fix(yaml): Don't panic on multiple groups

Because we gradually build the `ArgGroup` as we parse the YAML, we don't
use `ArgGroup::new`.  Clap3 introduced an internal `id` in addition to
the public `name` and it appears that this custom initialization code
was not updated.

This shows the problem with publically exposing `impl Default`.
Choices:
- Remove `impl Default`
  - Always valid
  - Requires spreading invariants
  - Callers can't implement code the same way we do
- Add `ArgGroup::name`
  - Can be constructed in an invalid state
  - Centralizes invariants
  - A caller could implement code like the yaml logic

I decided to go with `ArgGroup::name`.

Fixes #2719

src/build/arg_group.rs

@@ -105,12 +105,22 @@ impl<'help> ArgGroup<'help> {
     /// # ;
     /// ```
     pub fn new<S: Into<&'help str>>(n: S) -> Self {
-        let name = n.into();
-        ArgGroup {
-            id: Id::from(&*name),
-            name,
-            ..ArgGroup::default()
+        ArgGroup::default().name(n)
     }
+
+    /// Sets the group name.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use clap::{App, ArgGroup};
+    /// ArgGroup::default().name("config")
+    /// # ;
+    /// ```
+    pub fn name<S: Into<&'help str>>(mut self, n: S) -> Self {
+        self.name = n.into();
+        self.id = Id::from(&self.name);
+        self
     }
 
     /// Adds an [argument] to this group by name
@@ -434,6 +444,7 @@ impl<'help> From<&'help Yaml> for ArgGroup<'help> {
                 .as_str()
                 .expect("failed to convert arg YAML name to str");
             a.name = name_str;
+            a.id = Id::from(&a.name);
             b.get(name_yaml)
                 .expect("failed to get name_str")
                 .as_hash()
@@ -457,7 +468,7 @@ impl<'help> From<&'help Yaml> for ArgGroup<'help> {
                 "conflicts_with" => yaml_vec_or_str!(a, v, conflicts_with),
                 "name" => {
                     if let Some(ys) = v.as_str() {
-                        a.name = ys;
+                        a = a.name(ys);
                     }
                     a
                 }

tests/examples.rs

@@ -40,8 +40,9 @@ fn examples_are_functional() {
         let help_output = run_example(example_name, &["--help"]);
         assert!(
             help_output.status.success(),
-            "{} --help exited with nonzero",
+            "{} --help exited with nonzero: {}",
             example_name,
+            String::from_utf8_lossy(&help_output.stderr),
         );
         assert!(
             !help_output.stdout.is_empty(),

tests/fixtures/multiple_groups.yaml

@@ -0,0 +1,21 @@
+name: app
+
+args:
+  - arg_1:
+      short: a
+  - arg_2:
+      short: b
+  - arg_3:
+      short: c
+  - arg_4:
+      short: d
+
+groups:
+  - group_1:
+      args:
+        - arg_1
+        - arg_2
+  - group_2:
+      args:
+        - arg_3
+        - arg_4

tests/yaml.rs

@@ -256,3 +256,11 @@ fn arg_field_not_string() {
     let yml = load_yaml!("fixtures/arg_field_not_string.yaml");
     Arg::from(yml);
 }
+
+#[test]
+fn multiple_groups() {
+    let yml = load_yaml!("fixtures/multiple_groups.yaml");
+    let matches = App::from(yml).try_get_matches_from(&["app", "-a", "-c"]);
+    eprintln!("{:?}", matches);
+    assert!(matches.is_ok());
+}