the target is named by mcpp, and a sixth patch site was found by enumerating - #27
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`__libunwind_config.h` sizes `unw_context_t` by whether the target is
Windows. It read `__CYGWIN__`, deliberately, because it is INSTALLED --- an
application's own compile reads it, and this package's private define never
reaches that compile. mcpp 2026.9.21.1 states the fact under its own name, so
the header reads `__mcpp_target_windows__` first and still accepts the old one:
correct on an engine from either side of that release, no flag day.
THE SIXTH SITE WAS NOT ON ANY LIST. `compiler-rt/lib/builtins/int_lib.h`
selects how `COMPILER_RT_ALIAS` is spelt:
#if defined(__ELF__) || defined(__MINGW32__) || ... || defined(__CYGWIN__)
#elif defined(__APPLE__)
#elif defined(_WIN32) || defined(__UEFI__)
#else
#error Unsupported target
It works today BY ACCIDENT. `_WIN32` and `__MINGW32__` are suppressed by
`[c-abi] presents = "posix"` and `__ELF__` is false, so `__CYGWIN__` is the
only operand holding that branch up. Withdraw it and every branch goes false:
`#error Unsupported target`, one release from now.
This file is NOT installed --- it is compiled only by this package's own build
--- so it takes the private define, which is the rule the other five patches
in this tree follow and the reason `__libunwind_config.h` does the opposite.
Found by enumerating the readers rather than fixing the two that were known.
The first shape of this ecosystem change was stopped by cross-verification
precisely because a set was compared by sampling; doing it again here would
have moved the failure from a `static_assert` to an `#error`, one release
later. PATCHES.md records the site and adds `__CYGWIN__` to the grep list that
the next person changing this tree is told to run.
Both packages move onto mcpp's own name for the target in the same step, because both reasons are the same reason: an INSTALLED header is read by an application's own compile, and a package-private define never reaches it. THE PIN MOVES WITH IT, AND THE ORDER IS THE ONE THIS ECOSYSTEM LEARNED THE HARD WAY TODAY. openkal-musl 0.19.0 is registered in the index before this pin is merged; a consumer merged ahead of its dependency's registration made mcpp's own `openkal-cross` job fail earlier in this wave, and the consumer's own CI could not see it --- that CI substitutes working trees for the whole graph, so it cannot observe a missing registration by construction.
Project-owned macros are upper case --- `NDEBUG` and every other one are --- while lower case belongs to the compiler's own predefines (`__linux__`), which mcpp supplies but does not own. 2026.9.21.1 spelt it lower; the release that withdraws `__CYGWIN__` renames it in the same change, so these headers are written against the spelling they will keep. The lower-case name is not read here and does not need to be: it existed for one release and nothing consumed it. Two operands cover every engine --- one up to and including 2026.9.21.1 defines `__CYGWIN__`, the one that withdraws it defines `__MCPP_TARGET_WINDOWS__` --- so these headers have no flag day and the releases may land in either order. `int_lib.h` is unaffected by the spelling: it is not installed, so it takes the package-private `OPENKAL_TARGET_WINDOWS`, which is the rule this tree already follows and the reason `__libunwind_config.h` does the opposite.
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What changes
__libunwind_config.hsizesunw_context_tby whether the target is Windows. It read__CYGWIN__deliberately — this header is INSTALLED, an application own compile reads it, and this package private define never reaches that compile.mcpp 2026.9.21.1 states the fact under its own name, so the header reads
__mcpp_target_windows__first and still accepts the old one: correct on an engine from either side of that release, no flag day.A sixth patch site, found by enumerating rather than fixing the known two
compiler-rt/lib/builtins/int_lib.hselects howCOMPILER_RT_ALIASis spelt:It works today by accident.
_WIN32and__MINGW32__are suppressed by[c-abi] presents = "posix",__ELF__is false —__CYGWIN__is the only operand holding that branch up. Withdraw it and every branch goes false:#error Unsupported target, one release from now.This file is not installed — compiled only by this package own build — so it takes the private define
OPENKAL_TARGET_WINDOWS, which is the rule the other five patches in this tree follow, and the reason__libunwind_config.hdoes the opposite.PATCHES.mdrecords the site and adds__CYGWIN__to the grep list the next person changing this tree is told to run.Why enumeration mattered here
The first shape of this ecosystem change was stopped by cross-verification precisely because a set had been compared by sampling — the measurement counted third-party readers of
__CYGWIN__and not this repository own. Fixing only the two known headers would have moved the failure from astatic_assertto an#error, one release later.Sequence
__mcpp_target_<os>__(additive)__CYGWIN__