[lldb] Skip libc++ category tests on Darwin when no in-tree libc++ is built

[medismailben]

canRunLibcxxTests() previously short-circuited with "libc++ always present" for all Darwin targets, meaning the "libc++" test category was never skipped on macOS — even when LLDB_HAS_LIBCXX is OFF and no --libcxx-include-dir / --libcxx-library-dir are passed to dotest. The tests would silently run against the system libc++ instead of an in-tree build, producing results inconsistent with what the suite is designed to validate.

This fixes canRunLibcxxTests() to apply the same libcxx_include_dir / libcxx_library_dir guard on Darwin that Linux already uses. When those dirs are absent (i.e. no in-tree libc++ was built), the function returns False and checkLibcxxSupport() appends libc++ to skip_categories — skipping those tests exactly as Linux does.

Also downgrade the CMakeLists.txt SEND_ERROR to a WARNING for LLDB_HAS_LIBCXX=OFF, so downstreams that intentionally skip the runtimes build can still keep LLDB_INCLUDE_TESTS=ON for the tests they actually want to run (Shell tests, non-libc++ API tests, etc.), and update the message to accurately say the libc++ tests will be skipped.

[llvmorg-github-actions]

@llvm/pr-subscribers-lldb

Author: Med Ismail Bennani (medismailben)

Changes

The cmake check that fails when LLDB_HAS_LIBCXX is OFF is too strict: the test infrastructure already handles the missing-libcxx case gracefully. lit.cfg.py only adds --libcxx-include-dir / --libcxx-library-dir to the dotest command when config.has_libcxx is true; otherwise dotest falls back to whatever libc++ the compiler picks up by default (system libc++ on Darwin).

Dotest also has a category-based skip path: checkLibcxxSupport() appends "libc++" to configuration.skip_categories when libc++ isn't available and the user hasn't explicitly asked for it, so the @<!-- -->add_test_categories(["libc++"]) tests are filtered out automatically. On Darwin canRunLibcxxTests() always returns True (SDK libc++ is present), so libc++ tests still run there — against the SDK's libc++.

Hard-failing the cmake configure means downstreams that intentionally opt out of building libc++ in-tree can't keep LLDB_INCLUDE_TESTS=ON to get the ninja check-lldb-* targets for the tests they do want to run (Shell tests, non-libc++ API tests, cloud-device tests, etc.).

Downgrade SEND_ERROR to WARNING and rephrase the message so it reflects the actual fallback behavior instead of demanding the user either enable libcxx or disable tests outright.

---

Full diff: https://github.com/llvm/llvm-project/pull/199262.diff

1 Files Affected:

• (modified) lldb/test/CMakeLists.txt (+9-6)

diff --git a/lldb/test/CMakeLists.txt b/lldb/test/CMakeLists.txt
index ed9dcd11cc6cd..fdfa1fa83efef 100644
--- a/lldb/test/CMakeLists.txt
+++ b/lldb/test/CMakeLists.txt
@@ -205,13 +205,16 @@ if(TARGET clang)
             "LLDB_TEST_LIBCXX_ROOT_DIR to it.")
       endif()
     else()
-      # We require libcxx for the test suite, so if we aren't building it,
-      # provide a helpful error about how to resolve the situation.
+      # The LLDB test suite uses libc++ for many tests. If it isn't being
+      # built in-tree, warn and let the suite fall back to whatever libc++
+      # the compiler picks up by default (the system one on Darwin); the
+      # libc++-specific tests will be skipped or run against system libc++.
       if(NOT LLDB_HAS_LIBCXX)
-        message(SEND_ERROR
-          "LLDB test suite requires libc++, but it is currently disabled. "
-          "Please add `libcxx` to `LLVM_ENABLE_RUNTIMES` or disable tests via "
-          "`LLDB_INCLUDE_TESTS=OFF`.")
+        message(WARNING
+          "LLDB test suite normally uses an in-tree libc++; it is currently "
+          "disabled. libc++-specific tests will be skipped or fall back to the "
+          "system libc++. Add `libcxx` to `LLVM_ENABLE_RUNTIMES` to enable "
+          "the full test surface.")
       endif()
     endif()
   endif()

[Michael137]

I'd just keep the old phrasing tbh. I'm not sure users that aren't deeply familiar with the API test suite will understand the skip/fall back comment.

Do we skip all the data formatter tests if no local libc++ is available? If we're downgrading this to a warning, I think we should make sure that they are being skipped. Otherwise it just gives a false sense of security to someone working on the data formatters

[bulbazord]

They should be skipped if they're all annotated as libc++ tests (based on what Ismail wrote in the description). @medismailben Can you verify that?

[medismailben]

Yeah, I need to test that on linux since macOS will always fallback to the system libcxx.

[medismailben]

I built lldb on linux without LLVM_ENABLE_RUNTIMES, so it linked against the system libstdc++:

$ ldd bin/lldb | grep c++
        libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x00007fe93f000000)

All the tests that have @add_test_categories(["libc++"]) are skipped.

[medismailben]

@Michael137 I also went back to the old phrasing with some minor adjustments.

[Michael137]

macOS will always fallback to the system libcxx

This was what i'm worried about. We would still run the tests in the libcxx category even without a locally built libc++?

[medismailben]

True, on macOS we were still running libc++ category tests using the system libcxx when we couldn't find an in-tree one. I didn't notice previously because on my machine all the tests were passing so I thought that actually the libc++ were skipped correctly.

[JDevlieghere]

There should still be a way to silence this warning. Right now the only options are to enable libc++ (which may not be possible/desirable) or to disable all the tests. What if I just want to run the non-libc++ specific tests? Something like LLDB_(DIS/EN)ABLE_LIBCXX_TESTS. Then maybe you can make that the canonical variable to key off elsewhere too?

[JDevlieghere]

Proposed wording:

LLDB's libc++ specific tests will be skipped. Add libcxx to LLVM_ENABLE_RUNTIMES to get full coverage, pass LLDB_ENABLE_LIBCXX_TESTS=OFF to disable the libc++ test and this warning, or disable all tests with LLDB_INCLUDE_TESTS=OFF.

[medismailben]

What if I just want to run the non-libc++ specific tests?

I don't really see the point of adding LLDB_ENABLE_LIBCXX_TESTS. Instead of setting that flag, the user should just not build libcxx (i.e. remove it from LLVM_ENABLE_RUNTIMES). To me it makes things more confusing.

[JDevlieghere]

There should still be a way to silence this warning. Right now the only options are to enable libc++ (which may not be possible/desirable) or to disable all the tests. What if I just want to run the non-libc++ specific tests? Something like LLDB_(DIS/EN)ABLE_LIBCXX_TESTS. Then maybe you can make that the canonical variable to key off elsewhere too?

[JDevlieghere]

Proposed wording:

LLDB's libc++ specific tests will be skipped. Add libcxx to LLVM_ENABLE_RUNTIMES to get full coverage, pass LLDB_ENABLE_LIBCXX_TESTS=OFF to disable the libc++ test and this warning, or disable all tests with LLDB_INCLUDE_TESTS=OFF.