// =============================================================================
// Copyright (c) 2025 - 2026 Haixing Hu.
//
// SPDX-License-Identifier: Apache-2.0
//
// Licensed under the Apache License, Version 2.0.
// =============================================================================
//! Classification independent of the source and requested value types.
/// Identifies why a read could not produce a concrete value.
///
/// The reason is independent of the source and target types. Inspect the
/// accompanying [`crate::ValueMissing`] accessors for those types and for a
/// collection index when one is available.