fix(help): Render partially optional values with []

Fixes: #4847

clap_builder/src/builder/arg.rs

@@ -4363,8 +4363,19 @@ impl Arg {
         }
 
         debug_assert!(self.is_takes_value_set());
+
         for (n, val_name) in val_names.iter().enumerate() {
-            let arg_name = if self.is_positional() && (num_vals.min_values() == 0 || !required) {
+            let value_is_required = if self.is_positional() {
+                required && (num_vals.min_values() != 0)
+            } else {
+                // If all values are optional, the [] get rendered by the caller:
+                //    --foo[=<bar>]
+                // In this case, treat as required.
+                let required = self.get_min_vals() == 0;
+                required || (n < num_vals.min_values())
+            };
+
+            let arg_name = if !value_is_required {
                 format!("[{val_name}]")
             } else {
                 format!("<{val_name}>")
@@ -4646,7 +4657,7 @@ mod test {
             .value_names(["file", "name"]);
         o._build();
 
-        assert_eq!(o.to_string(), "-o <file> <name>...");
+        assert_eq!(o.to_string(), "-o <file> [name]...");
     }
 
     #[test]

tests/builder/help.rs

@@ -2845,3 +2845,24 @@ fn display_name_subcommand_explicit() {
         Some("child.display")
     );
 }
+
+#[test]
+fn issue_4847_usage() {
+    static USAGE_WITH_GROUP: &str = "\
+Usage: deno [OPTIONS]
+
+Options:
+      --example <REQUIRED> [OPTIONAL]  issue 4847
+  -h, --help                           Print help
+";
+
+    let cmd = clap::Command::new("hello").bin_name("deno").arg(
+        Arg::new("example")
+            .long("example")
+            .num_args(1..=2)
+            .help("issue 4847")
+            .value_names(&["REQUIRED", "OPTIONAL"]),
+    );
+
+    utils::assert_output(cmd, "deno --help", USAGE_WITH_GROUP, false);
+}