refactor: Clarify intent of default_value_if

While this doesn't help in the public API (yet), this at least clarifies
the intent in the implementation.

This is building towards #3020.

src/build/arg/arg_predicate.rs

@@ -0,0 +1,14 @@
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub(crate) enum ArgPredicate<'help> {
+    IsPresent,
+    Equals(&'help std::ffi::OsStr),
+}
+
+impl<'help> From<Option<&'help std::ffi::OsStr>> for ArgPredicate<'help> {
+    fn from(other: Option<&'help std::ffi::OsStr>) -> Self {
+        match other {
+            Some(other) => Self::Equals(other),
+            None => Self::IsPresent,
+        }
+    }
+}

src/build/arg/mod.rs

@@ -1,3 +1,4 @@
+mod arg_predicate;
 #[cfg(debug_assertions)]
 pub mod debug_asserts;
 mod possible_value;
@@ -9,6 +10,7 @@ mod value_hint;
 pub use self::possible_value::PossibleValue;
 pub use self::settings::{ArgFlags, ArgSettings};
 pub use self::value_hint::ValueHint;
+pub(crate) use arg_predicate::ArgPredicate;
 
 // Std
 use std::{
@@ -92,7 +94,7 @@ pub struct Arg<'help> {
     pub(crate) validator_os: Option<Arc<Mutex<ValidatorOs<'help>>>>,
     pub(crate) val_delim: Option<char>,
     pub(crate) default_vals: Vec<&'help OsStr>,
-    pub(crate) default_vals_ifs: Vec<(Id, Option<&'help OsStr>, Option<&'help OsStr>)>,
+    pub(crate) default_vals_ifs: Vec<(Id, ArgPredicate<'help>, Option<&'help OsStr>)>,
     pub(crate) default_missing_vals: Vec<&'help OsStr>,
     #[cfg(feature = "env")]
     pub(crate) env: Option<(&'help OsStr, Option<OsString>)>,
@@ -3486,7 +3488,8 @@ impl<'help> Arg<'help> {
         val: Option<&'help OsStr>,
         default: Option<&'help OsStr>,
     ) -> Self {
-        self.default_vals_ifs.push((arg_id.into(), val, default));
+        self.default_vals_ifs
+            .push((arg_id.into(), val.into(), default));
         self.takes_value(true)
     }
 

src/build/mod.rs

@@ -12,3 +12,4 @@ pub use self::{
     arg::{Arg, ArgFlags, ArgSettings, PossibleValue, ValueHint},
     arg_group::ArgGroup,
 };
+pub(crate) use arg::ArgPredicate;

src/parse/parser.rs

@@ -1325,10 +1325,11 @@ impl<'help, 'app> Parser<'help, 'app> {
             if matcher.get(&arg.id).is_none() {
                 for (id, val, default) in arg.default_vals_ifs.iter() {
                     let add = if let Some(a) = matcher.get(id) {
-                        if let Some(v) = val {
-                            a.vals_flatten().any(|value| v == value)
-                        } else {
-                            true
+                        match val {
+                            crate::build::ArgPredicate::Equals(v) => {
+                                a.vals_flatten().any(|value| v == value)
+                            }
+                            crate::build::ArgPredicate::IsPresent => true,
                         }
                     } else {
                         false