cmArgumentParser: Model maybe-missing string with wrapper type

Bindings to `std::string` require one value.  Some clients have been
filtering `keywordsMissingValue` to support keywords that tolerate a
missing value.  Offer them a type-safe way to achieve this instead.

1 files changed, 5 insertions, 0 deletions

diff --git a/Source/cmArgumentParserTypes.h b/Source/cmArgumentParserTypes.h
index 7d4ce6fcdc..9afa5c7166 100644
--- a/Source/cmArgumentParserTypes.h
+++ b/Source/cmArgumentParserTypes.h
@@ -7,6 +7,11 @@
 namespace ArgumentParser {
 
 template <typename T>
+struct Maybe : public T
+{
+};
+
+template <typename T>
 struct MaybeEmpty : public T
 {
 };