Test that case conversions are idempotent

Author: Bodigrim

tests/Tests/Properties/Text.hs

@@ -128,6 +128,11 @@ t_toTitle_1stNotLower = and . notLow . T.toTitle . T.filter stable . T.filter (n
           -- https://en.wikipedia.org/wiki/Georgian_Extended
           isGeorgian c = c >= '\4256' && c < '\4352'
 
+t_toUpper_idempotent t = T.toUpper (T.toUpper t) === T.toUpper t
+t_toLower_idempotent t = T.toLower (T.toLower t) === T.toLower t
+t_toTitle_idempotent t = T.toTitle (T.toTitle t) === T.toTitle t
+t_toCaseFold_idempotent t = T.toCaseFold (T.toCaseFold t) === T.toCaseFold t
+
 ascii_toLower (ASCIIString xs) = map C.toLower xs === T.unpack (T.toLower (T.pack xs))
 ascii_toUpper (ASCIIString xs) = map C.toUpper xs === T.unpack (T.toUpper (T.pack xs))
 ascii_toCaseFold (ASCIIString xs) = map C.toLower xs === T.unpack (T.toCaseFold (T.pack xs))
@@ -331,6 +336,10 @@ testText =
         testProperty "tl_toUpper_upper" tl_toUpper_upper,
         testProperty "t_toTitle_title" t_toTitle_title,
         testProperty "t_toTitle_1stNotLower" t_toTitle_1stNotLower,
+        testProperty "t_toUpper_idempotent" t_toUpper_idempotent,
+        testProperty "t_toLower_idempotent" t_toLower_idempotent,
+        testProperty "t_toTitle_idempotent" t_toTitle_idempotent,
+        testProperty "t_toCaseFold_idempotent" t_toCaseFold_idempotent,
         testProperty "ascii_toLower" ascii_toLower,
         testProperty "ascii_toUpper" ascii_toUpper,
         testProperty "ascii_toTitle" ascii_toTitle,