Rollup merge of #73485 - estebank:dedup-preds, r=nikomatsakis

Perform obligation deduplication to avoid buggy `ExistentialMismatch`

Address #59326.

src/test/ui/issues/issue-59326.rs

@@ -0,0 +1,26 @@
+// check-pass
+trait Service {
+    type S;
+}
+
+trait Framing {
+    type F;
+}
+
+impl Framing for () {
+    type F = ();
+}
+
+trait HttpService<F: Framing>: Service<S = F::F> {}
+
+type BoxService = Box<dyn HttpService<(), S = ()>>;
+
+fn build_server<F: FnOnce() -> BoxService>(_: F) {}
+
+fn make_server<F: Framing>() -> Box<dyn HttpService<F, S = F::F>> {
+    unimplemented!()
+}
+
+fn main() {
+    build_server(|| make_server())
+}