vim-patch:8.2.1517: cannot easily get the character under the cursor

Problem:    Cannot easily get the character under the cursor.
Solution:   Add the {chars} argument to strpart().
https://github.com/vim/vim/commit/6c53fca02301ff871cddc1c74c388e23e53a424a

4 files changed, 32 insertions, 10 deletions

diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 448885296d..ca03ee0374 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2409,7 +2409,8 @@ str2list({expr} [, {utf8}])	List	convert each character of {expr} to
 str2nr({expr} [, {base}])	Number	convert String to Number
 strchars({expr} [, {skipcc}])	Number	character length of the String {expr}
 strcharpart({str}, {start} [, {len}])
-				String	{len} characters of {str} at {start}
+				String	{len} characters of {str} at
+					character {start}
 strdisplaywidth({expr} [, {col}]) Number display length of the String {expr}
 strftime({format} [, {time}])	String	time in specified format
 strgetchar({str}, {index})	Number	get char {index} from {str}
@@ -2417,8 +2418,9 @@ stridx({haystack}, {needle} [, {start}])
 				Number	index of {needle} in {haystack}
 string({expr})			String	String representation of {expr} value
 strlen({expr})			Number	length of the String {expr}
-strpart({str}, {start} [, {len}])
-				String	{len} characters of {str} at {start}
+strpart({str}, {start} [, {len} [, {chars}]])
+				String	{len} bytes/chars of {str} at
+					byte {start}
 strridx({haystack}, {needle} [, {start}])
 				Number	last index of {needle} in {haystack}
 strtrans({expr})		String	translate string to make it printable
@@ -2906,7 +2908,8 @@ byte2line({byte})					*byte2line()*
 
 byteidx({expr}, {nr})					*byteidx()*
 		Return byte index of the {nr}'th character in the string
-		{expr}.  Use zero for the first character, it returns zero.
+		{expr}.  Use zero for the first character, it then returns
+		zero.
 		This function is only useful when there are multibyte
 		characters, otherwise the returned value is equal to {nr}.
 		Composing characters are not counted separately, their byte
@@ -8438,14 +8441,19 @@ strlen({expr})	The result is a Number, which is the length of the String
 		{expr} in bytes.
 		If the argument is a Number it is first converted to a String.
 		For other types an error is given.
-		If you want to count the number of multi-byte characters use
+		If you want to count the number of multibyte characters use
 		|strchars()|.
 		Also see |len()|, |strdisplaywidth()| and |strwidth()|.
 
-strpart({src}, {start} [, {len}])			*strpart()*
+strpart({src}, {start} [, {len} [, {chars}]])			*strpart()*
 		The result is a String, which is part of {src}, starting from
 		byte {start}, with the byte length {len}.
-		To count characters instead of bytes use |strcharpart()|.
+		When {chars} is present and TRUE then {len} is the number of
+		characters positions (composing characters are not counted
+		separately, thus "1" means one base character and any
+		following composing characters).
+		To count {start} as characters instead of bytes use
+		|strcharpart()|.
 
 		When bytes are selected which do not exist, this doesn't
 		result in an error, the bytes are simply omitted.
@@ -8457,8 +8465,8 @@ strpart({src}, {start} [, {len}])			*strpart()*
 			strpart("abcdefg", 3)	    == "defg"
 
 <		Note: To get the first character, {start} must be 0.  For
-		example, to get three bytes under and after the cursor: >
-			strpart(getline("."), col(".") - 1, 3)
+		example, to get the character under the cursor: >
+			strpart(getline("."), col(".") - 1, 1, v:true)
 <
 strridx({haystack}, {needle} [, {start}])			*strridx()*
 		The result is a Number, which gives the byte index in
diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index a5544435d2..be16ddd7f6 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -336,7 +336,7 @@ return {
     stridx={args={2, 3}},
     string={args=1},
     strlen={args=1},
-    strpart={args={2, 3}},
+    strpart={args={2, 4}},
     strridx={args={2, 3}},
     strtrans={args=1},
     strwidth={args=1},
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 18e514b9a0..8e0e420eb0 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -9946,6 +9946,16 @@ static void f_strpart(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     len = slen - n;
   }
 
+  if (argvars[2].v_type != VAR_UNKNOWN && argvars[3].v_type != VAR_UNKNOWN) {
+    int off;
+
+    // length in characters
+    for (off = n; off < (int)slen && len > 0; len--) {
+      off += utfc_ptr2len((char_u *)p + off);
+    }
+    len = off - n;
+  }
+
   rettv->v_type = VAR_STRING;
   rettv->vval.v_string = (char_u *)xmemdupz(p + n, (size_t)len);
 }
diff --git a/src/nvim/testdir/test_functions.vim b/src/nvim/testdir/test_functions.vim
index 51689db9c4..6b45ac61d1 100644
--- a/src/nvim/testdir/test_functions.vim
+++ b/src/nvim/testdir/test_functions.vim
@@ -334,6 +334,10 @@ func Test_strpart()
 
   call assert_equal('lép', strpart('éléphant', 2, 4))
   call assert_equal('léphant', strpart('éléphant', 2))
+
+  call assert_equal('é', strpart('éléphant', 0, 1, 1))
+  call assert_equal('ép', strpart('éléphant', 3, 2, v:true))
+  call assert_equal('ó', strpart('cómposed', 1, 1, 1))
 endfunc
 
 func Test_tolower()