cgen: fix assigning a new value to the mut sumtype receiver (#13204)

vlib/v/gen/c/assign.v

@@ -435,7 +435,12 @@ fn (mut g Gen) gen_assign_stmt(node ast.AssignStmt) {
 						if op_overloaded {
 							g.op_arg(val, op_expected_right, val_type)
 						} else {
-							g.expr_with_cast(val, val_type, var_type)
+							exp_type := if left.is_auto_deref_var() {
+								var_type.deref()
+							} else {
+								var_type
+							}
+							g.expr_with_cast(val, val_type, exp_type)
 						}
 					}
 				}

vlib/v/tests/mut_receiver_of_sumtype_test.v

@@ -0,0 +1,17 @@
+pub type Foo = Bar | Baz
+
+fn (mut f Foo) set_baz() {
+	f = Baz{}
+}
+
+pub struct Bar {}
+
+pub struct Baz {}
+
+fn test_mut_receiver_of_sumtype() {
+	mut x := Foo(Bar{})
+	x.set_baz()
+
+	println(x)
+	assert '$x' == 'Foo(Baz{})'
+}