fix: skip --sysroot for std module precompile on macOS

Root cause: on macOS, xlings LLVM's clang++.cfg already contains a
--sysroot for the macOS SDK. When mcpp additionally passes --sysroot
on the command line during std module precompilation, it changes the
C header include ordering. The macOS SDK's ___wctype.h references
_CTYPE_A (defined in _ctype.h), but the duplicate --sysroot flag
prevents _ctype.h from being included transitively during module
purview compilation, causing "undeclared identifier '_CTYPE_A'" errors.

Fix: detect apple/darwin target triple in stdmod.cppm and skip the
--sysroot flag for std module precompile. The cfg's own --sysroot
handles macOS SDK discovery correctly for this compilation mode.
Regular compilation (flags.cppm) and linking still use the probed
sysroot as before.

Also restore probe_sysroot() to its original logic (xcrun fallback)
since the issue was in stdmod.cppm passing sysroot, not in probing it.

Author: Sunrisepeak

src/toolchain/probe.cppm

@@ -254,15 +254,6 @@ probe_target_triple(const std::filesystem::path& compilerBin,
 std::filesystem::path
 probe_sysroot(const std::filesystem::path& compilerBin,
               const std::string& envPrefix) {
-    // macOS: prefer xcrun's SDK path over -print-sysroot.
-    // xlings LLVM bakes --sysroot into clang++.cfg at install time, and
-    // -print-sysroot echoes that back. If the CLT/Xcode SDK was updated
-    // after the LLVM package was cached, the cfg-baked path may be stale
-    // or point to an incompatible SDK.  xcrun always returns the currently
-    // active SDK, so mcpp passes it on the command line to override the cfg.
-    if (auto sdk = mcpp::platform::macos::sdk_path())
-        return *sdk;
-
     auto r = run_capture(std::format("{}{} -print-sysroot {}",
                                      envPrefix,
                                      mcpp::xlings::shq(compilerBin.string()),
@@ -271,6 +262,12 @@ 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.
+    // The sysroot is used for regular compilation flags (flags.cppm) but
+    // skipped for std module precompilation on macOS (stdmod.cppm) to
+    // avoid breaking SDK internal header dependencies.
+    if (auto sdk = mcpp::platform::macos::sdk_path())
+        return *sdk;
     return {};
 }
 

src/toolchain/stdmod.cppm

@@ -94,7 +94,18 @@ std::expected<StdModule, StdModError> ensure_built(
 
     std::string sysroot_flag;
     if (!tc.sysroot.empty()) {
-        sysroot_flag = std::format(" --sysroot='{}'", tc.sysroot.string());
+        // On macOS, skip --sysroot for std module precompilation.
+        // xlings LLVM ships a clang++.cfg with a --sysroot that is tuned to
+        // work for normal compilation. Adding a second --sysroot (even the
+        // identical value) changes header include ordering and breaks macOS
+        // SDK internal dependencies — ___wctype.h references _CTYPE_A which
+        // is defined in _ctype.h, but the duplicate --sysroot flag causes
+        // the C runtime header to not be included transitively during module
+        // purview compilation.  Let the cfg's own --sysroot handle it.
+        bool skip_sysroot = tc.targetTriple.find("apple") != std::string::npos
+                         || tc.targetTriple.find("darwin") != std::string::npos;
+        if (!skip_sysroot)
+            sysroot_flag = std::format(" --sysroot='{}'", tc.sysroot.string());
     }
 
     bool std_cached = std::filesystem::exists(sm.bmiPath) && std::filesystem::exists(sm.objectPath);