add regression test

src/test/pretty/gat-bounds.pp

@@ -0,0 +1,25 @@
+// Check that associated types print generic parameters and where clauses.
+// See issue #67509.
+
+// pretty-compare-only
+// pp-exact:gat-bounds.pp
+
+#![feature(generic_associated_types)]
+
+trait X {
+    type
+    Y<T>: Trait
+    where
+    Self: Sized;
+}
+
+impl X for () {
+    type
+    Y<T>
+    where
+    Self: Sized
+    =
+    u32;
+}
+
+fn main() { }

src/test/pretty/gat-bounds.rs

@@ -0,0 +1,17 @@
+// Check that associated types print generic parameters and where clauses.
+// See issue #67509.
+
+// pretty-compare-only
+// pp-exact:gat-bounds.pp
+
+#![feature(generic_associated_types)]
+
+trait X {
+    type Y<T>: Trait where Self: Sized;
+}
+
+impl X for () {
+    type Y<T> where Self: Sized = u32;
+}
+
+fn main() { }