Case 77: Internal `detail::` templated base class layout change¶

Field	Value
Verdict	🔴 BREAKING
Category	Breaking
Platforms	Linux, macOS, Windows
Flags	ABI break, API break
Detected `ChangeKind`s	`internal_type_leaks_via_public_api`
Source files	browse on GitHub

Category: Internal-leak | Verdict: BREAKING

What breaks¶

This mirrors the actual oneDAL pattern from cpp/oneapi/dal/algo/knn/common.hpp:

namespace mylib::detail {
template <typename Task> class descriptor_base { /* ... */ };
}
template <typename Task = task::classification>
class knn_descriptor : public detail::descriptor_base<Task> { /* ... */ };

detail::descriptor_base is a class template. v2 adds an int max_iter_ field, which grows every instantiation simultaneously:

sizeof(knn_descriptor<task::classification>) grows
sizeof(knn_descriptor<task::regression>) grows
The offset of neighbor_count_ in every knn_descriptor<Task> shifts

Difference from case74¶

	case74	case77
Detail base	non-template class	class template
Reachability edge	nominal `base` lookup	template-instantiation traversal
Affected public types	one (`knn_descriptor`)	every `knn_descriptor<Task>` instantiation
Detector code path	direct base name match	template-argument expansion

case74 verifies the simple inheritance edge. case77 verifies that the leak reachability walker follows template-instantiation edges into detail::.

Why abicheck catches it¶

type_size_changed / type_field_added fire on each instantiation of detail::descriptor_base<...>. The internal_type_leaks_via_public_api overlay (abicheck/internal_leak.py) walks reachability from public symbols and finds the chain knn_descriptor<task::classification> → base:detail::descriptor_base<task::classification>. The overlay path is what reviewers can't dismiss as "internal-only".

Code diff¶

// v1
namespace mylib::detail {
template <typename Task>
class descriptor_base {
public:
    int class_count_;
};
}

// v2 — single new field, but multiplied across every instantiation
namespace mylib::detail {
template <typename Task>
class descriptor_base {
public:
    int class_count_;
    int max_iter_;        // NEW
};
}

Real-world reference¶

cpp/oneapi/dal/algo/knn/common.hpp declares:

template <typename Float = float,
          typename Method = method::by_default,
          typename Task = task::by_default,
          typename Distance = oneapi::dal::minkowski_distance::descriptor<Float>>
class descriptor : public detail::descriptor_base<Task>;

A single field added to oneDAL's detail::descriptor_base<Task> would break the binary layout of every shipped algorithm descriptor.

Case 77: Internal detail:: templated base class layout change¶