add known-bug test for unsoundness issue rust-lang#108425

Part of the resolution to rust-lang#105107

tests/ui/type-alias-impl-trait/issue-108425-impl-generics-unsoundness.rs

@@ -0,0 +1,25 @@
+// known-bug: #108425
+// check-pass
+#![feature(type_alias_impl_trait)]
+use std::fmt::Display;
+
+type Opaque<'a> = impl Sized + 'static;
+fn define<'a>() -> Opaque<'a> {}
+
+trait Trait {
+    type Assoc: Display;
+}
+impl<'a> Trait for Opaque<'a> {
+    type Assoc = &'a str;
+}
+
+// ======= Exploit =======
+
+fn extend<T: Trait + 'static>(s: T::Assoc) -> Box<dyn Display> {
+    Box::new(s)
+}
+
+fn main() {
+    let val = extend::<Opaque<'_>>(&String::from("blah blah blah"));
+    println!("{}", val);
+}