Treat nil diagnostic writers as no-output entries

execute_external_test.go

@@ -469,3 +469,39 @@ func TestExecuteNormalizesOutcomeDefaultsAndPreservesDomainValues(t *testing.T)
 		}
 	}
 }
+
+func TestNilDiagnosticWritersHaveNoOutputAndDoNotHideCauses(t *testing.T) {
+	cause := errors.New("operation failed")
+	for _, base := range []commandkit.Outcome{commandkit.Success(), commandkit.Domain("needs action", 2, nil), commandkit.Failure(cause)} {
+		base.Diagnostics = []commandkit.Diagnostic{{}, {Severity: configkit.SeverityWarning}, {Severity: configkit.SeverityError}}
+		command, err := commandkit.NewCommand(commandkit.CommandSpec[struct{}]{Name: "run", Run: func(commandkit.Invocation, struct{}) commandkit.Outcome { return base }})
+		if err != nil {
+			t.Fatal(err)
+		}
+		app, err := commandkit.New(commandkit.Identity{Name: "example"}, command)
+		if err != nil {
+			t.Fatal(err)
+		}
+		var diagnostics bytes.Buffer
+		result := app.Execute(context.Background(), []string{"run"}, commandkit.Streams{Err: &diagnostics})
+		want := commandkit.DeliveryNone
+		if base.Cause != nil {
+			want = commandkit.DeliveryComplete
+		}
+		if result.Kind != base.Kind || result.Status != base.Status || result.Domain != base.Domain || result.Cause != base.Cause || result.DeliveryError != nil || result.DiagnosticDelivery != want {
+			t.Fatalf("result=%#v base=%#v", result, base)
+		}
+		if base.Cause == nil && diagnostics.Len() != 0 {
+			t.Fatalf("unexpected output=%q", diagnostics.String())
+		}
+		if base.Cause != nil && strings.Count(diagnostics.String(), cause.Error()) != 1 {
+			t.Fatalf("cause output=%q", diagnostics.String())
+		}
+		if base.Cause != nil {
+			result = app.Execute(context.Background(), []string{"run"}, commandkit.Streams{Err: failingWriter{io.ErrClosedPipe}})
+			if !errors.Is(result.Cause, cause) || !errors.Is(result.DeliveryError, io.ErrClosedPipe) || result.DiagnosticDelivery != commandkit.DeliveryIncomplete {
+				t.Fatalf("failed delivery=%#v", result)
+			}
+		}
+	}
+}