Provide a BisonWrap for default construction (#761)

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/syntax/bison_wrap.h

@@ -0,0 +1,41 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef EXECUTABLE_SEMANTICS_SYNTAX_BISON_WRAP_H_
+#define EXECUTABLE_SEMANTICS_SYNTAX_BISON_WRAP_H_
+
+#include <optional>
+
+#include "common/check.h"
+
+namespace Carbon {
+
+// Bison requires that types be default initializable for use with its variant
+// semantics. This wraps arbitrary types to support a default constructor, while
+// still requiring they be properly initialized.
+template <typename T>
+class BisonWrap {
+ public:
+  // Assigning a value initializes the wrapper.
+  BisonWrap& operator=(T&& rhs) {
+    val = std::move(rhs);
+    return *this;
+  }
+
+  // Support transparent conversion to the wrapped type, erroring if not
+  // initialized.
+  operator T() {
+    CHECK(val.has_value());
+    T ret = std::move(*val);
+    val.reset();
+    return ret;
+  }
+
+ private:
+  std::optional<T> val;
+};
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_SYNTAX_BISON_WRAP_H_