B> for F {
    fn call(&self, a: A) -> B {
        <Self as Fn(A) -> B>::call(self, (a,))
    }
}

fn main() {}

---

   Compiling playground v0.0.1 (file:///playground)
error[E0229]: associated type bindings are not allowed here
 --> src/main.rs:9:27
  |
9 |         <Self as Fn(A) -> B>::call(self, (a,))
  |                           ^ associated type not allowed here

error: aborting due to previous error

I'm closing this because the ICE is fixed and supporting type bindings in value paths (let x = Trait<AssocTy = u8>::method();) is a separate problem, please open a new lang-team issue if you are interested in supporting it.