Add new test cases for optional pair of arguments

optparse-applicative.cabal

@@ -36,6 +36,8 @@ extra-source-files:  CHANGELOG.md
                      tests/helponemptysub.err.txt
                      tests/formatting.err.txt
                      tests/nested.err.txt
+                     tests/optional.err.txt
+                     tests/nested_optional.err.txt
                      tests/subparsers.err.txt
 
 homepage:            https://github.com/pcapriotti/optparse-applicative

tests/nested_optional.err.txt

@@ -0,0 +1,7 @@
+Usage: nested_optional --a A [--b0 B0 [--b1 B1]]
+
+Available options:
+  --a A                    value a
+  --b0 B0                  value b0
+  --b1 B1                  value b1
+  -h,--help                Show this help text

tests/optional.err.txt

@@ -0,0 +1,6 @@
+Usage: optional [--a A --b B]
+
+Available options:
+  --a A                    value a
+  --b B                    value b
+  -h,--help                Show this help text

tests/test.hs

@@ -146,6 +146,37 @@ prop_alt_help = once $
       i = info (p <**> helper) idm
   in checkHelpText "alt" i ["--help"]
 
+prop_optional_help :: Property
+prop_optional_help = once $
+  let p :: Parser (Maybe (String, String))
+      p = optional ((,)
+                    <$> strOption ( long "a"
+                                    <> metavar "A"
+                                    <> help "value a" )
+                    <*> strOption ( long "b"
+                                    <> metavar "B"
+                                    <> help "value b" ) )
+      i = info (p <**> helper) idm
+  in checkHelpText "optional" i ["--help"]
+
+prop_nested_optional_help :: Property
+prop_nested_optional_help = once $
+  let p :: Parser (String, Maybe (String, Maybe String))
+      p = (,) <$>
+          (strOption ( long "a"
+                       <> metavar "A"
+                       <> help "value a" ) ) <*>
+          (optional
+           ((,) <$>
+            (strOption ( long "b0"
+                         <> metavar "B0"
+                         <> help "value b0" ) ) <*>
+            (optional (strOption ( long "b1"
+                                   <> metavar "B1"
+                                   <> help "value b1" )))))
+      i = info (p <**> helper) idm
+  in checkHelpText "nested_optional" i ["--help"]
+
 prop_nested_commands :: Property
 prop_nested_commands = once $
   let p3 :: Parser String