fix(parser): Dont report missing flags as present

When we switched flags to `ArgAction::SetTrue`,
we overlooked updating `args_present`.
Because of the default value for `ArgAction::SetTrue`,
`args_present` will always report true when a flag is defined.

I went with the trivial implementation for now.  We could proactively
track this but was unsure about correctness vs overhead and so I thought
I'd have those using it pay for it.

Looking over uses on github, this will fix a couple bugs but should
otherwise be unnoticeable.

Fixes #5860

clap_builder/src/parser/matches/arg_matches.rs

@@ -575,7 +575,9 @@ impl ArgMatches {
     ///     .unwrap();
     /// assert!(! m.args_present());
     pub fn args_present(&self) -> bool {
-        !self.args.is_empty()
+        self.args
+            .values()
+            .any(|v| v.source().map(|s| s.is_explicit()).unwrap_or(false))
     }
 
     /// Report where argument value came from

clap_builder/src/util/flat_map.rs

@@ -118,6 +118,10 @@ impl<K: PartialEq + Eq, V> FlatMap<K, V> {
         self.keys.iter()
     }
 
+    pub(crate) fn values(&self) -> std::slice::Iter<'_, V> {
+        self.values.iter()
+    }
+
     pub(crate) fn iter(&self) -> Iter<'_, K, V> {
         Iter {
             keys: self.keys.iter(),

tests/builder/arg_matches.rs

@@ -99,7 +99,7 @@ fn args_present_flag() {
     let c = Command::new("test").arg(Arg::new("flag").long("flag").action(ArgAction::SetTrue));
 
     let m = c.clone().try_get_matches_from(["test"]).unwrap();
-    assert!(m.args_present());
+    assert!(!m.args_present());
 
     let m = c.clone().try_get_matches_from(["test", "--flag"]).unwrap();
     assert!(m.args_present());