checker: fix sumtype assign error (fix #7988) (#8655)

vlib/v/checker/checker.v

@@ -1199,6 +1199,9 @@ fn (mut c Checker) fail_if_immutable(expr ast.Expr) (string, token.Position) {
 						c.error('`$typ_sym.kind` can not be modified', expr.pos)
 					}
 				}
+				.aggregate {
+					c.fail_if_immutable(expr.expr)
+				}
 				else {
 					c.error('unexpected symbol `$typ_sym.kind`', expr.pos)
 				}

vlib/v/tests/sumtype_assign_test.v

@@ -0,0 +1,46 @@
+struct Foo {
+mut:
+	num int
+}
+
+struct Bar {
+mut:
+	text string
+}
+
+struct Baz {
+mut:
+	text string
+}
+
+type FBB = Bar | Baz | Foo
+
+fn test_sumtype_assign() {
+	mut arr := []FBB{}
+	arr << Foo{
+		num: 22
+	}
+	arr << Bar{
+		text: 'bar'
+	}
+	arr << Baz{
+		text: 'baz'
+	}
+	mut results := []string{}
+	for a in arr {
+		match mut a {
+			Bar, Baz {
+				a.text = 'I am ' + a.text
+				println(a.text)
+				results << a.text
+			}
+			Foo {
+				a.num = 11
+				results << 'Num is $a.num'
+			}
+		}
+	}
+	assert results[0] == 'Num is 11'
+	assert results[1] == 'I am bar'
+	assert results[2] == 'I am baz'
+}