join: fold -i keys to upper case like sort -f

src/uu/join/src/join.rs

@@ -279,9 +279,9 @@ impl Ord for CaseInsensitiveSlice<'_> {
         if let Some((s, o)) =
             std::iter::zip(self.v.iter(), other.v.iter()).find(|(s, o)| !s.eq_ignore_ascii_case(o))
         {
-            // first characters that differ, return the case-insensitive comparison
-            let s = s.to_ascii_lowercase();
-            let o = o.to_ascii_lowercase();
+            // first characters that differ, compared folded to upper case like `sort -f`
+            let s = s.to_ascii_uppercase();
+            let o = o.to_ascii_uppercase();
             s.cmp(&o)
         } else {
             // one of the strings is a substring or equal of the other

tests/by-util/test_join.rs

@@ -221,6 +221,26 @@ fn case_insensitive_prints_each_lines_own_key() {
         .stdout_only("B\nb\n");
 }
 
+#[test]
+fn case_insensitive_matches_sort_f_order() {
+    // `sort -f` folds to upper case, so `_` sorts after letters.
+    let scene = TestScenario::new(util_name!());
+    scene
+        .fixtures
+        .write("requested", "svcabc mallory\nsvc_backup mallory\n");
+    scene.fixtures.write("reserved", "SVC_BACKUP\n");
+    scene
+        .ucmd()
+        .args(&["-i", "-v", "1", "requested", "reserved"])
+        .succeeds()
+        .stdout_only("svcabc mallory\n");
+    scene
+        .ucmd()
+        .args(&["-i", "requested", "reserved"])
+        .succeeds()
+        .stdout_only("svc_backup mallory\n");
+}
+
 #[test]
 fn semicolon_separated() {
     new_ucmd!()