Backed out changeset 958d228b8b00 (bug 1929511) for causing build bustages in RegExpShim.h.

js/src/irregexp/RegExpShim.h

@@ -244,6 +244,21 @@ template <typename... Args>
   return mozilla::AddToHash(aHash, aArgs...);
 }
 
+template <typename T>
+class Optional {
+  mozilla::Maybe<T> inner_;
+
+ public:
+  Optional() = default;
+  Optional(T t) { inner_.emplace(t); }
+
+  bool has_value() const { return inner_.isSome(); }
+  const T& value() const { return inner_.ref(); }
+
+  T* operator->() { return &inner_.ref(); }
+  T& operator*() { return inner_.ref(); }
+};
+
 namespace bits {
 
 inline uint64_t CountTrailingZeros(uint64_t value) {
@@ -1105,15 +1120,13 @@ inline bool IsEitherUnicode(RegExpFlags flags) {
   return flags.unicode() || flags.unicodeSets();
 }
 
-inline std::optional<RegExpFlag> TryRegExpFlagFromChar(char c) {
+inline base::Optional<RegExpFlag> TryRegExpFlagFromChar(char c) {
   RegExpFlag flag;
 
   // The parser only calls this after verifying that it's a supported flag.
-  if (JS::MaybeParseRegExpFlag(c, &flag)) {
-    return flag;
-  }
+  MOZ_ALWAYS_TRUE(JS::MaybeParseRegExpFlag(c, &flag));
 
-  return std::optional<RegExpFlag>{};
+  return base::Optional(flag);
 }
 
 inline bool operator==(const RegExpFlags& lhs, const int& rhs) {