syntax: fix bug in new alternation literal analysis

In the rewrite of this area of the code, I got part of the logic wrong.

This bug was thankfully detected by OSS-fuzz before the release went
out!

Ref https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=58173

Author: BurntSushi

regex-syntax/src/hir/mod.rs

@@ -2072,8 +2072,8 @@ impl Properties {
     /// concatenation of only `Literal`s or an alternation of only `Literal`s.
     ///
     /// For example, `f`, `foo`, `a|b|c`, and `foo|bar|baz` are alternation
-    /// literals, but `f+`, `(foo)`, `foo()`, ``
-    /// are not (even though that contain sub-expressions that are literals).
+    /// literals, but `f+`, `(foo)`, `foo()`, and the empty pattern are not
+    /// (even though that contain sub-expressions that are literals).
     #[inline]
     pub fn is_alternation_literal(&self) -> bool {
         self.0.alternation_literal
@@ -2211,7 +2211,7 @@ impl Properties {
                 props.static_explicit_captures_len = None;
             }
             props.alternation_literal =
-                props.alternation_literal && p.is_alternation_literal();
+                props.alternation_literal && p.is_literal();
             if !min_poisoned {
                 if let Some(xmin) = p.minimum_len() {
                     if props.minimum_len.map_or(true, |pmin| xmin < pmin) {

regex-syntax/src/hir/translate.rs

@@ -3480,6 +3480,7 @@ mod tests {
         assert!(!props(r"a|[b]").is_alternation_literal());
         assert!(!props(r"(?:a)|b").is_alternation_literal());
         assert!(!props(r"a|(?:b)").is_alternation_literal());
+        assert!(!props(r"(?:z|xx)@|xx").is_alternation_literal());
     }
 
     // This tests that the smart Hir::concat constructor simplifies the given

tests/regression_fuzz.rs

@@ -29,3 +29,12 @@ fn big_regex_fails_to_compile() {
     let pat = "[\u{0}\u{e}\u{2}\\w~~>[l\t\u{0}]p?<]{971158}";
     assert!(regex_new!(pat).is_err());
 }
+
+// This was caught while on master but before a release went out(!).
+//
+// See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=58173
+#[test]
+fn todo() {
+    let pat = "(?:z|xx)@|xx";
+    assert!(regex_new!(pat).is_ok());
+}