Case 224: Header-Only Template Default Added¶

Field	Value
Verdict	✅ NO_CHANGE
Category	No Change
Classification	Rule
Platforms	Linux, macOS, Windows
Flags	—
Detected `ChangeKind`s	—
Source files	`catalog/cases/case224_header_only_template_default_added/`
Rule family	`header-only-template-default-added`
Subject	Template and generic-programming issues

Category: Template ABI | Verdict: 🟢 NO_CHANGE (castxml) / 🟠 API_BREAK (clang)

Verdict and consumer impact¶

v1 declares template <typename T, typename U> struct Box; with no default for U. v2 adds U = int. Source that already spells both arguments still compiles. Source that relies on the single-argument form Box<char> starts to compile and selects Box<char, int>, so the meaning of existing code can change. No symbol depends on the default, so there is no binary break.

On the default castxml lane the run reads NO_CHANGE, with the template-default assurance gap. clang records that v1 had no default and that v2 has int, and reports template_param_default_changed as API_BREAK.

Old/new diff¶

v1.h	v2.h
`template <typename T, typename U> struct Box;`	`template <typename T, typename U = int> struct Box;`

abicheck command¶

cat > .abicheck.yml <<'EOF'
compile:
  frontend: castxml
  lang: c++
EOF
abicheck dump -H v1.h --config .abicheck.yml --depth headers -o old.json
abicheck dump -H v2.h --config .abicheck.yml --depth headers -o new.json
abicheck compare old.json new.json

Switch frontend: clang to see the clang result.

Expected abicheck finding¶

castxml (default): Verdict: NO_CHANGE, Assurance: partial with the gap template-parameter defaults (castxml cannot record them).

clang: Verdict: API_BREAK, template_param_default_changed.

Minimum evidence¶

min_evidence: L2, the public header.

Why abicheck catches it¶

The None-to-spelling transition is a difference in the recorded per-parameter defaults (compare/template_param_defaults.py). It is compared like any other spelling change, and it is never a clean match with an UNKNOWN side.

Runtime failure demonstration¶

None, as in case223.

Safe redesign¶

Add the default only when every existing spelling is also still valid. Otherwise put the new default on a new alias.

Source files¶

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