Add regression test for rust-lang#119529

Author: GuillaumeGomez

tests/rustdoc/issue-119529-middle-const-ty.rs

@@ -0,0 +1,9 @@
+// This is a regression test for <https://github.com/rust-lang/rust/issues/119529>.
+
+#![crate_name = "foo"]
+
+use std::cmp::PartialEq;
+
+// @has 'foo/type.Dyn.html'
+// @has - '//*[@class="rust item-decl"]' 'pub type Dyn<Rhs> = dyn PartialEq<Rhs>;'
+pub type Dyn<Rhs> = dyn PartialEq<Rhs>;