fix UBSan false positives with captureless lambdas in C++20#666
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kris-jusiak merged 1 commit intoMay 23, 2026
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which AI agent are you using for these PRs? |
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Claude. |
Might want to leave in the "Co-authored-by" for future ref. |
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We are using this library in production so it's my duty to make sure its well maintained. |
Two changes:
1. zero_wrapper_impl::operator(): in C++20, captureless lambdas are
default-constructible so we can call TExpr{}(args...) directly
instead of the reinterpret_cast<const TExpr&>(*this) hack.
The reinterpret_cast violates strict aliasing and triggers
-fsanitize=undefined "insufficient space" when the wrapper has
zero size (via __BOOST_SML_ZERO_SIZE_ARRAY). In C++14/17 the
existing approach is preserved.
2. transition(always, ...): pass always by value instead of
const-reference. always{} temporaries bound to const& trigger
the same UBSan check when the optimizer recycles their stack slot.
Since always has no state, passing by value is equivalent.
Fixes boost-ext#627
PavelGuzenfeld
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Two sources of
-fsanitize=undefinednoise in C++20 builds:1.
zero_wrapper_impl::operator()The existing implementation uses
reinterpret_cast<const TExpr&>(*this)to call a captureless lambda stored in a zero-size wrapper. This violates strict aliasing and triggers UBSan "reference binding with insufficient space" because the wrapper has zero size (via__BOOST_SML_ZERO_SIZE_ARRAY).In C++20, captureless lambdas are default-constructible, so
TExpr{}(args...)works directly — no cast needed, no UBSan. The C++14/17 path is unchanged.2.
transition(always, ...)constructorsalways{}temporaries passed asconst always&trigger the same UBSan check when the optimizer recycles the stack slot. Sincealwaysis stateless, passing by value is equivalent and avoids the reference binding.Reproducer: compile
example/data.cppwith-std=c++20 -fsanitize=undefined -O1.Fixes #627
Co-authored-by claude.