checker: fix generic array of sumtype push operation (#17268)

vlib/v/checker/infix.v

@@ -476,7 +476,7 @@ fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 								right_pos)
 						}
 					} else {
-						right_value_type := c.table.value_type(right_type)
+						right_value_type := c.table.value_type(c.unwrap_generic(right_type))
 						if !c.table.is_sumtype_or_in_variant(left_value_type, ast.mktyp(right_value_type)) {
 							c.error('cannot append `${right_sym.name}` to `${left_sym.name}`',
 								right_pos)

vlib/v/tests/generic_array_of_sumtype_push_test.v

@@ -0,0 +1,19 @@
+type SumType = int | string
+
+fn template_fn[T](a []T) []T {
+	mut b := []T{}
+	b << a
+	return b
+}
+
+fn test_generic_array_of_sumtype_push() {
+	ret1 := template_fn([SumType(1)])
+	println(ret1)
+	assert ret1.len == 1
+	assert ret1[0] == SumType(1)
+
+	ret2 := template_fn([SumType('hello')])
+	println(ret2)
+	assert ret2.len == 1
+	assert ret2[0] == SumType('hello')
+}