If an option with a default value is underused, invoke it more

spec/default_spec.lua

@@ -42,6 +42,15 @@ describe("tests related to default values", function()
          assert.same({foo = "bar"}, args)
       end)
 
+      it("handles underused option with default value correctly", function()
+         local parser = Parser()
+         parser:option "-o" "--output"
+            :count(1)
+            :default "a.out"
+         local args = parser:parse{}
+         assert.same({output = "a.out"}, args)
+      end)
+
       it("doesn't use default if option is not invoked", function()
          local parser = Parser()
          parser:option("-f", "--foo", {

src/argparse.lua

@@ -761,9 +761,16 @@ function Parser:_parse(args, errhandler)
    end
 
    for _, option in ipairs(options) do
-      assert_(invocations[option] >= option._mincount,
-         "option '%s' must be used at least %d times", option._name, option._mincount
-      )
+      if invocations[option] < option._mincount then
+         if option._default then
+            while invocations[option] < option._mincount do
+               invoke(option)
+               close(option)
+            end
+         else
+            error_("option '%s' must be used at least %d times", option._name, option._mincount)
+         end
+      end
    end
 
    for _, callback in ipairs(com_callbacks) do