Constants of type Never used in switch statements do not trigger error for use of uninitialized constant, leading to runtime crash

[schuelermine]

Description

When a constant is declared with type Never and subsequently used as the subject of a switch statement, the compiler fails to diagnose the use of an uninitialized constant, leading to a runtime crash.

Reproduction

let x: Never
switch x { }

I will provide the crash message shortly; at the moment, I am not at my computer.

Expected behavior

The compiler should produce an error.

Environment

I believe it is 5.10.1, but I am not sure as I am not at my computer. I downloaded the release about a week ago. I will report the version as soon as I am back at my computer.

I was running Swift in an Ubuntu 24.04 container. Swift was installed to /opt/swift.

Additional information

I was informed this is not intentional in this thread on the forum.

[schuelermine]

In lieu of the full error message on my computer I have reproduced the error on Android (Termux):

$ swift run
Building for debugging...
[8/8] Linking swift-crash
Build complete! (3.77s)
fish: Job 1, 'swift run' terminated by signal SIGBUS (Misaligned address error)

$ swiftc -version
Swift version 5.10.1 (swift-5.10.1-RELEASE)
Target: aarch64-unknown-linux-android24

[schuelermine]

The issue also occurs with a production build.

[hamishknight]

Looks like a SILGen issue, for this:

func foo() {
  let x: Never
  switch x {}
}

we emit:

sil hidden [ossa] @$s4main3fooyyF : $@convention(thin) () -> () {
bb0:
  %0 = alloc_stack [var_decl] $Never, let, name "x" // user: %1
  %1 = mark_uninitialized [var] %0 : $*Never
  unreachable                                     // id: %2
}

with no use of the variable, so definite initialization doesn't diagnose it.