Case 225: Header-Only Template Default Removed¶

Field	Value
Verdict	🟠 API_BREAK
Category	API Break
Classification	Rule
Platforms	Linux, macOS, Windows
Flags	API break
Detected `ChangeKind`s	`template_param_default_changed`
Source files	`catalog/cases/case225_header_only_template_default_removed/`
Rule family	`header-only-template-default-removed`
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 = int> struct Box;. v2 removes the default. Existing source that writes the one-argument form Box<char> stops compiling. That is a source break, not a binary break, because no symbol depends on the default.

castxml cannot record defaults. Its validator-lane result is NO_CHANGE with the template-default gap, so it does not see this source break. clang records the removal and reports template_param_default_changed as API_BREAK. Read the castxml result as a known blind spot, not a clean pass.

Old/new diff¶

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

clang: Verdict: API_BREAK, template_param_default_changed.

Minimum evidence¶

min_evidence: L2, the public header.

Why abicheck catches it¶

The recorded default for U goes from int to None. That is a spelling change, detected the same way as case224.

Runtime failure demonstration¶

None. Compilation fails for consumers that use the defaulted form.

Safe redesign¶

Keep the default. If it must go, deprecate it first and provide an explicit alias.

Source files¶

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