diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index ec816d1..869c75f 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -16,3 +16,148 @@
   call assert_equal(123456789, str2nr('123456789'))
   call assert_equal(-123456789, str2nr('-123456789'))
 endfunc
+
+func Test_tolower()
+  call assert_equal("", tolower(""))
+
+  " Test with all printable ASCII characters.
+  call assert_equal(' !"#$%&''()*+,-./0123456789:;<=>?@abcdefghijklmnopqrstuvwxyz[\]^_`abcdefghijklmnopqrstuvwxyz{|}~',
+          \ tolower(' !"#$%&''()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~'))
+
+  if !has('multi_byte')
+    return
+  endif
+
+  " Test with a few uppercase diacritics.
+  call assert_equal("aàáâãäåāăąǎǟǡả", tolower("AÀÁÂÃÄÅĀĂĄǍǞǠẢ"))
+  call assert_equal("bḃḇ", tolower("BḂḆ"))
+  call assert_equal("cçćĉċč", tolower("CÇĆĈĊČ"))
+  call assert_equal("dďđḋḏḑ", tolower("DĎĐḊḎḐ"))
+  call assert_equal("eèéêëēĕėęěẻẽ", tolower("EÈÉÊËĒĔĖĘĚẺẼ"))
+  call assert_equal("fḟ ", tolower("FḞ "))
+  call assert_equal("gĝğġģǥǧǵḡ", tolower("GĜĞĠĢǤǦǴḠ"))
+  call assert_equal("hĥħḣḧḩ", tolower("HĤĦḢḦḨ"))
+  call assert_equal("iìíîïĩīĭįiǐỉ", tolower("IÌÍÎÏĨĪĬĮİǏỈ"))
+  call assert_equal("jĵ", tolower("JĴ"))
+  call assert_equal("kķǩḱḵ", tolower("KĶǨḰḴ"))
+  call assert_equal("lĺļľŀłḻ", tolower("LĹĻĽĿŁḺ"))
+  call assert_equal("mḿṁ", tolower("MḾṀ"))
+  call assert_equal("nñńņňṅṉ", tolower("NÑŃŅŇṄṈ"))
+  call assert_equal("oòóôõöøōŏőơǒǫǭỏ", tolower("OÒÓÔÕÖØŌŎŐƠǑǪǬỎ"))
+  call assert_equal("pṕṗ", tolower("PṔṖ"))
+  call assert_equal("q", tolower("Q"))
+  call assert_equal("rŕŗřṙṟ", tolower("RŔŖŘṘṞ"))
+  call assert_equal("sśŝşšṡ", tolower("SŚŜŞŠṠ"))
+  call assert_equal("tţťŧṫṯ", tolower("TŢŤŦṪṮ"))
+  call assert_equal("uùúûüũūŭůűųưǔủ", tolower("UÙÚÛÜŨŪŬŮŰŲƯǓỦ"))
+  call assert_equal("vṽ", tolower("VṼ"))
+  call assert_equal("wŵẁẃẅẇ", tolower("WŴẀẂẄẆ"))
+  call assert_equal("xẋẍ", tolower("XẊẌ"))
+  call assert_equal("yýŷÿẏỳỷỹ", tolower("YÝŶŸẎỲỶỸ"))
+  call assert_equal("zźżžƶẑẕ", tolower("ZŹŻŽƵẐẔ"))
+
+  " Test with a few lowercase diacritics, which should remain unchanged.
+  call assert_equal("aàáâãäåāăąǎǟǡả", tolower("aàáâãäåāăąǎǟǡả"))
+  call assert_equal("bḃḇ", tolower("bḃḇ"))
+  call assert_equal("cçćĉċč", tolower("cçćĉċč"))
+  call assert_equal("dďđḋḏḑ", tolower("dďđḋḏḑ"))
+  call assert_equal("eèéêëēĕėęěẻẽ", tolower("eèéêëēĕėęěẻẽ"))
+  call assert_equal("fḟ", tolower("fḟ"))
+  call assert_equal("gĝğġģǥǧǵḡ", tolower("gĝğġģǥǧǵḡ"))
+  call assert_equal("hĥħḣḧḩẖ", tolower("hĥħḣḧḩẖ"))
+  call assert_equal("iìíîïĩīĭįǐỉ", tolower("iìíîïĩīĭįǐỉ"))
+  call assert_equal("jĵǰ", tolower("jĵǰ"))
+  call assert_equal("kķǩḱḵ", tolower("kķǩḱḵ"))
+  call assert_equal("lĺļľŀłḻ", tolower("lĺļľŀłḻ"))
+  call assert_equal("mḿṁ ", tolower("mḿṁ "))
+  call assert_equal("nñńņňŉṅṉ", tolower("nñńņňŉṅṉ"))
+  call assert_equal("oòóôõöøōŏőơǒǫǭỏ", tolower("oòóôõöøōŏőơǒǫǭỏ"))
+  call assert_equal("pṕṗ", tolower("pṕṗ"))
+  call assert_equal("q", tolower("q"))
+  call assert_equal("rŕŗřṙṟ", tolower("rŕŗřṙṟ"))
+  call assert_equal("sśŝşšṡ", tolower("sśŝşšṡ"))
+  call assert_equal("tţťŧṫṯẗ", tolower("tţťŧṫṯẗ"))
+  call assert_equal("uùúûüũūŭůűųưǔủ", tolower("uùúûüũūŭůűųưǔủ"))
+  call assert_equal("vṽ", tolower("vṽ"))
+  call assert_equal("wŵẁẃẅẇẘ", tolower("wŵẁẃẅẇẘ"))
+  call assert_equal("ẋẍ", tolower("ẋẍ"))
+  call assert_equal("yýÿŷẏẙỳỷỹ", tolower("yýÿŷẏẙỳỷỹ"))
+  call assert_equal("zźżžƶẑẕ", tolower("zźżžƶẑẕ"))
+
+  " According to https://twitter.com/jifa/status/625776454479970304
+  " Ⱥ (U+023A) and Ⱦ (U+023E) are the *only* code points to increase
+  " in length (2 to 3 bytes) when lowercased. So let's test them.
+  call assert_equal("ⱥ ⱦ", tolower("Ⱥ Ⱦ"))
+endfunc
+
+func Test_toupper()
+  call assert_equal("", toupper(""))
+
+  " Test with all printable ASCII characters.
+  call assert_equal(' !"#$%&''()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`ABCDEFGHIJKLMNOPQRSTUVWXYZ{|}~',
+          \ toupper(' !"#$%&''()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~'))
+
+  if !has('multi_byte')
+    return
+  endif
+
+  " Test with a few lowercase diacritics.
+  call assert_equal("AÀÁÂÃÄÅĀĂĄǍǞǠẢ", toupper("aàáâãäåāăąǎǟǡả"))
+  call assert_equal("BḂḆ", toupper("bḃḇ"))
+  call assert_equal("CÇĆĈĊČ", toupper("cçćĉċč"))
+  call assert_equal("DĎĐḊḎḐ", toupper("dďđḋḏḑ"))
+  call assert_equal("EÈÉÊËĒĔĖĘĚẺẼ", toupper("eèéêëēĕėęěẻẽ"))
+  call assert_equal("FḞ", toupper("fḟ"))
+  call assert_equal("GĜĞĠĢǤǦǴḠ", toupper("gĝğġģǥǧǵḡ"))
+  call assert_equal("HĤĦḢḦḨẖ", toupper("hĥħḣḧḩẖ"))
+  call assert_equal("IÌÍÎÏĨĪĬĮǏỈ", toupper("iìíîïĩīĭįǐỉ"))
+  call assert_equal("JĴǰ", toupper("jĵǰ"))
+  call assert_equal("KĶǨḰḴ", toupper("kķǩḱḵ"))
+  call assert_equal("LĹĻĽĿŁḺ", toupper("lĺļľŀłḻ"))
+  call assert_equal("MḾṀ ", toupper("mḿṁ "))
+  call assert_equal("NÑŃŅŇŉṄṈ", toupper("nñńņňŉṅṉ"))
+  call assert_equal("OÒÓÔÕÖØŌŎŐƠǑǪǬỎ", toupper("oòóôõöøōŏőơǒǫǭỏ"))
+  call assert_equal("PṔṖ", toupper("pṕṗ"))
+  call assert_equal("Q", toupper("q"))
+  call assert_equal("RŔŖŘṘṞ", toupper("rŕŗřṙṟ"))
+  call assert_equal("SŚŜŞŠṠ", toupper("sśŝşšṡ"))
+  call assert_equal("TŢŤŦṪṮẗ", toupper("tţťŧṫṯẗ"))
+  call assert_equal("UÙÚÛÜŨŪŬŮŰŲƯǓỦ", toupper("uùúûüũūŭůűųưǔủ"))
+  call assert_equal("VṼ", toupper("vṽ"))
+  call assert_equal("WŴẀẂẄẆẘ", toupper("wŵẁẃẅẇẘ"))
+  call assert_equal("ẊẌ", toupper("ẋẍ"))
+  call assert_equal("YÝŸŶẎẙỲỶỸ", toupper("yýÿŷẏẙỳỷỹ"))
+  call assert_equal("ZŹŻŽƵẐẔ", toupper("zźżžƶẑẕ"))
+
+  " Test that uppercase diacritics, which should remain unchanged.
+  call assert_equal("AÀÁÂÃÄÅĀĂĄǍǞǠẢ", toupper("AÀÁÂÃÄÅĀĂĄǍǞǠẢ"))
+  call assert_equal("BḂḆ", toupper("BḂḆ"))
+  call assert_equal("CÇĆĈĊČ", toupper("CÇĆĈĊČ"))
+  call assert_equal("DĎĐḊḎḐ", toupper("DĎĐḊḎḐ"))
+  call assert_equal("EÈÉÊËĒĔĖĘĚẺẼ", toupper("EÈÉÊËĒĔĖĘĚẺẼ"))
+  call assert_equal("FḞ ", toupper("FḞ "))
+  call assert_equal("GĜĞĠĢǤǦǴḠ", toupper("GĜĞĠĢǤǦǴḠ"))
+  call assert_equal("HĤĦḢḦḨ", toupper("HĤĦḢḦḨ"))
+  call assert_equal("IÌÍÎÏĨĪĬĮİǏỈ", toupper("IÌÍÎÏĨĪĬĮİǏỈ"))
+  call assert_equal("JĴ", toupper("JĴ"))
+  call assert_equal("KĶǨḰḴ", toupper("KĶǨḰḴ"))
+  call assert_equal("LĹĻĽĿŁḺ", toupper("LĹĻĽĿŁḺ"))
+  call assert_equal("MḾṀ", toupper("MḾṀ"))
+  call assert_equal("NÑŃŅŇṄṈ", toupper("NÑŃŅŇṄṈ"))
+  call assert_equal("OÒÓÔÕÖØŌŎŐƠǑǪǬỎ", toupper("OÒÓÔÕÖØŌŎŐƠǑǪǬỎ"))
+  call assert_equal("PṔṖ", toupper("PṔṖ"))
+  call assert_equal("Q", toupper("Q"))
+  call assert_equal("RŔŖŘṘṞ", toupper("RŔŖŘṘṞ"))
+  call assert_equal("SŚŜŞŠṠ", toupper("SŚŜŞŠṠ"))
+  call assert_equal("TŢŤŦṪṮ", toupper("TŢŤŦṪṮ"))
+  call assert_equal("UÙÚÛÜŨŪŬŮŰŲƯǓỦ", toupper("UÙÚÛÜŨŪŬŮŰŲƯǓỦ"))
+  call assert_equal("VṼ", toupper("VṼ"))
+  call assert_equal("WŴẀẂẄẆ", toupper("WŴẀẂẄẆ"))
+  call assert_equal("XẊẌ", toupper("XẊẌ"))
+  call assert_equal("YÝŶŸẎỲỶỸ", toupper("YÝŶŸẎỲỶỸ"))
+  call assert_equal("ZŹŻŽƵẐẔ", toupper("ZŹŻŽƵẐẔ"))
+
+  call assert_equal("ⱥ ⱦ", tolower("Ⱥ Ⱦ"))
+endfunc
+
+