Regression in Error conversion from Infallible · Issue #66757 · rust-lang/rust (original) (raw)

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Description

The following code:

use std::convert::{TryFrom, Infallible};

struct E;

impl From for E { fn from(_: Infallible) -> E { E } }

fn foo() -> Result<(), E> { u32::try_from(1u32)?; Ok(()) }

compiles on stable but fails to compile on nightly with:

error[E0277]: `?` couldn't convert the error to `E`
  --> src/lib.rs:13:24
   |
13 |     u32::try_from(1u32)?;
   |                        ^ the trait `std::convert::From<()>` is not implemented for `E`
   |
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
   = help: the following implementations were found:
             <E as std::convert::From<!>>
   = note: required by `std::convert::From::from`

This was discovered through bytecodealliance/wasmtime#633