if ... as T < ... && ... >= ... is treated as const generics

[alianse777]

I'm not sure if it is expected behavior or not.

I tried this code:

fn main() {
    if 1 as f64 < 0.0 && 1 >= 0 {
        
    }
}

I expected to see this happen: the code compiles without errors

Instead, this happened:

Compiling playground v0.0.1 (/playground)
error: expressions must be enclosed in braces to be used as const generic arguments
 --> src/main.rs:3:19
  |
3 |     if 1 as f64 < 0.0 && 1 >= 0 {
  |                   ^^^^^^^^
  |
help: enclose the `const` expression in braces
  |
3 |     if 1 as f64 < { 0.0 && 1 } >= 0 {
  |                   +          +

error[E0109]: const arguments are not allowed for this type
 --> src/main.rs:3:19
  |
3 |     if 1 as f64 < 0.0 && 1 >= 0 {
  |                   ^^^^^^^^ const argument not allowed

error[E0308]: mismatched types
 --> src/main.rs:3:8
  |
3 |     if 1 as f64 < 0.0 && 1 >= 0 {
  |        ^^^^^^^^^^^^^^^^^^^^^^^^ expected `bool`, found `()`
  |
help: you might have meant to use pattern matching
  |
3 |     if let 1 as f64 < 0.0 && 1 >= 0 {
  |        +++

error[E0308]: mismatched types
 --> src/main.rs:3:19
  |
3 |     if 1 as f64 < 0.0 && 1 >= 0 {
  |                   ^^^ expected `bool`, found floating-point number

error[E0308]: mismatched types
 --> src/main.rs:3:26
  |
3 |     if 1 as f64 < 0.0 && 1 >= 0 {
  |                          ^ expected `bool`, found integer

Some errors have detailed explanations: E0109, E0308.
For more information about an error, try `rustc --explain E0109`.
error: could not compile `playground` due to 5 previous errors

rustc 1.54.0 stable.

If I change to (1 as f64) < 0.0 && 1 >= 0 it compiles.

Link to playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=48dbdf8fc80dde053da67f6b3db3d143

[goldwind-ting]

If I change to if 1 as f64 <= 0.0 && 1 >= 0 it also compiles, why is that?

[alianse777]

If I change to if 1 as f64 <= 0.0 && 1 >= 0 it also compiles, why is that?

Because of <= instead of <

[SNCPlay42]

Output on rust 1.48.0 (godbolt):

error: `<` is interpreted as a start of generic arguments for `f64`, not a comparison
 --> <source>:2:17
  |
2 |     if 1 as f64 < 0.0 && 1 >= 0 {
  |        -------- ^ ------ interpreted as generic arguments
  |        |        |
  |        |        not interpreted as comparison
  |        help: try comparing the cast value: `(1 as f64)`

error: aborting due to previous error

[fmease]

Sadly, this seems to have regressed again (stable 1.75, nightly 1.77):

error: expressions must be enclosed in braces to be used as const generic arguments
 --> src/main.rs:2:19
  |
2 |     if 1 as f64 < 0.0 && 1 >= 0 {
  |                   ^^^^^^^^
  |
help: enclose the `const` expression in braces
  |
2 |     if 1 as f64 < { 0.0 && 1 } >= 0 {
  |                   +          +

error[E0109]: const arguments are not allowed on builtin type `f64`
 --> src/main.rs:2:19
  |
2 |     if 1 as f64 < 0.0 && 1 >= 0 {
  |             ---   ^^^^^^^^ const argument not allowed
  |             |
  |             not allowed on builtin type `f64`
  |
help: primitive type `f64` doesn't have generic parameters
  |
2 -     if 1 as f64 < 0.0 && 1 >= 0 {
2 +     if 1 as f64= 0 {
  |

error[E0308]: mismatched types
 --> src/main.rs:2:8
  |
2 |     if 1 as f64 < 0.0 && 1 >= 0 {
  |        ^^^^^^^^^^^^^^^^^^^^^^^^ expected `bool`, found `()`

error[E0308]: mismatched types
 --> src/main.rs:2:19
  |
2 |     if 1 as f64 < 0.0 && 1 >= 0 {
  |                   ^^^ expected `bool`, found floating-point number

error[E0308]: mismatched types
 --> src/main.rs:2:26
  |
2 |     if 1 as f64 < 0.0 && 1 >= 0 {
  |                          ^ expected `bool`, found integer

[estebank]

Triage: no change.