Allow absent version headers below structural controls

facade_source_version_mode_test.go

@@ -170,6 +170,68 @@ func TestFacadeSourceVersionAbsenceKeepsCompleteEvidencePath(t *testing.T) {
 	}
 }
 
+func TestFacadeSourceVersionMissingHeaderDoesNotOverlapControl(t *testing.T) {
+	t.Parallel()
+	for _, test := range []struct {
+		name     string
+		mode     configkit.SourceVersionMode
+		control  string
+		wantCode configkit.Code
+	}{
+		{"optional omission", configkit.SourceVersionModeIfPresent, "base", ""},
+		{"required omission", configkit.SourceVersionModeRequired, "base", configkit.CodeVersionError},
+		{"invalid control", configkit.SourceVersionModeIfPresent, "true", configkit.CodeInheritanceInvalid},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			main := configkit.Bytes("main", []byte("{}\n"))
+			if test.mode == configkit.SourceVersionModeRequired {
+				main = configkit.NoSource()
+			}
+			checks := 0
+			path := configkit.Keys("entries", "child", "extends", "version")
+			loaded, err := configkit.LoadLayered(configkit.Input{Source: main, Destination: configkit.Keys("main")}, configkit.LayeredOptions[map[string]any]{
+				SourceVersionMode: test.mode,
+				Definition: configkit.Definition[map[string]any]{Version: &configkit.VersionRule{
+					Path: path, Check: func(version configkit.Version, _ *configkit.Validation) error {
+						checks++
+						if version.Present {
+							t.Error("checker received a nonexistent declaration")
+						}
+						return nil
+					},
+				}},
+				Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
+					Source: configkit.Bytes("underlay", []byte("entries:\n  base: {value: good}\n  child: {extends: "+test.control+"}\n")), Destination: configkit.Keys("under"),
+				}}}},
+			})
+			if checks != 1 {
+				t.Errorf("checker ran %d times, want absent main only", checks)
+			}
+			if test.wantCode != "" {
+				var failure *configkit.LoadError
+				if loaded != nil || !errors.As(err, &failure) {
+					t.Fatal("invalid input did not fail safely")
+				}
+				if diagnostics := failure.Diagnostics(); len(diagnostics) != 1 || diagnostics[0].Code != test.wantCode {
+					t.Fatalf("diagnostics=%#v", diagnostics)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			child := loaded.Config["under"].(map[string]any)["entries"].(map[string]any)["child"].(map[string]any)
+			if child["value"] != "good" {
+				t.Fatal("valid sibling inheritance was not applied")
+			}
+			check := loaded.Sources()[1].VersionCheck
+			if check == nil || check.Version.Present || check.Path.String() != path.String() {
+				t.Fatal("optional omission evidence was not retained")
+			}
+		})
+	}
+}
+
 func (reader *sourceVersionModeReader) Read(data []byte) (int, error) {
 	reader.reads++
 	return reader.Reader.Read(data)