Flattening a subcommand now panics with a meaningful error message

structopt-derive/src/lib.rs

@@ -559,7 +559,8 @@ fn gen_from_clap_enum(name: &Ident) -> TokenStream {
  quote! {
  fn from_clap(matches: &::structopt::clap::ArgMatches) -> Self {
  <#name as ::structopt::StructOptInternal>::from_subcommand(matches.subcommand())
- .unwrap()
+ .expect("structopt misuse: You likely tried to #[flatten] a struct \
+ that contains #[subcommand]. This is forbidden.")
  }
  }
 }

tests/flatten.rs

@@ -127,3 +127,27 @@ fn merge_subcommands_with_flatten() {
  Opt::from_iter(&["test", "command2", "43"])
  );
 }
+
+#[test]
+#[should_panic = "structopt misuse: You likely tried to #[flatten] a struct \
+ that contains #[subcommand]. This is forbidden."]
+fn subcommand_in_flatten() {
+ #[derive(Debug, StructOpt)]
+ pub enum Struct1 {
+ #[structopt(flatten)]
+ Struct1(Struct2),
+ }
+
+ #[derive(Debug, StructOpt)]
+ pub struct Struct2 {
+ #[structopt(subcommand)]
+ command_type: Enum3,
+ }
+
+ #[derive(Debug, StructOpt)]
+ pub enum Enum3 {
+ Command { args: Vec<String> },
+ }
+
+ Struct1::from_iter(&["test", "command", "foo"]);
+}