Auto merge of #40224 - nikomatsakis:issue-39808, r=eddyb

change the strategy for diverging types

The new strategy is as follows. First, the `!` type is assigned
in two cases:

- a block with a diverging statement and no tail expression (e.g.,
  `{return;}`);
- any expression with the type `!` is considered diverging.

Second, we track when we are in a diverging state, and we permit a value
of any type to be coerced **into** `!` if the expression that produced
it is diverging. This means that `fn foo() -> ! { panic!(); 22 }`
type-checks, even though the block has a type of `usize`.

Finally, coercions **from** the `!` type to any other are always
permitted.

Fixes #39808.
Fixes #39984.

1 files changed, 28 insertions, 0 deletions

diff --git a/src/test/ui/reachable/expr_array.rs b/src/test/ui/reachable/expr_array.rs
new file mode 100644
index 00000000000..00e8be07725
--- /dev/null
+++ b/src/test/ui/reachable/expr_array.rs
@@ -0,0 +1,28 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(unused_variables)]
+#![allow(unused_assignments)]
+#![allow(dead_code)]
+#![deny(unreachable_code)]
+#![feature(never_type)]
+#![feature(type_ascription)]
+
+fn a() {
+    // the `22` is unreachable:
+    let x: [usize; 2] = [return, 22];
+}
+
+fn b() {
+    // the `array is unreachable:
+    let x: [usize; 2] = [22, return];
+}
+
+fn main() { }