Add another case of fallback to () avoid breakage

This adds src/test/ui/never_type/fallback-closure-ret.rs as a test case which
showcases the failure mode fixed by this commit.

2 files changed, 53 insertions, 0 deletions

diff --git a/src/test/ui/never_type/fallback-closure-ret.rs b/src/test/ui/never_type/fallback-closure-ret.rs
new file mode 100644
index 00000000000..5c8ce48cbb0
--- /dev/null
+++ b/src/test/ui/never_type/fallback-closure-ret.rs
@@ -0,0 +1,23 @@
+// This test verifies that never type fallback preserves the following code in a
+// compiling state. This pattern is fairly common in the wild, notably seen in
+// wasmtime v0.16. Typically this is some closure wrapper that expects a
+// collection of 'known' signatures, and -> ! is not included in that set.
+//
+// This test is specifically targeted by the unit type fallback when
+// encountering a set of obligations like `?T: Foo` and `Trait::Projection =
+// ?T`. In the code below, these are `R: Bar` and `Fn::Output = R`.
+//
+// revisions: nofallback fallback
+// check-pass
+
+#![cfg_attr(fallback, feature(never_type_fallback))]
+
+trait Bar { }
+impl Bar for () {  }
+impl Bar for u32 {  }
+
+fn foo<R: Bar>(_: impl Fn() -> R) {}
+
+fn main() {
+    foo(|| panic!());
+}
diff --git a/src/test/ui/never_type/fallback-closure-wrap.rs b/src/test/ui/never_type/fallback-closure-wrap.rs
new file mode 100644
index 00000000000..af0577ac060
--- /dev/null
+++ b/src/test/ui/never_type/fallback-closure-wrap.rs
@@ -0,0 +1,30 @@
+// This is a minified example from Crater breakage observed when attempting to
+// stabilize never type, nstoddard/webgl-gui @ 22f0169f.
+//
+// This particular test case currently fails as the inference to `()` rather
+// than `!` happens as a result of an `as` cast, which is not currently tracked.
+// Crater did not find many cases of this occuring, but it is included for
+// awareness.
+//
+// revisions: nofallback fallback
+//[nofallback] check-pass
+//[fallback] check-fail
+
+#![cfg_attr(fallback, feature(never_type_fallback))]
+
+use std::marker::PhantomData;
+
+fn main() {
+    let error = Closure::wrap(Box::new(move || {
+        //[fallback]~^ ERROR type mismatch resolving
+        panic!("Can't connect to server.");
+    }) as Box<dyn FnMut()>);
+}
+
+struct Closure<T: ?Sized>(PhantomData<T>);
+
+impl<T: ?Sized> Closure<T> {
+    fn wrap(data: Box<T>) -> Closure<T> {
+        todo!()
+    }
+}