parser: warn if PostfixExpr followed by `]` or `)` token (#6214)

vlib/v/parser/pratt.v

@@ -261,6 +261,11 @@ pub fn (mut p Parser) expr(precedence int) ast.Expr {
 			}
 		} else if p.tok.kind in [.inc, .dec] {
 			// Postfix
+			// detect `f(x++)`, `a[x++]`
+			if p.peek_tok.kind in [.rpar, .rsbr] &&
+				p.mod !in ['builtin', 'regex', 'strconv'] { // temp
+				p.warn_with_pos('`$p.tok.kind` operator can only be used as a statement', p.peek_tok.position())
+			}
 			node = ast.PostfixExpr{
 				op: p.tok.kind
 				expr: node

vlib/v/parser/tests/postfix_err.out

@@ -0,0 +1,21 @@
+vlib/v/parser/tests/postfix_err.v: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.v: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.v: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.vv

@@ -0,0 +1,11 @@
+fn f(i int) {}
+
+fn test_postfix() {
+	mut x := 1
+	_ = (x++)
+	x--, x-- // OK
+	f(x++)
+	a := [x]
+	_ = a[x--]
+}
+