cmArgumentParser: Model maybe-empty and non-empty lists with wrapper types

Previously bindings to `std::vector<std::string>` required at least one
value.  Some clients have been filtering `keywordsMissingValue` to
support keywords followed by empty lists.  Instead, require clients to
specify whether a keyword's list can be empty as part of the binding
type.

1 files changed, 19 insertions, 0 deletions

diff --git a/Source/cmArgumentParserTypes.h b/Source/cmArgumentParserTypes.h
new file mode 100644
index 0000000..7d4ce6f
--- /dev/null
+++ b/Source/cmArgumentParserTypes.h
@@ -0,0 +1,19 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#pragma once
+
+#include "cmConfigure.h" // IWYU pragma: keep
+
+namespace ArgumentParser {
+
+template <typename T>
+struct MaybeEmpty : public T
+{
+};
+
+template <typename T>
+struct NonEmpty : public T
+{
+};
+
+} // namespace ArgumentParser