checker, cgen: fix for_in_mut iterator val (#12563)

vlib/v/checker/checker.v

@@ -3395,7 +3395,14 @@ pub fn (mut c Checker) selector_expr(mut node ast.SelectorExpr) ast.Type {
 
 	old_selector_expr := c.inside_selector_expr
 	c.inside_selector_expr = true
-	typ := c.expr(node.expr)
+	mut typ := c.expr(node.expr)
+	if node.expr.is_auto_deref_var() {
+		if mut node.expr is ast.Ident {
+			if mut node.expr.obj is ast.Var {
+				typ = node.expr.obj.typ
+			}
+		}
+	}
 	c.inside_selector_expr = old_selector_expr
 	c.using_new_err_struct = using_new_err_struct_save
 	if typ == ast.void_type_idx {
@@ -4857,7 +4864,10 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 			if next_fn.params.len != 1 {
 				c.error('iterator method `next()` must have 0 parameters', node.cond.position())
 			}
-			val_type := next_fn.return_type.clear_flag(.optional)
+			mut val_type := next_fn.return_type.clear_flag(.optional)
+			if node.val_is_mut {
+				val_type = val_type.ref()
+			}
 			node.cond_type = typ
 			node.kind = sym.kind
 			node.val_type = val_type

vlib/v/gen/c/cgen.v

@@ -2183,7 +2183,7 @@ fn (mut g Gen) for_in_stmt(node ast.ForInStmt) {
 		val := if node.val_var in ['', '_'] { g.new_tmp_var() } else { node.val_var }
 		val_styp := g.typ(node.val_type)
 		if node.val_is_mut {
-			g.writeln('\t$val_styp* $val = ($val_styp*)${t_var}.data;')
+			g.writeln('\t$val_styp $val = ($val_styp)${t_var}.data;')
 		} else {
 			g.writeln('\t$val_styp $val = *($val_styp*)${t_var}.data;')
 		}

vlib/v/tests/for_in_mut_iterator_val_test.v

@@ -0,0 +1,50 @@
+const (
+	packets = {
+		0: &Packet{
+			pid: 2
+		}
+		1: &Packet{
+			pid: 1
+		}
+	}
+)
+
+struct Packet {
+	pid        int
+	handle     fn () string
+	restricted bool
+}
+
+struct Reader {
+	foo int
+	bar string
+mut:
+	index int
+}
+
+fn (mut p Reader) next() ?&Packet {
+	if p.index + 1 > packets.len {
+		return error('')
+	}
+	if p.index !in packets {
+		return error('')
+	}
+
+	p.index++
+	return packets[p.index - 1]
+}
+
+fn test_for_in_mut_interator_val() {
+	r := Reader{}
+	mut rets := []string{}
+
+	for mut packet in r {
+		println(packet.pid)
+		rets << '$packet.pid'
+	}
+
+	println(rets)
+	assert rets.len == 2
+	assert rets[0] == '2'
+	assert rets[1] == '1'
+}