parser: disallow index variable with range for (#6464)

vlib/v/parser/for.v

@@ -113,6 +113,9 @@ fn (mut p Parser) for_stmt() ast.Stmt {
 				typ: table.int_type
 				pos: val_var_pos
 			})
+			if key_var_name.len > 0 {
+				p.error_with_pos('cannot declare index variable with range `for`', key_var_pos)
+			}
 		} else {
 			// this type will be set in checker
 			p.scope.register(val_var_name, ast.Var{

vlib/v/parser/tests/for.out

@@ -0,0 +1,4 @@
+vlib/v/parser/tests/for.vv:1:5: error: cannot declare index variable with range `for`
+    1 | for i, k in 0..5 {
+      |     ^
+    2 | }

vlib/v/parser/tests/for.vv

@@ -0,0 +1,2 @@
+for i, k in 0..5 {
+}