Add regression test for TAIT in extern fn ICE with the new solver

tests/ui/lint/improper-ctypes/tait-extern-fn-next-solver-issue-156345.rs

@@ -0,0 +1,42 @@
+//@ compile-flags: -Znext-solver=globally
+//@ edition: 2021
+//@ check-pass
+
+// Regression test for <https://github.com/rust-lang/rust/issues/156345>.
+// An `extern "C" fn` taking a type alias impl trait argument used to ICE with
+// the new solver, leaving an entry in the `OpaqueTypeStorage`. Only the new
+// solver was affected.
+
+#![feature(type_alias_impl_trait)]
+#![allow(improper_ctypes_definitions)]
+
+struct Foo {
+    field: String,
+}
+
+type Tait = impl Sized;
+
+#[define_opaque(Tait)]
+extern "C" fn ice_cold(beverage: Tait) {
+    let Foo { field } = beverage;
+    let _ = field;
+}
+
+// A second reproducer from the same issue, with the opaque type in return
+// position behind a higher-ranked closure bound.
+struct Parser<H>(H);
+
+impl<H, T> Parser<H>
+where
+    H: for<'a> Fn(&'a str) -> T,
+{
+    fn new(handler: H) -> Parser<H> {
+        Parser(handler)
+    }
+
+    extern "C" fn many<'s>() -> Parser<impl for<'a> Fn(&'a str) + 's> {
+        Parser::new(|_| ())
+    }
+}
+
+fn main() {}