Add a test

tests/ui/parser/anon-enums-are-ambiguous.rs

@@ -0,0 +1,26 @@
+// check-pass
+
+macro_rules! test_expr {
+    ($expr:expr) => {};
+}
+
+macro_rules! test_ty {
+    ($a:ty | $b:ty) => {};
+}
+
+fn main() {
+    test_expr!(a as fn() -> B | C);
+    // Do not break the `|` operator.
+
+    test_expr!(|_: fn() -> B| C | D);
+    // Do not break `-> Ret` in closure args.
+
+    test_ty!(A | B);
+    // We can't support anon enums in arbitrary positions.
+
+    test_ty!(fn() -> A | B);
+    // Don't break fn ptrs.
+
+    test_ty!(impl Fn() -> A | B);
+    // Don't break parenthesized generics.
+}