Unable to manually implement FnOnce

[nikomatsakis]

This reddit thread reported a few examples of being unable to implement FnOnce manually. I haven't investigated enough to produce a standalone test case, but I wanted to record the issue.

[pfalabella]

standalone test case (could be reduced but I hope it's useful):

#![feature(overloaded_calls)]
// Implements http://rosettacode.org/wiki/Accumulator_factory
pub struct G<T, U> {
    n: T,
}

impl<T: Add<U, T> + Clone, U> FnMut<(U,), T> for G<T, U> {
    extern "rust-call" fn call_mut(&mut self, (i,):(U,)) -> T {
        self.n = self.n + i;
        self.n.clone()
    }
}

pub fn accum<T: Add<T, U> + Clone, U>(n: T) -> G<T, U> {
G { n: n }
}

pub fn main() {
    println!("{}", accumulate());
}

fn accumulate() -> f32 {
    let mut g = accum(1f32);
    g(5.);
    accum(3i32);
    g(2.3)
}

used to compile, but now it does not, with the following output:

$ rustc accum.rs
accum.rs:7:1: 12:2 error: conflicting implementations for trait `core::ops::FnMut` [E0119]
accum.rs:7 impl<T: Add<U, T> + Clone, U> FnMut<(U,), T> for G<T, U> {
accum.rs:8     extern "rust-call" fn call_mut(&mut self, (i,):(U,)) -> T {
accum.rs:9         self.n = self.n + i;
accum.rs:10         self.n.clone()
accum.rs:11     }
accum.rs:12 }
accum.rs:7:1: 12:2 note: conflicting implementation in crate `core`
accum.rs:7 impl<T: Add<U, T> + Clone, U> FnMut<(U,), T> for G<T, U> {
accum.rs:8     extern "rust-call" fn call_mut(&mut self, (i,):(U,)) -> T {
accum.rs:9         self.n = self.n + i;
accum.rs:10         self.n.clone()
accum.rs:11     }
accum.rs:12 }
error: aborting due to previous error

EDIT: reduced

pub struct G;

impl<T, U> FnMut<(U,), T> for G {
    extern "rust-call" fn call_mut(&mut self, (i,):(U,)) -> T {
        panic!()
    }
}

pub fn main() {} 

[netvl]

Very simple example:

#![feature(unboxed_closures, tuple_indexing)]

struct Chunk<T>(Box<FnOnce<(), T>+'static>);

impl<T> FnOnce<(), T> for Chunk<T> {
    extern "rust-call" fn call_once(self, arg: ()) -> T {
        self.0.call_once(arg)
    }
}

Compilation error:

lib.rs:5:1: 9:2 error: conflicting implementations for trait `core::ops::FnOnce` [E0119]
lib.rs:5 impl<T> FnOnce<(), T> for Chunk<T> {
lib.rs:6     extern "rust-call" fn call_once(self, arg: ()) -> T {
lib.rs:7         self.0.call_once(arg)
lib.rs:8     }
lib.rs:9 }
lib.rs:5:1: 9:2 note: conflicting implementation in crate `core`
lib.rs:5 impl<T> FnOnce<(), T> for Chunk<T> {
lib.rs:6     extern "rust-call" fn call_once(self, arg: ()) -> T {
lib.rs:7         self.0.call_once(arg)
lib.rs:8     }
lib.rs:9 }
error: aborting due to previous error

[nikomatsakis]

Thanks guys for the reduced examples! I'll try to look into this this week.

[nikomatsakis]

This is a special case of #19032.