Multiple candidate method resolution error

[philberty]

I tried this code: https://godbolt.org/z/KdfGEbxK8

#[lang = "add"]
pub trait Add<RHS = Self> {
    type Output;

    fn add(self, rhs: RHS) -> Self::Output;
}
macro_rules! add_impl {
    ($($t:ty)*) => ($(
        impl Add for $t {
            type Output = $t;

            fn add(self, other: $t) -> $t { self + other }
        }
    )*)
}

add_impl! { usize u8 u16 u32 u64  /*isize i8 i16 i32 i64*/  f32 f64 }

pub fn test() {
    let x:usize = 123;
    let mut i = 0;
    let bug = i + x;
}

I expected to see this happen: compile without error

Instead, this happened:

<source>:22:15: error: multiple candidates found for possible operator overload
   12 |             fn add(self, other: $t) -> $t { self + other }
      |             ~~ 
......
   22 |     let bug = i + x;
      |               ^

Its not obvious from the error message but adding in some debugging shows there are multiple candidates:

test.rs:147:15: error: multiple candidates found for possible operator overload
  147 |         while i + block_size <= len {
      |               ^
......
  288 |             fn add(self, other: $t) -> $t { self + other }
      |             ~~ 
crab1: note: [0x3fd0610] <integer> -> self
crab1: note: [0x3fd0d60] usize -> other
crab1: note: [0x3fdb5d0] fn (self usize,other usize,) -> usize
crab1: note: [0x3fe7be0] fn (self u8,other u8,) -> u8
crab1: note: [0x3ff4040] fn (self u16,other u16,) -> u16
crab1: note: [0x4001490] fn (self u32,other u32,) -> u32
crab1: note: [0x400f660] fn (self u64,other u64,) -> u64

Meta

• What version of Rust GCC were you using, git sha if possible. c021eef

[philberty]

Method selection needs to take into account the other arguments to help resolve this case

[CohenArthur]

This is the output of rustc -Zunpretty=hir,typed:

pub fn test() ({
                   let x: usize = (123 as usize);
                   let mut i = (0 as usize);
                   let bug = ((i as usize) + (x as usize) as usize);
               } as ())

[philberty]

Yeah i think the method selection is meant to see i + x and turn i into a usize.

[powerboat9]

What are you using to debug?

[philberty]

@powerboat9 i use -frust-debug but i also added in a few calls to TyTy::BaseType::debug to emit each of the candidates.

[philberty]

The fix here should also fix #1586