From 776fe37464e06455d78515865ccbf8e654b32c45 Mon Sep 17 00:00:00 2001
From: CreepySkeleton <creepy-skeleton@yandex.ru>
Date: Fri, 12 Jun 2020 15:34:56 +0300
Subject: [PATCH] Flattening a subcommand now panics with a meaningful error
 message

---
 structopt-derive/src/lib.rs |  3 ++-
 tests/flatten.rs            | 24 ++++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/structopt-derive/src/lib.rs b/structopt-derive/src/lib.rs
index a5cd7c26..87fadf26 100644
--- a/structopt-derive/src/lib.rs
+++ b/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.")
         }
     }
 }
diff --git a/tests/flatten.rs b/tests/flatten.rs
index f01e44e7..ddc0ef7b 100644
--- a/tests/flatten.rs
+++ b/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"]);
+}