fix: don't use xcrun SDK as sysroot for xlings LLVM on macOS

Root cause: probe_sysroot() falls back to xcrun --show-sdk-path on
macOS when the compiler doesn't report a sysroot via -print-sysroot.
For xlings-installed LLVM 20.1.7, this sets --sysroot to the Xcode
SDK path. Combined with -nostdinc++ from clang++.cfg, this breaks
C runtime header resolution — macOS SDK headers reference internal
macros (_CTYPE_A, etc.) that are only defined via default include
paths which --sysroot overrides.

Fix: remove the xcrun SDK fallback in probe_sysroot(). If the
compiler itself doesn't report a sysroot, none should be used.
The xcrun fallback was designed for Apple's system clang, not for
standalone xlings LLVM which provides its own libc++ headers.

Also revert the ci-macos.yml MCPP_HOME workaround — the real bug
is fixed, no workaround needed.

Author: Sunrisepeak

.github/workflows/ci-macos.yml

@@ -19,8 +19,6 @@ jobs:
     name: macOS ARM64 — xlings LLVM end-to-end
     runs-on: macos-15
     timeout-minutes: 30
-    env:
-      MCPP_HOME: /Users/runner/.mcpp
     steps:
       - uses: actions/checkout@v4
 
@@ -287,12 +285,9 @@ jobs:
 
       - name: Unit + integration tests via `mcpp test`
         run: |
-          # Use freshly-built mcpp with the shared sandbox (MCPP_HOME set
-          # via job-level env). MCPP_VENDORED_XLINGS ensures the sandbox
-          # uses the pre-installed xlings binary.
+          # Use freshly-built mcpp (has --mirror support)
           MCPP=$(find target -path "*/bin/mcpp" | head -1)
           MCPP=$(cd "$(dirname "$MCPP")" && pwd)/$(basename "$MCPP")
-          export MCPP_VENDORED_XLINGS="$XLINGS_BIN"
           "$MCPP" self config --mirror GLOBAL
           "$MCPP" test
 

src/toolchain/probe.cppm

@@ -262,9 +262,17 @@ probe_sysroot(const std::filesystem::path& compilerBin,
         auto s = trim_line(*r);
         if (!s.empty() && std::filesystem::exists(s)) return s;
     }
-    // macOS fallback: use xcrun to discover the SDK path
-    if (auto sdk = mcpp::platform::macos::sdk_path())
-        return *sdk;
+    // macOS: do NOT fall back to xcrun SDK path for xlings-installed LLVM.
+    // xlings LLVM uses -nostdinc++ (via clang++.cfg) to provide its own
+    // libc++ headers.  Combining --sysroot=<MacOSX.sdk> with -nostdinc++
+    // breaks C runtime header resolution (_CTYPE_A, memcpy, errno etc.
+    // become undeclared) because the SDK headers assume the C runtime
+    // macros are available via default include paths, which --sysroot
+    // overrides.
+    //
+    // If the compiler itself reports a sysroot (-print-sysroot), use it —
+    // that's an explicit choice by the toolchain.  But the xcrun SDK is
+    // meant for Apple's system clang, not for standalone xlings LLVM.
     return {};
 }