Throw exception on invalid positional argument

Fixes #169. It seems reasonable to throw an exception when an attempt is
made to parse into positional parameters that don't exist.

CHANGELOG.md

@@ -13,6 +13,7 @@ options. The project adheres to semantic versioning.
 ### Added
 
 * Iterator inputs to `parse_positional`.
+* Throw an exception if the option in `parse_positional` doesn't exist.
 
 ### Bug Fixes
 

include/cxxopts.hpp

@@ -1659,6 +1659,10 @@ ParseResult::consume_positional(std::string a)
         return true;
       }
     }
+    else
+    {
+      throw option_not_exists_exception(*m_next_positional);
+    }
     ++m_next_positional;
   }
 

test/options.cpp

@@ -216,6 +216,22 @@ TEST_CASE("No positional with extras", "[positional]")
   CHECK(argv[1] == std::string("a"));
 }
 
+TEST_CASE("Positional not valid", "[positional]") {
+  cxxopts::Options options("positional_invalid", "invalid positional argument");
+  options.add_options()
+      ("long", "a long option", cxxopts::value<std::string>())
+      ;
+
+  options.parse_positional("something");
+
+  Argv av({"foobar", "bar", "baz"});
+
+  char** argv = av.argv();
+  auto argc = av.argc();
+
+  CHECK_THROWS_AS(options.parse(argc, argv), cxxopts::option_not_exists_exception);
+}
+
 TEST_CASE("Empty with implicit value", "[implicit]")
 {
   cxxopts::Options options("empty_implicit", "doesn't handle empty");