cpreproc: "C include file not found" reports the last search dir, not the requested name

• Type: bug (diagnostic quality, cpreproc). Track A (shared core) / C-facing.
• Found: 2026-07-07 by Track T while triaging tstate regression test-core#261.

Symptom

When an #include "..." (or a source file passed with -I search) cannot be resolved, the error prints the LAST directory probed instead of the include name and the search list:

pascal26:1: error: C include file not found (/usr/lib/llvm-18/lib/clang/18/include/re.c)

Actual cause was library_candidates/tiny-regex-c/ being absent (gitignored corpus tree), but the message points at the clang system include dir — it cost a triage round-trip to see the real problem. Repro (with the corpus tree absent):

./compiler/pascal26 -Ilib/crtl/include -Ilibrary_candidates/tiny-regex-c test/crtl_tiny_regex_match.c /tmp/x

Wanted

error: C include file not found: "re.c" (searched: lib/crtl/include, library_candidates/tiny-regex-c, /usr/lib/llvm-18/lib/clang/18/include) — name first, search path list after. See compiler/cpreproc.inc around the hardcoded clang path (CPAppendRange(CPrepPath, '/usr/lib/llvm-18/...'), cpreproc.inc:1845) — the hardcoded llvm-18 path itself is also fragile on boxes with a different llvm major and may deserve its own probe/fallback.

Non-goals

Fixing test-core#261 itself — that was a watcher-box corpus gap; testmgr now self-skips corpus jobs when library_candidates/<tree> is absent.

Log

• 2026-07-08 — resolved, commit a8a75188.