Overflow evaluating a trait requirement

[taiheioki]

I tried this code (playground):

use std::ops::Add;

struct A<T>(T);

impl<T> Add<A<T>> for i32
where
    i32: Add<T, Output = T>,
{
    type Output = A<T>;

    fn add(self, rhs: A<T>) -> Self::Output {
        A(Add::add(self, rhs.0))
    }
}

fn main() {}

I think this code should compile. However, I got the following compile error:

error[E0275]: overflow evaluating the requirement `i32: Add<A<_>>`
  --> src/lib.rs:12:11
   |
12 | ...   A(Add::add(self, rhs.0))
   |         ^^^^^^^^
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`geometry`)
note: required for `i32` to implement `Add<A<A<_>>>`
  --> src/lib.rs:5:9
   |
5  | impl<T> Add<A<T>> for i32
   |         ^^^^^^^^^     ^^^
6  | where
7  |     i32: Add<T, Output = T>,
   |                 ---------- unsatisfied trait bound introduced here
   = note: 127 redundant requirements hidden
   = note: required for `i32` to implement `Add<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<A<_>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>`

Changing A(Add::add(self, rhs.0)) to A(self + rhs.0) or A(Add::<T>::add(self, rhs.0)) results in a successful compilation. Maybe related to https://github.com/rust-lang/rust/issues/39959?

Meta

rustc --version --verbose:

rustc 1.68.2 (9eb3afe9e 2023-03-27)
binary: rustc
commit-hash: 9eb3afe9ebe9c7d2b84b71002d44f4a0edac95e0
commit-date: 2023-03-27
host: aarch64-apple-darwin
release: 1.68.2
LLVM version: 15.0.6

[compiler-errors]

For what it's worth, this does compile with the new trait solver (-Ztrait-solver=next).

[npuichigo]

@compiler-errors

Cannot find the trait-solver flag in nightly.

Available unstable (nightly-only) flags:

    -Z allow-features          -- Allow *only* the listed unstable features
    -Z avoid-dev-deps          -- Avoid installing dev-dependencies if possible
    -Z binary-dep-depinfo      -- Track changes to dependency artifacts
    -Z bindeps                 -- Allow Cargo packages to depend on bin, cdylib, and staticlib crates, and use the artifacts built by those crates
    -Z build-std               -- Enable Cargo to compile the standard library itself as part of a crate graph compilation
    -Z build-std-features      -- Configure features enabled for the standard library itself when building the standard library
    -Z check-cfg               -- Specify scope of compile-time checking of `cfg` names/values
    -Z codegen-backend         -- Enable the `codegen-backend` option in profiles in .cargo/config.toml file
    -Z config-include          -- Enable the `include` key in config files
    -Z credential-process      -- Add a config setting to fetch registry authentication tokens by calling an external process
    -Z direct-minimal-versions -- Resolve minimal dependency versions instead of maximum (direct dependencies only)
    -Z doctest-xcompile        -- Compile and run doctests for non-host target using runner config
    -Z dual-proc-macros        -- Build proc-macros for both the host and the target
    -Z gitoxide                -- Use gitoxide for the given git interactions, or all of them if no argument is given
    -Z host-config             -- Enable the [host] section in the .cargo/config.toml file
    -Z lints                   -- Pass `[lints]` to the linting tools
    -Z minimal-versions        -- Resolve minimal dependency versions instead of maximum
    -Z msrv-policy             -- Enable rust-version aware policy within cargo
    -Z mtime-on-use            -- Configure Cargo to update the mtime of used files
    -Z no-index-update         -- Do not update the registry index even if the cache is outdated
    -Z panic-abort-tests       -- Enable support to run tests with -Cpanic=abort
    -Z profile-rustflags       -- Enable the `rustflags` option in profiles in .cargo/config.toml file
    -Z publish-timeout         -- Enable the `publish.timeout` key in .cargo/config.toml file
    -Z registry-auth           -- Authentication for alternative registries, and generate registry authentication tokens using asymmetric cryptography
    -Z rustdoc-map             -- Allow passing external documentation mappings to rustdoc
    -Z rustdoc-scrape-examples -- Allows Rustdoc to scrape code examples from reverse-dependencies
    -Z script                  -- Enable support for single-file, `.rs` packages
    -Z target-applies-to-host  -- Enable the `target-applies-to-host` key in the .cargo/config.toml file
    -Z unstable-options        -- Allow the usage of unstable options

Run with 'cargo -Z [FLAG] [COMMAND]'

[compiler-errors]

@npuichigo: Those are cargo's flags, but also I don't recommend using the new trait solver yet. It will probably cause you a lot more problems in other code.