Derived ArgEnum::from_str returns Err instead of panicking

ArgEnum is public so it is possible for users to call it with
a non-variant input string.

clap_derive/src/derives/arg_enum.rs

@@ -118,7 +118,7 @@ fn gen_from_str(lits: &[(TokenStream, Ident)]) -> TokenStream {
 
             match input {
                 #(val if func(val, #lit) => Ok(Self::#variant),)*
-                e => unreachable!("The impossible variant have been spotted: {}", e),
+                e => Err(format!("Invalid variant: {}", e)),
             }
         }
     }

clap_derive/tests/arg_enum.rs

@@ -7,7 +7,7 @@
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
-use clap::Clap;
+use clap::{ArgEnum, Clap};
 
 #[test]
 fn basic() {
@@ -309,3 +309,13 @@ fn vector() {
     );
     assert!(Opt::try_parse_from(&["", "-a", "fOo"]).is_err());
 }
+
+#[test]
+fn from_str_invalid() {
+    #[derive(Clap, PartialEq, Debug)]
+    enum ArgChoice {
+        Foo,
+    }
+
+    assert!(ArgChoice::from_str("bar", true).is_err());
+}