patch 9.2.1130: Vim9: assigning a Number to $VAR or @r is not an error

Problem:  Vim9: assigning a non-String value to an environment
          variable, a register or a String v: variable at the script
          level converts the value, while a :def function gives E1012.
Solution: Also check the type at the script level and give E1012

Using "@#" still takes a Number with "=".

Add Test_assign_string_only(), which checks that these fail with E1012
both in a :def function and at the script level: "= 123", "..= 123"
and "= true" on $XSTRING_ONLY, @a and v:errmsg, "@# ..= 1" and
"[$XSTRING_ONLY, @a] = [1, 2]".  It also checks that legacy :let still
converts a Number to a String for these three targets.

closes: #21364

Assisted-by: Claude
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/vim9.txt

@@ -1,4 +1,4 @@
-*vim9.txt*	For Vim version 9.2.  Last change: 2026 Sep 24
+*vim9.txt*	For Vim version 9.2.  Last change: 2026 Sep 25
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -4620,6 +4620,12 @@ in Vim9 script.
 	echo filter([1, 2], 0)
 	vim9cmd echo filter([1, 2], 0)	# E1024: Using a Number as a String
 <
+- Assigning a number to an environment variable, a register or a String
+  v: variable:
+>vim
+	let $FOO = 123
+	vim9cmd $FOO = 123	# E1012: Type mismatch; expected string but ...
+<
 - Not using a number where a number is expected:
 >vim
 	" In this example, legacy Vim script treats v:false as 0

src/eval.c

@@ -2417,6 +2417,20 @@ set_var_lval(
 		    return;
 		}
 
+		// In Vim9 script a String v: variable only takes a String.
+		if (in_vim9script() && STRNCMP(lp->ll_name, "v:", 2) == 0
+			&& tv.v_type == VAR_STRING)
+		{
+		    where_T	where = WHERE_INIT;
+
+		    if (check_typval_type(&t_string, rettv, where) == FAIL)
+		    {
+			clear_tv(&tv);
+			*endp = cc;
+			return;
+		    }
+		}
+
 		if ((di == NULL
 			 || (!var_check_ro(di->di_flags, lp->ll_name, FALSE)
 			   && !tv_check_lock(&di->di_tv, lp->ll_name, FALSE)))

src/evalvars.c

@@ -1991,6 +1991,18 @@ ex_let_one(
     if (check_typval_is_value(tv) == FAIL)
 	return NULL;
 
+    // In Vim9 script an environment variable and a register only take a
+    // String.  "@#" also takes a buffer number.
+    if (in_vim9script() && (*arg == '$' || *arg == '@')
+	    && !(*arg == '@' && arg[1] == '#' && tv->v_type == VAR_NUMBER
+					       && (op == NULL || *op == '=')))
+    {
+	where_T	where = WHERE_INIT;
+
+	if (check_typval_type(&t_string, tv, where) == FAIL)
+	    return NULL;
+    }
+
     if (*arg == '$')
     {
 	// ":let $VAR = expr": Set environment variable.
@@ -4327,6 +4339,16 @@ set_var_const(
 
 	// existing variable, need to clear the value
 
+	// In Vim9 script a String v: variable only takes a String.
+	if (ht == &vimvarht && di->di_tv.v_type == VAR_STRING
+							    && in_vim9script())
+	{
+	    where_T	where = WHERE_INIT;
+
+	    if (check_typval_type(&t_string, tv, where) == FAIL)
+		goto failed;
+	}
+
 	// Handle setting internal v: variables separately where needed to
 	// prevent changing the type.
 	int type_error = FALSE;

src/testdir/test_vim9_assign.vim

@@ -308,6 +308,33 @@ def Test_assign_register()
   v9.CheckDefFailure(['@a += 123'], 'E1012:')
 enddef
 
+" An environment variable, a register and a String v: variable only take a
+" String.
+def Test_assign_string_only()
+  for target in ['$XSTRING_ONLY', '@a', 'v:errmsg']
+    v9.CheckDefAndScriptFailure([target .. ' = 123'],
+      'E1012: Type mismatch; expected string but got number', 1)
+    v9.CheckDefAndScriptFailure([target .. ' ..= 123'],
+      'E1012: Type mismatch; expected string but got number', 1)
+    v9.CheckDefAndScriptFailure([target .. ' = true'],
+      'E1012: Type mismatch; expected string but got bool', 1)
+  endfor
+  v9.CheckDefAndScriptFailure(['[$XSTRING_ONLY, @a] = [1, 2]'],
+    'E1012: Type mismatch; expected string but got number', 1)
+  v9.CheckDefAndScriptFailure(['@# ..= 1'],
+    'E1012: Type mismatch; expected string but got number', 1)
+  v9.CheckDefAndScriptSuccess(['@# = bufnr()'])
+
+  # Legacy script converts the value.
+  legacy let $XSTRING_ONLY = 123
+  legacy let @a = 456
+  legacy let v:errmsg = 789
+  assert_equal('123', $XSTRING_ONLY)
+  assert_equal('456', @a)
+  assert_equal('789', v:errmsg)
+  unlet $XSTRING_ONLY
+enddef
+
 def Test_reserved_name()
   var more_names = ['null_job', 'null_channel']
   if !has('job')

src/version.c

@@ -763,6 +763,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1130,
 /**/
     1129,
 /**/