`i8::MIN as f32`: warning: attempted to negate with overflow, #[warn(const_err)] on by default

[japaric]

STR

// cast.rs
use std::i8;

fn main() {
    i8::MIN as f32;
}

$ rustc cast.rs
cast.rs:5:5: 5:19 warning: attempted to negate with overflow, #[warn(const_err)] on by default
cast.rs:5     i8::MIN as f32;
              ^~~~~~~~~~~~~~

The "evaluated" expression -128 as f32 doesn't trigger the warning. But if you expand the definition of i8::MIN i.e. ((-1 as i8) << 8 - 1) as f32 you also get a warning.

Neither of these three equivalent expressions should trigger a warning.

Metadata

$ rustc -V
rustc 1.10.0-nightly (cda7c1cf2 2016-04-27)

cc @oli-obk might be related to #33214

[oli-obk]

hmmm... I think const_eval is treating -1i8 << 7 as -(1i8 << 7), and 1i8 << 7 is -128, I'll look into it.