plans/objchange: when prior is null, computed attributes are unknown

plans/objchange/objchange.go

@@ -26,9 +26,10 @@ import (
 // block where _all_ attributes are computed.
 func ProposedNewObject(schema *configschema.Block, prior, config cty.Value) cty.Value {
 	if prior.IsNull() {
-		// This is the easy case... no prior value to merge, so we can just
-		// return the config as-is.
-		return config
+		// In this case, we will treat the prior value as unknown so that
+		// any computed attributes not overridden in config will show as
+		// unknown values, rather than null values.
+		prior = cty.UnknownVal(schema.ImpliedType())
 	}
 	if config.IsNull() || !config.IsKnown() {
 		// This is a weird situation, but we'll allow it anyway to free

plans/objchange/objchange_test.go

@@ -59,7 +59,7 @@ func TestProposedNewObject(t *testing.T) {
 			}),
 			cty.ObjectVal(map[string]cty.Value{
 				"foo": cty.StringVal("hello"),
-				"bar": cty.NullVal(cty.String),
+				"bar": cty.UnknownVal(cty.String),
 				"baz": cty.ObjectVal(map[string]cty.Value{
 					"boz": cty.StringVal("world"),
 				}),
@@ -432,7 +432,7 @@ func TestProposedNewObject(t *testing.T) {
 					}),
 					cty.ObjectVal(map[string]cty.Value{
 						"bar": cty.StringVal("bosh"),
-						"baz": cty.NullVal(cty.String),
+						"baz": cty.UnknownVal(cty.String),
 					}),
 				}),
 			}),