Default TypeList / TypeSet

Terraform provides a test example for using a default for TypeList:
https://github.com/hashicorp/terraform/blob/master/helper/schema/schema_test.go#L3624

However if I define one in a provider I get:
`
foo: Default is not valid for lists or sets
`

I have also tried using a DefaultFunc:
```go
"foo": &schema.Schema{
Type: schema.TypeSet,
Optional: true,
DefaultFunc: func() (interface{}, error) {
return []interface{}{"bar", "thing"}, nil
},
Elem: &schema.Schema{
Type: schema.TypeString,
},
},
```

Which compiles but doesn't provide a default, how would you go about achieving this?

Thanks!

Start with helper/schema/schema_test.go at the referenced default TypeList example, then trace validation for TypeList/TypeSet and DefaultFunc. Done means the SDK behavior matches the test example or clearly documents the supported way to configure a collection default, with a regression test covering it.

Written by the indexing model from the issue text.