fix(parser): Apply conditional defaults

Now that we can store constants for flags, we can apply defaults for
flags too.

Fixes #3294

src/parser/parser.rs

@@ -1385,12 +1385,7 @@ impl<'help, 'cmd> Parser<'help, 'cmd> {
     fn add_defaults(&self, matcher: &mut ArgMatcher) -> ClapResult<()> {
         debug!("Parser::add_defaults");
 
-        for arg in self.cmd.get_opts() {
-            debug!("Parser::add_defaults:iter:{}:", arg.name);
-            self.add_default_value(arg, matcher)?;
-        }
-
-        for arg in self.cmd.get_positionals() {
+        for arg in self.cmd.get_arguments() {
             debug!("Parser::add_defaults:iter:{}:", arg.name);
             self.add_default_value(arg, matcher)?;
         }

tests/builder/action.rs

@@ -0,0 +1,49 @@
+use clap::builder::ArgAction;
+use clap::Arg;
+use clap::Command;
+
+#[test]
+fn set_true_with_default_value_if_present() {
+    let cmd = Command::new("test")
+        .arg(
+            Arg::new("mammal")
+                .long("mammal")
+                .action(ArgAction::SetTrue)
+                .default_value_if("dog", None, Some("true")),
+        )
+        .arg(Arg::new("dog").long("dog").action(ArgAction::SetTrue));
+
+    let matches = cmd.clone().try_get_matches_from(["test", "--dog"]).unwrap();
+    assert_eq!(*matches.get_one::<bool>("dog").unwrap(), true);
+    assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), true);
+
+    let matches = cmd
+        .clone()
+        .try_get_matches_from(["test", "--mammal"])
+        .unwrap();
+    assert_eq!(matches.get_one::<bool>("dog"), None);
+    assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), true);
+}
+
+#[test]
+fn set_true_with_default_value_if_value() {
+    let cmd = Command::new("test")
+        .arg(
+            Arg::new("mammal")
+                .long("mammal")
+                .action(ArgAction::SetTrue)
+                .default_value_if("dog", Some("true"), Some("true")),
+        )
+        .arg(Arg::new("dog").long("dog").action(ArgAction::SetTrue));
+
+    let matches = cmd.clone().try_get_matches_from(["test", "--dog"]).unwrap();
+    assert_eq!(*matches.get_one::<bool>("dog").unwrap(), true);
+    assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), true);
+
+    let matches = cmd
+        .clone()
+        .try_get_matches_from(["test", "--mammal"])
+        .unwrap();
+    assert_eq!(matches.get_one::<bool>("dog"), None);
+    assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), true);
+}

tests/builder/main.rs

@@ -1,3 +1,4 @@
+mod action;
 mod app_from_crate;
 mod app_settings;
 mod arg_aliases;