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diff --git a/js/src/frontend/EitherParser.h b/js/src/frontend/EitherParser.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * A variant-like class abstracting operations on a Parser with a given
+ * ParseHandler but unspecified character type.
+ */
+
+#ifndef frontend_EitherParser_h
+#define frontend_EitherParser_h
+
+#include "mozilla/Utf8.h"
+#include "mozilla/Variant.h"
+
+#include <type_traits>
+#include <utility>
+
+#include "frontend/Parser.h"
+
+namespace js::frontend {
+
+class EitherParser final {
+  // Leave this as a variant, to promote good form until 8-bit parser
+  // integration.
+  mozilla::Variant<Parser<FullParseHandler, char16_t>* const,
+                   Parser<FullParseHandler, mozilla::Utf8Unit>* const>
+      parser;
+
+ public:
+  template <class Parser>
+  explicit EitherParser(Parser* parser) : parser(parser) {}
+
+  const ErrorReporter& errorReporter() const {
+    return parser.match([](auto* parser) -> const frontend::ErrorReporter& {
+      return parser->tokenStream;
+    });
+  }
+
+  void computeLineAndColumn(uint32_t offset, uint32_t* line,
+                            uint32_t* column) const {
+    return parser.match([offset, line, column](auto* parser) -> void {
+      parser->tokenStream.computeLineAndColumn(offset, line, column);
+    });
+  }
+
+  ParserAtomsTable& parserAtoms() {
+    auto& base = parser.match(
+        [](auto* parser) -> frontend::ParserSharedBase& { return *parser; });
+    return base.parserAtoms();
+  }
+};
+
+} /* namespace js::frontend */
+
+#endif /* frontend_EitherParser_h */