Compile error when providing explicit generic arguments where impl Trait appears in bound of generic type

[haileys]

I tried this code:

fn example<T: FromStr<Err = impl Debug>>() {
    todo!();
}

fn main() {
    example::<usize>();
}

I expected this to compile, however I received this error instead:

error[E0632]: cannot provide explicit generic arguments when `impl Trait` is used in argument position
 --> x.rs:9:15
  |
9 |     example::<usize>();
  |               ^^^^^ explicit generic argument not allowed

This error is unexpected, because impl Debug is not used in the argument position and is instead part of a trait bound on a named type parameter.

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (cf9cf7c92 2020-11-10)
binary: rustc
commit-hash: cf9cf7c923eb01146971429044f216a3ca905e06
commit-date: 2020-11-10
host: x86_64-unknown-linux-gnu
release: 1.49.0-nightly

[jonas-schievink]

The error is intentional, as it would not be clear if/where its value can be specified by the caller (same reasoning as for impl Trait in argument position). The message could be improved though.