Match expressions should sometimes have type !

[powerboat9]

Summary

Match expressions with no arms should give a value of type !

Reproducer

I tried this code:

pub enum X {}

pub fn foo(x: X) {
    let _a: i32 = match x {};
}

pub fn main() {}

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

• Nightly

Godbolt link

link

Actual behavior

<source>:4:5: error: mismatched types, expected 'i32' but got '()' [E0308]
    4 |     let _a: i32 = match x {};
      |     ^~~     ~~~   ~~~~~

Expected behavior

No error

GCC Version

GCCRS master on godbolt

[philberty]

Whats the logic for never type here ? @flip1995

[CohenArthur]

@philberty https://github.com/rust-lang/rust/blob/3d1dba830a564d1118361345d7ada47a05241f45/compiler/rustc_hir_typeck/src/_match.rs#L32-L36 I believe

so it should be pretty simple - if there are no arms, then it returns !. if there are no arms when there should be arms, then it's up to the exhaustiveness checker to report an error

[flip1995]

The best documentation I could find was a blog post by Niko. Also take a look at the linked RFC. I think this is something missing in the reference.

I think this paragraph from the RFC explains it best:

let foo: &'static str = match x {};

Reading this, it may be tempting to ask the question "what is the value of foo then?". Remember that this depends on the value of x. As there are no possible values of x it's a meaningless question and besides, the fact that x has type Never gives us a static guarantee that the match block will never be executed.

Because the scrutinee is an uninhabited type (which the never type is), it can never be created. So this match can never be executed. So the type of the expression is ! = never.