[steve-downey]

Summary

The 59 negative compile tests were registered through a three-argument add_fail_test macro local to tests/beman/expected/CMakeLists.txt. This PR turns it into a reusable CMake module, cmake/beman-xfail-compile.cmake, written in the shape of beman-install-library.cmake so it can move to bemanproject/infra unchanged. Which tests exist and what each one expects does not change: every regex ctest receives was checked byte-identical to the one the macro passed.

Three commits do the work, each with its own verification; the plan and its decision log are in docs/plan/extract-xfail-compile.md.

1. beman_add_xfail_compile_test replaces the macro. Keyword interface (SOURCE, LINK_LIBRARIES, COMPILE_DEFINITIONS, TARGET_PREFIX, LABELS, EXPECT), same per-test target names, same ctest names.
2. A driver script runs the nested build instead of relying on PASS_REGULAR_EXPRESSION. It tells apart "the source compiled" from "it failed, but not with the expected diagnostic", neither of which the bare regex could report. Tests get the label xfail-compile and a RESOURCE_LOCK, so ctest -j is safe and ctest -L xfail-compile runs just this suite. $<CONFIGURATION> becomes $<CONFIG>.
3. The regex lives in the test source. Each _fail.test.cpp carries // EXPECT: "<regex>" next to the ill-formed line; the module reads it at configure time, and a source without one fails configure naming the file. 35 files already matched, 15 carried an older subset, 9 had none; all 59 now carry the regex that was proven on gcc, clang, and MSVC. Long alternations are split across several EXPECT lines (joined with |) because clang-format reflows comments past column 119.

Demonstration

A real test under ctest -V now says which diagnostic it matched:

5: -- beman-xfail-compile-driver: beman.expected.tests.unexpected_array_fail failed as expected:
   .../include/beman/expected/unexpected.hpp:73:25: error: static assertion failed: unexpected<E>: E must not be an array type
1/1 Test #5: unexpected_array_fail ............   Passed    0.42 sec

A probe file that compiles cleanly, registered with the same regex:

1/1 Test #64: probe_compiles ...................***Failed    0.67 sec
  beman-xfail-compile-driver: beman.expected.tests.probe_compiles compiled,
  but was expected to fail with a diagnostic matching
      E must not be an array type

A probe file with a syntax error, which WILL_FAIL would have accepted:

1/1 Test #65: probe_syntax .....................***Failed    0.62 sec
  beman-xfail-compile-driver: beman.expected.tests.probe_syntax failed to
  compile, but not for the expected reason. No line matches
      E must not be an array type
  Build output: ...

Removing the EXPECT line from a source:

CMake Error at cmake/beman-xfail-compile.cmake:120 (message):
  beman_add_xfail_compile_test(unexpected_cvref_fail): no '// EXPECT: "<regex>"' line in
  .../tests/beman/expected/unexpected_cvref_fail.test.cpp; add one or pass EXPECT

Verification

Check	Result
Regex ctest receives vs the macro's, all 59	byte-identical
gcc 16, full suite	1250/1250
clang 23 with libstdc++, full suite	1249/1249 (before the rebase onto #107)
ctest -L xfail-compile -j 8, gcc 16	59/59, about half the serial time
Touch a test source	CMake re-runs before the build
pre-commit on changed files	clang-format, gersemi, codespell pass

Follow-up

The module is proposed to bemanproject/infra with identical content. Once it lands there, a small PR here bumps the infra/ pin, deletes cmake/beman-xfail-compile*.cmake, and changes one include() path.