llvm/polly/lib/External/isl/include/isl/maybe_templ.h

#include <isl/ctx.h>
#include <isl/maybe.h>

/* A structure that possibly contains a pointer to an object of type ISL_TYPE.
 * The pointer in "value" is only valid if "valid" is isl_bool_true.
 * Otherwise, "value" is set to NULL.
 */
struct ISL_MAYBE(ISL_TYPE) { … };
isl_maybe_isl_ast_expr;