ICE with self.variance.is_some()

[pnkfelix]

While trying to port the bug case from #4775 forward to more modern Rust syntax, I hit the following ICE:

% cat /tmp/n.rs
fn echo<T>(x:T) { io::println(fmt!("%?", x)); }

pub trait OpInt<'self> { fn call(&'self self, int, int) -> int; }

impl<'self> OpInt for &'self fn(int,int) -> int {
    fn call(&self, a:int, b:int) -> int {
        echo("I dont wanna die!");
        (*self)(a,b)
    }
}

fn squarei<'a>(x:int, op:&'a OpInt) -> int       { op.call(x, x) }

fn muli(x:int, y:int) -> int {
    echo("You will never get here.");
    x * y
}


fn main() {
    echo("Entered main");
    let f = |x,y|muli(x,y);
    {
        let g = &f;
        let h = g as &OpInt;
        let r = squarei(3, h); echo(r);
    }
}
% RUST_LOG=rustc=1,::rt::backtrace rustc /tmp/n.rs
rust: task failed at 'assertion failed: self.variance.is_some()', /Users/fklock/Dev/Mozilla/rust.git/src/librustc/middle/typeck/rscope.rs:179
error: internal compiler error: unexpected failure
note: the compiler hit an unexpected failure path. this is a bug
note: try running with RUST_LOG=rustc=1,::rt::backtrace to get further details and report the results to github.com/mozilla/rust/issues
rust: task failed at 'explicit failure', /Users/fklock/Dev/Mozilla/rust.git/src/librustc/rustc.rc:349
rust: domain main @0x7f8af2008410 root task failed
% 

[catamorphism]

Reproduced with b44953b ; nominating for milestone 5, production-ready

[graydon]

accepted for production-ready milestone

[pnkfelix]

(still a bug. assigning to self for investigation as part of grokking the type/borrow check code.)

[erickt]

Probably a subbug of #4846.

[flaper87]

This is an updated code for this issue. With the following code, it is not possible to reproduce this issue:

use std::io;
use std::fmt::Default;

fn echo<T: Default>(x: T) { io::println(format!("{}", x)); }

pub trait OpInt<'a> { fn call<'a>(&'a self, int, int) -> int; }

impl<'a> OpInt<'a> for 'a |int, int| -> int {
    fn call(&self, a:int, b:int) -> int {
        echo("I dont wanna die!");
        (*self)(a, b)
    }
}

fn squarei<'a>(x: int, op: &'a OpInt) -> int { op.call(x, x) }

fn muli(x:int, y:int) -> int {
    echo("You will never get here.");
    x * y
}


fn main() {
    echo("Entered main");
    let f = |x,y| muli(x,y);
    {
        let g = &f;
        let h = g as &OpInt;
        let r = squarei(3, h);
        echo(r);
    }
}

@pnkfelix Can we close this issue?

[alexcrichton]

A test should be checked in to close the issue.

[flaper87]

@alexcrichton +1, I'll submit a PR with the same example used here