[clang][Darwin] Prefer the toolchain-provided libc++.dylib if there is one

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -2846,11 +2846,49 @@ void AppleMachO::AddCXXStdlibLibArgs(const ArgList &Args,
   CXXStdlibType Type = GetCXXStdlibType(Args);
 
   switch (Type) {
-  case ToolChain::CST_Libcxx:
-    CmdArgs.push_back("-lc++");
+  case ToolChain::CST_Libcxx: {
+    // On Darwin, we prioritize a libc++ located in the toolchain to a libc++
+    // located in the sysroot. Unlike the driver for most other platforms, on
+    // Darwin we do that by explicitly passing the library path to the linker
+    // to avoid having to add the toolchain's `lib/` directory to the linker
+    // search path, which would make other libraries findable as well.
+    //
+    // Prefering the toolchain library over the sysroot library matches the
+    // behavior we have for headers, where we prefer headers in the toolchain
+    // over headers in the sysroot if there are any. Note that it's important
+    // for the header search path behavior to match the link-time search path
+    // behavior to ensure that we link the program against a library that
+    // matches the headers that were used to compile it.
+    //
+    // Otherwise, we end up compiling against some set of headers and then
+    // linking against a different library (which, confusingly, shares the same
+    // name) which may have been configured with different options, be at a
+    // different version, etc.
+    SmallString<128> InstallLib = llvm::sys::path::parent_path(getDriver().Dir);
+    llvm::sys::path::append(InstallLib, "lib"); // <install>/lib
+    auto Link = [&](StringRef Library) {
+      SmallString<128> Shared(InstallLib);
+      llvm::sys::path::append(Shared,
+                              SmallString<4>("lib") + Library + ".dylib");
+      SmallString<128> Static(InstallLib);
+      llvm::sys::path::append(Static, SmallString<4>("lib") + Library + ".a");
+      SmallString<32> Relative("-l");
+      Relative += Library;
+
+      if (getVFS().exists(Shared)) {
+        CmdArgs.push_back(Args.MakeArgString(Shared));
+      } else if (getVFS().exists(Static)) {
+        CmdArgs.push_back(Args.MakeArgString(Static));
+      } else {
+        CmdArgs.push_back(Args.MakeArgString(Relative));
+      }
+    };
+
+    Link("c++");
     if (Args.hasArg(options::OPT_fexperimental_library))
-      CmdArgs.push_back("-lc++experimental");
+      Link("c++experimental");
     break;
+  }
 
   case ToolChain::CST_Libstdcxx:
     // Unfortunately, -lstdc++ doesn't always exist in the standard search path;

clang/test/Driver/darwin-header-search-libcxx.cpp

@@ -92,7 +92,7 @@
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1-NOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
 
-// Make sure that using -nostdinc, -nostdinc++ or -nostdlib will drop both the toolchain
+// Make sure that using -nostdinc, -nostdinc++ or -nostdlibinc will drop both the toolchain
 // C++ include path and the sysroot one.
 //
 // RUN: %clang -### %s -fsyntax-only 2>&1 \
@@ -116,7 +116,7 @@
 // RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
 // RUN:     -stdlib=platform \
 // RUN:     -nostdinc++ \
-// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
 // RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
 // RUN:               --check-prefix=CHECK-LIBCXX-NOSTDINCXX %s
 // CHECK-LIBCXX-NOSTDINCXX: "-cc1"

clang/test/Driver/darwin-link-libcxx.cpp

@@ -0,0 +1,81 @@
+// UNSUPPORTED: system-windows
+
+// Tests to check that we link against the toolchain-provided libc++ built library when it is provided.
+// This is required to prefer the toolchain's libc++ over the system's libc++, which matches the behavior
+// we have for header search paths.
+
+// When libc++.dylib is NOT in the toolchain, we should use -lc++ and fall back to the libc++
+// in the sysroot.
+//
+// (1) Without -fexperimental-library.
+// RUN: %clangxx -### %s 2>&1                                                   \
+// RUN:     --target=x86_64-apple-darwin                                        \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_no_libcxx    \
+// RUN:               --check-prefix=CHECK-1 %s
+// CHECK-1: "/usr/bin/ld"
+// CHECK-1: "-lc++"
+// CHECK-1-NOT: "[[TOOLCHAIN]]/usr/lib"
+//
+// (2) With -fexperimental-library.
+// RUN: %clangxx -### %s 2>&1                                                   \
+// RUN:     --target=x86_64-apple-darwin                                        \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN:     -fexperimental-library                                              \
+// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_no_libcxx    \
+// RUN:               --check-prefix=CHECK-2 %s
+// CHECK-2: "/usr/bin/ld"
+// CHECK-2: "-lc++" "-lc++experimental"
+// CHECK-2-NOT: "[[TOOLCHAIN]]/usr/lib"
+
+// When we have libc++.dylib in the toolchain, it should be used over the one in the sysroot.
+// There are a few cases worth testing.
+//
+// (1) Without -fexperimental-library.
+// RUN: %clangxx -### %s 2>&1                                                   \
+// RUN:     --target=x86_64-apple-darwin                                        \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin           \
+// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain              \
+// RUN:               --check-prefix=CHECK-3 %s
+// CHECK-3: "/usr/bin/ld"
+// CHECK-3: "[[TOOLCHAIN]]/usr/lib/libc++.dylib"
+// CHECK-3-NOT: "-lc++"
+//
+// (2) With -fexperimental-library.
+// RUN: %clangxx -### %s 2>&1                                                   \
+// RUN:     --target=x86_64-apple-darwin                                        \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin           \
+// RUN:     -fexperimental-library                                              \
+// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain              \
+// RUN:               --check-prefix=CHECK-4 %s
+// CHECK-4: "/usr/bin/ld"
+// CHECK-4: "[[TOOLCHAIN]]/usr/lib/libc++.dylib"
+// CHECK-4: "[[TOOLCHAIN]]/usr/lib/libc++experimental.a"
+// CHECK-4-NOT: "-lc++"
+// CHECK-4-NOT: "-lc++experimental"
+
+// When we have libc++.a in the toolchain instead of libc++.dylib, it should be
+// used over the one in the sysroot.
+//
+// (1) Without -fexperimental-library.
+// RUN: %clangxx -### %s 2>&1                                                   \
+// RUN:     --target=x86_64-apple-darwin                                        \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_static/usr/bin    \
+// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_static       \
+// RUN:               --check-prefix=CHECK-5 %s
+// CHECK-5: "/usr/bin/ld"
+// CHECK-5: "[[TOOLCHAIN]]/usr/lib/libc++.a"
+// CHECK-5-NOT: "-lc++"
+//
+// (2) With -fexperimental-library.
+// RUN: %clangxx -### %s 2>&1                                                   \
+// RUN:     --target=x86_64-apple-darwin                                        \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_static/usr/bin    \
+// RUN:     -fexperimental-library                                              \
+// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_static       \
+// RUN:               --check-prefix=CHECK-6 %s
+// CHECK-6: "/usr/bin/ld"
+// CHECK-6: "[[TOOLCHAIN]]/usr/lib/libc++.a"
+// CHECK-6: "[[TOOLCHAIN]]/usr/lib/libc++experimental.a"
+// CHECK-6-NOT: "-lc++"
+// CHECK-6-NOT: "-lc++experimental"

clang/test/Driver/experimental-library-flag.cpp

@@ -18,6 +18,8 @@
 // CHECK: -fexperimental-library
 
 // Depending on the stdlib in use, we should (or not) pass -lc++experimental.
-// CHECK-LIBCXX: -lc++experimental
-// CHECK-LIBSTDCXX-NOT: -lc++experimental
-// CHECK-NOSTDLIB-NOT: -lc++experimental
+// Note that we don't check for `-lc++experimental` specifically, since some targets
+// like Darwin pass the path to the library explicitly instead of using `-lx`.
+// CHECK-LIBCXX: c++experimental
+// CHECK-LIBSTDCXX-NOT: c++experimental
+// CHECK-NOSTDLIB-NOT: c++experimental