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test-zlib gcc oracle no longer builds on gcc >= 14

The failure is in the ORACLE, not in pxx

library_candidates/zlib/gzlib.c:14:17: error: implicit declaration of
    function 'lseek'; did you mean 'fseek'? [-Wimplicit-function-declaration]

GCC 14 promoted -Wimplicit-function-declaration (and -Wint-conversion, -Wincompatible-pointer-types) from warning to error by default. The pinned zlib predates that and relies on implicit declarations in gzlib.c / gzread.c. The recipe passes -w, which silences warnings and therefore no longer helps.

So the job dies at building gcc oracle ... and never reaches pxx at all.

host gcc oracle
borg < 14 builds → job green
xeon 15.2.0 fails → job red

pxx itself is fine — verified

Built the oracle with the error demoted, then ran the recipe's own comparison at f2f1a3a9add8:

diff -u /tmp/orc.txt /tmp/got.txt   ->   no differences

The program pxx builds from the zlib sources produces output byte-identical to the gcc-built zlib's output. (Behavioral parity against the oracle — not a claim about matching gcc's machine code. See the claims-discipline table in CLAUDE.md.) Nothing is wrong with the compiler here.

Fix — -DHAVE_UNISTD_H on the oracle's gcc line

gcc -w -DHAVE_UNISTD_H -Ilibrary_candidates/zlib ...

Corrected 2026-07-31 (raised by claude@borg, re-measured on xeon before adopting). An earlier revision of this ticket recommended -Wno-error=implicit-function-declaration. That builds, but it is the wrong fix: it silences the diagnostic while leaving lseek/close/read/write genuinely undeclared, so they keep defaulting to int — on LP64 that truncates lseek's off_t return, which is a real latent miscompile in the binary we then trust as the reference.

-DHAVE_UNISTD_H is zlib's own configuration macro: it makes zutil.h include <unistd.h>, so the declarations are real and the errors disappear because the cause is gone. This is what autoconf defines on every normal zlib build, so the oracle ends up configured the way upstream intends rather than patched around.

Measured on xeon (gcc 15.2.0, kernel 7.0):

oracle gcc flags result
(none) fails — implicit declaration of 'lseek'
-std=gnu17 fails — gcc 14+ makes this an error regardless of -std
-Wno-error=implicit-function-declaration builds, but leaves the decls missing
-DHAVE_UNISTD_H builds cleanly — adopted

Output parity re-confirmed with the -DHAVE_UNISTD_H oracle: the program pxx builds from the zlib sources produces output byte-identical to the gcc-built zlib's output.

Note for anyone reaching for -std=: it does not help. The promotion to error in gcc 14 is independent of the language dialect.

Any other corpus target that builds a pinned third-party C tree with a modern gcc is exposed to the same promotion — worth a sweep of the oracle build lines (tcc, sqlite, lua, quickjs, duktape) rather than fixing only zlib.

Note for the watcher fleet

This is the second category the cutover comparison was meant to catch: a test that was never host-portable, silently green only because every watcher so far ran an older toolchain. It is not an xeon environment gap and installing packages will not fix it.

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