fix: try_map loses some of what is expected in error when used sequentially

Author: Hedgehogo

.gitignore

@@ -1,5 +1,6 @@
 # IDE projects
 .idea/
+.vscode/
 
 # Project output
 /target

src/combinator.rs

@@ -630,25 +630,38 @@ where
         // Remove the pre-inner alt, to be reinserted later so we always preserve it
         let old_alt = inp.errors.alt.take();
 
-        let out = self.parser.go::<Emit>(inp)?;
+        let res = self.parser.go::<Emit>(inp);
         let span = inp.span_since(&before);
         let new_alt = inp.errors.alt.take();
 
-        match (self.mapper)(out, span) {
+        match res {
             Ok(out) => {
-                // If successful, reinsert the original alt and then apply the new alt on top of it, since both are valid
-                inp.errors.alt = old_alt;
-                if let Some(new_alt) = new_alt {
-                    inp.add_alt_err(&before.inner, new_alt.err);
+                match (self.mapper)(out, span) {
+                    Ok(out) => {
+                        // If successful, reinsert the original alt and then apply the new alt on top of it, since both are valid
+                        inp.errors.alt = old_alt;
+                        if let Some(new_alt) = new_alt {
+                            inp.add_alt_err(&new_alt.pos, new_alt.err);
+                        }
+                        Ok(M::bind(|| out))
+                    }
+
+                    Err(err) => {
+                        // If unsuccessful, reinsert the original alt but replace the new alt with the mapper error (since it overrides it)
+                        inp.errors.alt = old_alt;
+                        inp.add_alt_err(&before.inner, err);
+                        Err(())
+                    }
                 }
-                Ok(M::bind(|| out))
             }
-            Err(err) => {
-                // If unsuccessful, reinsert the original alt but replace the new alt with the mapper error (since it overrides it)
+
+            Err(_) => {
                 inp.errors.alt = old_alt;
-                inp.add_alt_err(&before.inner, err);
+                if let Some(new_alt) = new_alt {
+                    inp.add_alt_err(&new_alt.pos, new_alt.err);
+                }
                 Err(())
-            }
+            },
         }
     }
 

src/lib.rs

@@ -3812,6 +3812,34 @@ mod tests {
         );
     }
 
+    #[test]
+    fn try_map() {
+        use crate::{DefaultExpected, LabelError};
+
+        let parser = group((
+                just("a").or_not(),
+                just("b").try_map(|_, _| Ok(())).or_not(),
+                just::<_, &str, extra::Err<Rich<_>>>("c"),
+            ))
+            .ignored();
+
+        assert_eq!(
+            parser.parse("").into_output_errors(),
+            (
+                None,
+                vec![LabelError::<&str, _>::expected_found(
+                    vec![
+                        DefaultExpected::Token('a'.into()),
+                        DefaultExpected::Token('b'.into()),
+                        DefaultExpected::Token('c'.into()),
+                    ],
+                    None,
+                    SimpleSpan::new((), 0..0)
+                )]
+            )
+        );
+    }
+
     #[test]
     fn zero_size_custom_failure() {
         fn my_custom<'src>() -> impl Parser<'src, &'src str, ()> {