Add regression test for rust-lang#66757

Author: Nicholas Matsakis

src/test/ui/never_type/never-value-fallback-issue-66757.rs

@@ -0,0 +1,29 @@
+// Regression test for #66757
+//
+// Test than when you have a `!` value (e.g., the local variable
+// never) and an uninferred variable (here the argument to `From`) it
+// doesn't fallback to `()` but rather `!`.
+//
+// run-pass
+
+#![feature(never_type)]
+
+// FIXME(#67225) -- this should be true even without the fallback gate.
+#![feature(never_type_fallback)]
+
+struct E;
+
+impl From<!> for E {
+    fn from(_: !) -> E {
+        E
+    }
+}
+
+#[allow(unreachable_code)]
+#[allow(dead_code)]
+fn foo(never: !) {
+    <E as From<!>>::from(never);  // Ok
+    <E as From<_>>::from(never);  // Inference fails here
+}
+
+fn main() { }