diff --git a/src/test/ui/issues/issue-54966.rs b/src/test/ui/issues/issue-54966.rs
new file mode 100644
index 000000000000..0ed3c4b3ca94
--- /dev/null
+++ b/src/test/ui/issues/issue-54966.rs
@@ -0,0 +1,6 @@
+// issue-54966: ICE returning an unknown type with impl FnMut
+
+fn generate_duration() -> Oper<impl FnMut()> {}
+//~^ ERROR cannot find type `Oper` in this scope
+
+fn main() {}
diff --git a/src/test/ui/issues/issue-54966.stderr b/src/test/ui/issues/issue-54966.stderr
new file mode 100644
index 000000000000..aa9a61cb592d
--- /dev/null
+++ b/src/test/ui/issues/issue-54966.stderr
@@ -0,0 +1,9 @@
+error[E0412]: cannot find type `Oper` in this scope
+  --> $DIR/issue-54966.rs:3:27
+   |
+LL | fn generate_duration() -> Oper<impl FnMut()> {}
+   |                           ^^^^ not found in this scope
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0412`.