test: add tests for gu/gU behavior in Turkish locale

test/functional/normal/lang_spec.lua

@@ -0,0 +1,59 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear, insert, eq = helpers.clear, helpers.insert, helpers.eq
+local execute, expect = helpers.execute, helpers.expect
+local feed, eval = helpers.feed, helpers.eval
+local exc_exec = helpers.exc_exec
+
+describe('gu and gU', function()
+  before_each(clear)
+
+  it('works in any locale with default casemap', function()
+    eq('internal,keepascii', eval('&casemap'))
+    insert("iI")
+    feed("VgU")
+    expect("II")
+    feed("Vgu")
+    expect("ii")
+  end)
+
+  describe('works in Turkish locale', function()
+    if helpers.pending_win32(pending) then return end
+
+    clear()
+    if eval('has("mac")') ~= 0 then
+      pending("not yet on macOS", function() end)
+      return
+    end
+
+    local err = exc_exec('lang ctype tr_TR.UTF-8')
+    if err ~= 0 then
+      pending("Locale tr_TR.UTF-8 not supported", function() end)
+      return
+    end
+
+    before_each(function()
+      execute('lang ctype tr_TR.UTF-8')
+    end)
+
+    it('with default casemap', function()
+      eq('internal,keepascii', eval('&casemap'))
+      -- expect ASCII behavior
+      insert("iI")
+      feed("VgU")
+      expect("II")
+      feed("Vgu")
+      expect("ii")
+    end)
+
+    it('with casemap=""', function()
+      execute('set casemap=')
+      -- expect Turkish locale behavior
+      insert("iI")
+      feed("VgU")
+      expect("İI")
+      feed("Vgu")
+      expect("iı")
+    end)
+
+  end)
+end)