patch 8.2.1461: Vim9: string indexes are counted in bytes

Problem:    Vim9: string indexes are counted in bytes.
Solution:   Use character indexes. (closes #6574)
diff --git a/src/eval.c b/src/eval.c
index 70d5b34..5a61a50 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -3718,6 +3718,10 @@
 		    else
 			s = vim_strnsave(s + n1, n2 - n1 + 1);
 		}
+		else if (in_vim9script())
+		{
+		    s = char_from_string(s, n1);
+		}
 		else
 		{
 		    // The resulting variable is a string of a single
@@ -5285,6 +5289,30 @@
 }
 
 /*
+ * Return the character "str[index]" where "index" is the character index.  If
+ * "index" is out of range NULL is returned.
+ */
+    char_u *
+char_from_string(char_u *str, varnumber_T index)
+{
+    size_t	    nbyte = 0;
+    varnumber_T	    nchar = index;
+    size_t	    slen;
+
+    if (str == NULL || index < 0)
+	return NULL;
+    slen = STRLEN(str);
+    while (nchar > 0 && nbyte < slen)
+    {
+	nbyte += MB_CPTR2LEN(str + nbyte);
+	--nchar;
+    }
+    if (nbyte >= slen)
+	return NULL;
+    return vim_strnsave(str + nbyte, MB_CPTR2LEN(str + nbyte));
+}
+
+/*
  * Handle:
  * - expr[expr], expr[expr:expr] subscript
  * - ".name" lookup
diff --git a/src/proto/eval.pro b/src/proto/eval.pro
index a528d3e..c2df7a2 100644
--- a/src/proto/eval.pro
+++ b/src/proto/eval.pro
@@ -59,6 +59,7 @@
 int eval_isnamec(int c);
 int eval_isnamec1(int c);
 int eval_isdictc(int c);
+char_u *char_from_string(char_u *str, varnumber_T index);
 int handle_subscript(char_u **arg, typval_T *rettv, evalarg_T *evalarg, int verbose);
 int item_copy(typval_T *from, typval_T *to, int deep, int copyID);
 void echo_one(typval_T *rettv, int with_space, int *atstart, int *needclr);
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 15c26fd..182295c 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -2075,12 +2075,28 @@
 enddef
 
 def Test_expr7_subscript()
-  let text = 'abcdef'
-  assert_equal('', text[-1])
-  assert_equal('a', text[0])
-  assert_equal('e', text[4])
-  assert_equal('f', text[5])
-  assert_equal('', text[6])
+  let lines =<< trim END
+    let text = 'abcdef'
+    assert_equal('', text[-1])
+    assert_equal('a', text[0])
+    assert_equal('e', text[4])
+    assert_equal('f', text[5])
+    assert_equal('', text[6])
+
+    text = 'ábçdëf'
+    assert_equal('', text[-999])
+    assert_equal('', text[-1])
+    assert_equal('á', text[0])
+    assert_equal('b', text[1])
+    assert_equal('ç', text[2])
+    assert_equal('d', text[3])
+    assert_equal('ë', text[4])
+    assert_equal('f', text[5])
+    assert_equal('', text[6])
+    assert_equal('', text[999])
+  END
+  CheckDefSuccess(lines)
+  CheckScriptSuccess(['vim9script'] + lines)
 enddef
 
 def Test_expr7_subscript_linebreak()
diff --git a/src/version.c b/src/version.c
index 06c6995..1353360 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1461,
+/**/
     1460,
 /**/
     1459,
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 2945379..5131108 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -2233,7 +2233,6 @@
 
 	    case ISN_STRINDEX:
 		{
-		    char_u	*s;
 		    varnumber_T	n;
 		    char_u	*res;
 
@@ -2245,7 +2244,6 @@
 			emsg(_(e_stringreq));
 			goto on_error;
 		    }
-		    s = tv->vval.v_string;
 
 		    tv = STACK_TV_BOT(-1);
 		    if (tv->v_type != VAR_NUMBER)
@@ -2259,12 +2257,9 @@
 		    // The resulting variable is a string of a single
 		    // character.  If the index is too big or negative the
 		    // result is empty.
-		    if (n < 0 || n >= (varnumber_T)STRLEN(s))
-			res = NULL;
-		    else
-			res = vim_strnsave(s + n, 1);
 		    --ectx.ec_stack.ga_len;
 		    tv = STACK_TV_BOT(-1);
+		    res = char_from_string(tv->vval.v_string, n);
 		    vim_free(tv->vval.v_string);
 		    tv->vval.v_string = res;
 		}