Remove invalid suggestion involving Fn trait bound

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -1604,6 +1604,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
                         && !snippet.ends_with('>')
                         && !generics.has_impl_trait()
+                        && !self.tcx.fn_trait_kind_from_lang_item(*def_id).is_some()
                     {
                         // FIXME: To avoid spurious suggestions in functions where type arguments
                         // where already supplied, we check the snippet to make sure it doesn't

src/test/ui/traits/issue-85735.rs

@@ -0,0 +1,13 @@
+// Regression test for the invalid suggestion in #85735 (the
+// underlying issue #21974 still exists here).
+
+trait Foo {}
+impl<'a, 'b, T> Foo for T
+where
+    T: FnMut(&'a ()),
+    //~^ ERROR: type annotations needed [E0283]
+    T: FnMut(&'b ()),
+{
+}
+
+fn main() {}

src/test/ui/traits/issue-85735.stderr

@@ -0,0 +1,16 @@
+error[E0283]: type annotations needed
+  --> $DIR/issue-85735.rs:7:8
+   |
+LL |     T: FnMut(&'a ()),
+   |        ^^^^^^^^^^^^^ cannot infer type for type parameter `T`
+   | 
+  ::: $SRC_DIR/core/src/ops/function.rs:LL:COL
+   |
+LL | pub trait FnMut<Args>: FnOnce<Args> {
+   | ----------------------------------- required by this bound in `FnMut`
+   |
+   = note: cannot satisfy `T: FnMut<(&'a (),)>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0283`.