fix(builder): UnknownValueParser shouldn't error on flag absense

Fixes #5079

clap_builder/src/builder/value_parser.rs

@@ -2228,8 +2228,23 @@ impl TypedValueParser for UnknownArgumentValueParser {
         &self,
         cmd: &crate::Command,
         arg: Option<&crate::Arg>,
-        _value: &std::ffi::OsStr,
+        value: &std::ffi::OsStr,
     ) -> Result<Self::Value, crate::Error> {
+        TypedValueParser::parse_ref_(self, cmd, arg, value, ValueSource::CommandLine)
+    }
+
+    fn parse_ref_(
+        &self,
+        cmd: &crate::Command,
+        arg: Option<&crate::Arg>,
+        _value: &std::ffi::OsStr,
+        source: ValueSource,
+    ) -> Result<Self::Value, crate::Error> {
+        match source {
+            ValueSource::DefaultValue => {
+                TypedValueParser::parse_ref_(&StringValueParser::new(), cmd, arg, _value, source)
+            }
+            ValueSource::EnvVariable | ValueSource::CommandLine => {
                 let arg = match arg {
                     Some(arg) => arg.to_string(),
                     None => "..".to_owned(),
@@ -2256,6 +2271,8 @@ impl TypedValueParser for UnknownArgumentValueParser {
                 Err(err)
             }
         }
+    }
+}
 
 /// Register a type with [value_parser!][crate::value_parser!]
 ///

tests/builder/error.rs

@@ -263,7 +263,7 @@ fn unknown_argument_flag() {
     ]);
 
     let res = cmd.clone().try_get_matches_from(["test"]);
-    assert!(res.is_err());
+    assert!(res.is_ok());
 
     let res = cmd.try_get_matches_from(["test", "--ignored"]);
     assert!(res.is_err());