Box<dyn std::error::Error> fails to satisfy TryFrom::Error: std::error::Error #149086
Description
I tried this code:
fn main() { foo::<Bar>(); } fn foo<T>() -> Result<T, T::Error> where T: TryFrom<i64, Error: std::error::Error>, { todo!(); } struct Bar; impl TryFrom<i64> for Bar { type Error = Box<dyn std::error::Error>; fn try_from(_: i64) -> Result<Self, Self::Error> { Ok(Self) } }I expected to see this happen: It compile success.
Instead, this happened:
error[E0277]: the size for values of type `(dyn std::error::Error + 'static)` cannot be known at compilation time --> src/main.rs:2:11 | 2 | foo::<Bar>(); | ^^^ doesn't have a size known at compile-time | = help: the trait `Sized` is not implemented for `(dyn std::error::Error + 'static)` = help: the trait `std::error::Error` is implemented for `Box<E>` = note: required for `Box<(dyn std::error::Error + 'static)>` to implement `std::error::Error` note: required by a bound in `foo` --> src/main.rs:7:28 | 5 | fn foo<T>() -> Result<T, T::Error> | --- required by a bound in this function 6 | where 7 | T: TryFrom<i64, Error: std::error::Error>, | ^^^^^^^^^^^^^^^^^ required by this bound in `foo` For more information about this error, try `rustc --explain E0277`. Meta
rustc --version --verbose:
rustc 1.91.0 (f8297e351 2025-10-28) binary: rustc commit-hash: f8297e351a40c1439a467bbbb6879088047f50b3 commit-date: 2025-10-28 host: x86_64-unknown-linux-gnu release: 1.91.0 LLVM version: 21.1.2