Fix ICE in rustdoc when merging generic and where bounds of an Fn with an output

src/librustdoc/clean/simplify.rs

@@ -118,7 +118,7 @@ pub fn merge_bounds(
                 });
             }
             PP::Parenthesized { ref mut output, .. } => match output {
-                Some(o) => assert!(o == rhs),
+                Some(o) => assert_eq!(o, rhs),
                 None => if *rhs != clean::Type::Tuple(Vec::new()) {
                     *output = Some(rhs.clone());
                 }

src/test/rustdoc/auxiliary/issue-57180.rs

@@ -0,0 +1,16 @@
+// compile-flags: -Cmetadata=aux
+
+pub trait Trait {
+}
+
+pub struct Struct<F>
+{
+    _p: ::std::marker::PhantomData<F>,
+}
+
+impl<F: Fn() -> u32>
+Trait for Struct<F>
+    where
+        F: Fn() -> u32,
+{
+}

src/test/rustdoc/issue-57180.rs

@@ -0,0 +1,7 @@
+// aux-build:issue-57180.rs
+
+extern crate issue_57180;
+use issue_57180::Trait;
+
+fn main() {
+}