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Case 216: Overload Added Beside an initializer_list Overload

Field Value
Verdict 🟢 COMPATIBLE
Category Addition (Compatible)
Classification Rule
Platforms Linux, macOS
Flags —
Detected ChangeKinds func_added
Source files catalog/cases/case216_overload_added_beside_initializer_list/
Rule family overload-added-beside-initializer-list
Subject Safe changes correctly not flagged

Category: Overloads | Verdict: ✅ COMPATIBLE

Verdict and consumer impact

Negative control for case111. mylib::pick(int) and mylib::pick(std::initializer_list<int>) exist; v2 adds mylib::pick(double). pick(int) and pick(double) both accept {} by the identity conversion, so judged as a pair they look ambiguous. But the initializer_list overload ranks better for a braced argument ([over.ics.rank]/3.1), so mylib::pick({}) selects it before and after: GCC and Clang return 2 both ways. Before the 2026-10-11 re-audit fix, abicheck reported overload_ambiguity_introduced here.

Old/new diff

v1.h v2.h
int pick(int);
int pick(std::initializer_list<int>);		 same, plus int pick(double);

abicheck command

g++ -shared -fPIC -g v1.cpp -o libv1.so
g++ -shared -fPIC -g v2.cpp -o libv2.so
abicheck compare libv1.so libv2.so --header old=v1.h --header new=v2.h

Expected abicheck finding

Verdict: COMPATIBLE (exit 0)
  func_added  mylib::pick(double)
  (no overload_ambiguity_introduced)

Same result with the CastXML and the Clang header backend (ABICHECK_AST_FRONTEND=castxml|clang).

Minimum evidence

L0 (symbol table): the new pick(double) export alone shows the addition. The negative claim -- no overload_ambiguity_introduced -- is about the header tier, where the overload witness is computed.

Why abicheck catches it

The ambiguity witness is checked against the whole overload set: another overload taking std::initializer_list at the {} position wins the call, so no ambiguity is claimed.

Runtime failure demonstration

g++ app.cpp -L. -l:libv1.so -Wl,-rpath,. -o app && ./app       # pick({}) = 2
sed 's/v1.h/v2.h/' app.cpp > app2.cpp && g++ app2.cpp v2.cpp -o app2 && ./app2   # pick({}) = 2

Safe redesign

Nothing to fix: this is a compatible addition.

Source files

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

See also: Compatibility Catalog · All COMPATIBLE cases · Category: Addition (Compatible) · Rule: Overload added beside an initializer_list overload · Subject: Safe changes correctly not flagged.