config/configupgrade: Test to show that list unwrapping works for sets

This was already working but we had no tests to prove it.

configs/configschema/decoder_spec.go

@@ -19,11 +19,7 @@ func (b *Block) DecoderSpec() hcldec.Spec {
 	}
 
 	for name, attrS := range b.Attributes {
-		ret[name] = &hcldec.AttrSpec{
-			Name:     name,
-			Type:     attrS.Type,
-			Required: attrS.Required,
-		}
+		ret[name] = attrS.decoderSpec(name)
 	}
 
 	for name, blockS := range b.BlockTypes {
@@ -103,3 +99,11 @@ func (b *Block) DecoderSpec() hcldec.Spec {
 
 	return ret
 }
+
+func (a *Attribute) decoderSpec(name string) hcldec.Spec {
+	return &hcldec.AttrSpec{
+		Name:     name,
+		Type:     a.Type,
+		Required: a.Required,
+	}
+}

configs/configupgrade/test-fixtures/valid/redundant-list/input/redundant-list.tf

@@ -22,6 +22,18 @@ resource "test_instance" "bad4" {
   security_groups = ["${list("a", "b", "c")}"]
 }
 
+resource "test_instance" "bad5" {
+  security_groups = ["${test_instance.bad1.subnet_ids}"] # this one references a set
+}
+
+resource "test_instance" "bad6" {
+  subnet_ids = ["${test_instance.bad1.security_groups}"] # this one defines a set
+}
+
+resource "test_instance" "bad7" {
+  subnet_ids = ["${test_instance.bad1.*.id}"] # this one defines a set
+}
+
 # The rest of these should keep the same amount of list-ness
 
 resource "test_instance" "ok1" {

configs/configupgrade/test-fixtures/valid/redundant-list/want/redundant-list.tf

@@ -30,6 +30,18 @@ resource "test_instance" "bad4" {
   security_groups = ["a", "b", "c"]
 }
 
+resource "test_instance" "bad5" {
+  security_groups = test_instance.bad1.subnet_ids # this one references a set
+}
+
+resource "test_instance" "bad6" {
+  subnet_ids = test_instance.bad1.security_groups # this one defines a set
+}
+
+resource "test_instance" "bad7" {
+  subnet_ids = test_instance.bad1.*.id # this one defines a set
+}
+
 # The rest of these should keep the same amount of list-ness
 
 resource "test_instance" "ok1" {