unboxed closures + overloaded calls ICE: overloaded call method wasn't in method map

[huonw]

#![feature(unboxed_closures, overloaded_calls)]

fn main() {
    let unboxed = |&mut:| {};
    unboxed();
}

error: internal compiler error: overloaded call method wasn't in method map
note: the compiler hit an unexpected failure path. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
task 'rustc' failed at 'Box<Any>', /build/rust-git/src/rust/src/libsyntax/ast_util.rs:784

cc @pcwalton