fix(derive): Fix the problem where the build fails due to the ambiguous type of map

> This PR closes #490. Please refer to #490 for the detail of the problem. Let me know if you want to make `convert_type` a function.

This is a port of https://github.com/TeXitoi/structopt/pull/491

This is part of #2809

clap_derive/src/derives/args.rs

@@ -542,6 +542,13 @@ fn gen_parsers(
     let flag = *attrs.parser().kind == ParserKind::FromFlag;
     let occurrences = *attrs.parser().kind == ParserKind::FromOccurrences;
     let name = attrs.cased_name();
+    let convert_type = match **ty {
+        Ty::Vec | Ty::Option => sub_type(&field.ty).unwrap_or(&field.ty),
+        Ty::OptionOption | Ty::OptionVec => {
+            sub_type(&field.ty).and_then(sub_type).unwrap_or(&field.ty)
+        }
+        _ => &field.ty,
+    };
     // Use `quote!` to give this identifier the same hygiene
     // as the `arg_matches` parameter definition. This
     // allows us to refer to `arg_matches` within a `quote_spanned` block
@@ -584,7 +591,7 @@ fn gen_parsers(
         Ty::OptionVec => quote_spanned! { ty.span()=>
             if #arg_matches.is_present(#name) {
                 Some(#arg_matches.#values_of(#name)
-                     .map(|v| v.map(#parse).collect())
+                     .map(|v| v.map::<#convert_type, _>(#parse).collect())
                      .unwrap_or_else(Vec::new))
             } else {
                 None
@@ -600,7 +607,7 @@ fn gen_parsers(
 
             quote_spanned! { ty.span()=>
                 #arg_matches.#values_of(#name)
-                    .map(|v| v.map(#parse).collect())
+                    .map(|v| v.map::<#convert_type, _>(#parse).collect())
                     .unwrap_or_else(Vec::new)
             }
         }

clap_derive/tests/issues.rs

@@ -76,3 +76,32 @@ fn issue_324() {
     let help = get_long_help::<Opt>();
     assert!(help.contains("MY_VERSION"));
 }
+
+#[test]
+fn issue_490() {
+    use clap::Clap;
+    use std::iter::FromIterator;
+    use std::str::FromStr;
+
+    struct U16ish;
+    impl FromStr for U16ish {
+        type Err = ();
+        fn from_str(_: &str) -> Result<Self, Self::Err> {
+            unimplemented!()
+        }
+    }
+    impl<'a> FromIterator<&'a U16ish> for Vec<u16> {
+        fn from_iter<T: IntoIterator<Item = &'a U16ish>>(_: T) -> Self {
+            unimplemented!()
+        }
+    }
+
+    #[derive(Clap, Debug)]
+    struct Opt {
+        opt_vec: Vec<u16>,
+        #[clap(long)]
+        opt_opt_vec: Option<Vec<u16>>,
+    }
+
+    // Assert that it compiles
+}