Match guards are silently ignored #4957

Summary

Match guards are parsed and name resolved, but never type checked or compiled. An arm with a guard is taken whenever its pattern matches, and the guard never runs. Pattern support (#2082) lists guards as not done yet, but right now they compile without any error, so the program just behaves differently.

Reproducer

I tried this code:

#![feature(no_core)]
#![no_core]

fn never() -> bool {
    false
}

fn main() -> i32 {
    match 0 {
        0 if never() => 1,
        _ => 0,
    }
}

Does the code make use of any (1.49) nightly feature ?

• Nightly

Godbolt link

No response

Actual behavior

The program exits with 1, and never is never called. A guard that isn't a bool, like 0 if 5 => ..., is accepted too.

Expected behavior

The guard runs, is false, and the program exits with 0.

GCC Version

36b5a15