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

3 files changed, 15 insertions, 1 deletions

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()