fix: disable zero-size-array min-size trick by default to eliminate UBSan violations (#249)

[PavelGuzenfeld]

Problem

Issue #249: Clang and GCC UBSan report 30+ "insufficient object size" runtime errors at -O2 (and higher) for any SM that uses lambda guards or actions. The root cause is the BOOST_SML_DETAIL_ZERO_SIZE_ARRAY zero-length array extension used inside pool and wrapper types.

At optimisation level -O2+, the compiler sees that every struct containing a zero-length array member has zero usable bytes and may legally place multiple such objects at the same or overlapping memory addresses. UBSan then catches every dereference as an insufficient-size access.

Reproducer (any TCP-like FSM with lambda guards/actions):

# 30+ runtime errors like:
test/ft/issue_249_ubsan.cpp:39:5: runtime error: member access within address 0x… which
  does not point to an object of type 'pool<…>'
SUMMARY: UBSan: undefined-behavior …

The existing workaround — -DBOOST_SML_CFG_DISABLE_MIN_SIZE — has been known since the issue was filed in 2020 but forces users to discover and apply it manually.

Fix

Flip the default for Clang and GCC: make the static_assert(true) branch (no zero-length array) the default, matching what MSVC and IAR already do.

Flag	Behaviour
(neither)	default — no zero-length arrays, no UBSan violations
BOOST_SML_CFG_ENABLE_MIN_SIZE	new — opt-in to the zero-size-array trick for minimal object sizes
BOOST_SML_CFG_DISABLE_MIN_SIZE	legacy — backward-compat alias; same as the new default; takes priority over ENABLE_MIN_SIZE

Changes

• include/boost/sml.hpp: In the Clang and GCC blocks, change the condition on BOOST_SML_DETAIL_ZERO_SIZE_ARRAY from
  #if !defined(BOOST_SML_CFG_DISABLE_MIN_SIZE) → #if defined(BOOST_SML_CFG_ENABLE_MIN_SIZE) && !defined(BOOST_SML_CFG_DISABLE_MIN_SIZE)
  with a brief comment explaining the rationale.

• test/ft/sizeof.cpp: All size assertions assume objects are zero-sized, which requires the min-size trick. Guard the entire file with
  #if !defined(_MSC_VER) && defined(BOOST_SML_CFG_ENABLE_MIN_SIZE).

• test/ft/issue_249_ubsan.cpp: New regression test — a 4-state TCP-like FSM with lambda guards and actions. Includes // cppcheck-suppress missingIncludeSystem to suppress the Codacy cppcheck false positive (the cloud runner invokes cppcheck per-file without include paths). Verified UBSan-clean with
  clang-18 -std=c++17 -O2 -fsanitize=undefined,address and
  clang-18 -std=c++20 -O2 -fsanitize=undefined,address.

• example/hello_world.cpp: Remove static_assert(1 == sizeof(sm)). The assertion documented a size property that depended on the zero-size-array trick; it was never an API guarantee. The example demonstrates correctness of transitions, guards, and actions — not object size.

Verification

Clang UBSan (full ft suite, C++17 and C++20 at -O2)

PASS=29 FAIL=0  total UBSan runtime errors=0   # C++17
PASS=29 FAIL=0  total UBSan runtime errors=0   # C++20

GCC 14 (full suite including examples, C++20)

56/56 tests passed

MSVC 19.51.36244 (C++20)

=== Results: PASS=34 FAIL=0 ===

Relation

Companion CI config in PR #689 (can be merged independently in either order).

Closes #249.

[PavelGuzenfeld]

Removed due to UB.