parser, checker: fix postfix expr check (fix #14852) (#14857)

vlib/v/checker/checker.v

@@ -628,6 +628,9 @@ fn (mut c Checker) fail_if_immutable(expr_ ast.Expr) (string, token.Pos) {
 		ast.PrefixExpr {
 			to_lock, pos = c.fail_if_immutable(expr.right)
 		}
+		ast.PostfixExpr {
+			to_lock, pos = c.fail_if_immutable(expr.expr)
+		}
 		ast.SelectorExpr {
 			if expr.expr_type == 0 {
 				return '', pos

vlib/v/parser/expr.v

@@ -457,7 +457,7 @@ pub fn (mut p Parser) expr_with_left(left ast.Expr, precedence int, is_stmt_iden
 			if p.peek_tok.kind in [.rpar, .rsbr] {
 				if !p.inside_ct_if_expr {
 					p.warn_with_pos('`$p.tok.kind` operator can only be used as a statement',
-						p.peek_tok.pos())
+						p.tok.pos())
 				}
 			}
 			if p.tok.kind in [.inc, .dec] && p.prev_tok.line_nr != p.tok.line_nr {

vlib/v/parser/tests/postfix_err.out

@@ -1,21 +0,0 @@
-vlib/v/parser/tests/postfix_err.vv:5:10: warning: `++` operator can only be used as a statement
-    3 | fn test_postfix() {
-    4 |     mut x := 1
-    5 |     _ = (x++)
-      |             ^
-    6 |     x--, x-- // OK
-    7 |     f(x++)
-vlib/v/parser/tests/postfix_err.vv:7:7: warning: `++` operator can only be used as a statement
-    5 |     _ = (x++)
-    6 |     x--, x-- // OK
-    7 |     f(x++)
-      |          ^
-    8 |     a := [x]
-    9 |     _ = a[x--]
-vlib/v/parser/tests/postfix_err.vv:9:11: warning: `--` operator can only be used as a statement
-    7 |     f(x++)
-    8 |     a := [x]
-    9 |     _ = a[x--]
-      |              ^
-   10 | }
-   11 |

vlib/v/parser/tests/postfix_err_a.out

@@ -0,0 +1,21 @@
+vlib/v/parser/tests/postfix_err_a.vv:5:8: warning: `++` operator can only be used as a statement
+    3 | fn test_postfix() {
+    4 |     mut x := 1
+    5 |     _ = (x++)
+      |           ~~
+    6 |     x--, x-- // OK
+    7 |     f(x++)
+vlib/v/parser/tests/postfix_err_a.vv:7:5: warning: `++` operator can only be used as a statement
+    5 |     _ = (x++)
+    6 |     x--, x-- // OK
+    7 |     f(x++)
+      |        ~~
+    8 |     a := [x]
+    9 |     _ = a[x--]
+vlib/v/parser/tests/postfix_err_a.vv:9:9: warning: `--` operator can only be used as a statement
+    7 |     f(x++)
+    8 |     a := [x]
+    9 |     _ = a[x--]
+      |            ~~
+   10 | }
+   11 |

vlib/v/parser/tests/postfix_err_b.out

@@ -0,0 +1,14 @@
+vlib/v/parser/tests/postfix_err_b.vv:7:14: warning: `++` operator can only be used as a statement
+    5 |     for _ in 0..3 {
+    6 |         unsafe {
+    7 |             *(arrayptr++) = 0
+      |                       ~~
+    8 |         }
+    9 |     }
+vlib/v/parser/tests/postfix_err_b.vv:3:18: warning: cannot cast a fixed array (use e.g. `&arr[0]` instead)
+    1 | fn main(){
+    2 |     mut array := [3]int{}
+    3 |     mut arrayptr := &int(array)
+      |                     ~~~~~~~~~~~
+    4 |
+    5 |     for _ in 0..3 {

vlib/v/parser/tests/postfix_err_b.vv

@@ -0,0 +1,10 @@
+fn main(){
+	mut array := [3]int{}
+	mut arrayptr := &int(array)
+
+	for _ in 0..3 {
+		unsafe {
+			*(arrayptr++) = 0
+		}
+	}
+}