Case 122 — Uninstantiated Template Signature Change (documented gap)¶

Field	Value
Verdict	✅ NO_CHANGE
Category	No Change
Platforms	Linux, macOS, Windows
Flags	—
Detected `ChangeKind`s	—
Source files	`examples/case122_template_signature_uninstantiated/`

Verdict: ⚪ NO_CHANGE (abicheck — documented limitation) The real change is a source/API break that abicheck cannot detect in any mode.

What changes¶

Version	Declaration
v1	`template <typename T> T clamp(T value, int lo, int hi);`
v2	`template <typename T> T clamp(T value, long lo, long hi);`

The function template's parameter types change (int → long). The library instantiates nothing — it ships the template header-only and exports only an ordinary library_version() function.

What breaks¶

A consumer writing clamp<int>(x, a, b) resolves the call against the new parameter types and emits a different mangled symbol on its own side; overload resolution and deduction can also change. For users of the template this is a real source/ABI break.

Why this case exists — the hard limit of binary analysis¶

This change is invisible to every abicheck mode:

Object / DWARF mode — the library binary is byte-identical (no instantiation is emitted), so there is nothing to compare.
Header / castxml mode — castxml does not emit uninstantiated template declarations into its AST output, so the signature change is not modelled.

This is the fundamental boundary of comparing built artifacts: code that never becomes a symbol (uninstantiated templates, never-included inline code) leaves no trace a binary or castxml comparison can observe. A pure source-AST tool that diffs the headers directly (e.g. a Clang-based source comparator) can see it — the two approaches are complementary. See Limitations → Source-only changes.

Reproduce manually¶

g++ -shared -fPIC -g v1.cpp -o libtpl_v1.so
g++ -shared -fPIC -g v2.cpp -o libtpl_v2.so
abicheck compare libtpl_v1.so libtpl_v2.so \
    --old-header v1.h --new-header v2.h   # → NO_CHANGE (documented gap)

How to mitigate¶

For ABI-sensitive templates, ship explicit instantiations (template class Foo<int>;) so the instantiation becomes a real symbol abicheck can track (see case17_template_abi), or guard the public template API with a source-level (header-diff) check in addition to the binary comparison.

Source files¶

CMakeLists.txt
app.cpp
v1.cpp
v1.h
v2.cpp
v2.h