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, 7 insertions, 0 deletions

diff --git a/Source/cmArgumentParser.cxx b/Source/cmArgumentParser.cxx
index 1bb9051..f0c4cfc 100644
--- a/Source/cmArgumentParser.cxx
+++ b/Source/cmArgumentParser.cxx
@@ -46,6 +46,13 @@ void Instance::Bind(std::string& val)
   this->ExpectValue = true;
 }
 
+void Instance::Bind(Maybe<std::string>& val)
+{
+  this->CurrentString = &val;
+  this->CurrentList = nullptr;
+  this->ExpectValue = false;
+}
+
 void Instance::Bind(MaybeEmpty<std::vector<std::string>>& val)
 {
   this->CurrentString = nullptr;