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

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

This match arm is reached.

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

#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

Auto merge of #43532 - petrochenkov:pgargs, r=nikomatsakis

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.

Navigation Menu

Repository navigation

error: internal compiler error: /checkout/src/librustc_typeck/check/mod.rs:4419: impossible case reached #43431

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

error: internal compiler error: /checkout/src/librustc_typeck/check/mod.rs:4419: impossible case reached #43431

Description

Activity

estebank commented on Jul 26, 2017

petrochenkov commented on Jul 26, 2017

petrochenkov commented on Jul 28, 2017

petrochenkov commented on Oct 2, 2017

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions