parser: improve error messages of 'for val in array' (#14459)

vlib/v/parser/for.v

@@ -125,7 +125,8 @@ fn (mut p Parser) for_stmt() ast.Stmt {
 				is_stack_obj: true
 			})
 		} else if p.scope.known_var(val_var_name) {
-			return p.error('redefinition of value iteration variable `$val_var_name`')
+			return p.error_with_pos('redefinition of value iteration variable `$val_var_name`, use `for ($val_var_name in array) {` if you want to check for a condition instead',
+				val_var_pos)
 		}
 		p.check(.key_in)
 		if p.tok.kind == .name && p.tok.lit in [key_var_name, val_var_name] {

vlib/v/parser/tests/for_val_in_array_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/for_val_in_array_err.vv:5:6: error: redefinition of value iteration variable `val`, use `for (val in array) {` if you want to check for a condition instead
+    3 | fn main() {
+    4 |     val := `+`
+    5 |     for val in [TokenValue(`+`), TokenValue(`-`)] {
+      |         ~~~
+    6 |         println("ok")
+    7 |         break

vlib/v/parser/tests/for_val_in_array_err.vv

@@ -0,0 +1,9 @@
+type TokenValue = rune | u64
+
+fn main() {
+	val := `+`
+	for val in [TokenValue(`+`), TokenValue(`-`)] {
+		println("ok")
+		break
+	}
+}