[clang] Match -isysroot behaviour with system compiler on Darwin

See discussion in https://reviews.llvm.org/D89001

I updated test to match actual behavior of
/Applications/Xcode_14.3.1_14E300b.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang
after that modified upstream to match the test.

Differential Revision: https://reviews.llvm.org/D157283

Author: Dmitry Polukhin

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -2465,24 +2465,34 @@ void DarwinClang::AddClangCXXStdlibIncludeArgs(
   //        Also check whether this is used for setting library search paths.
   ToolChain::AddClangCXXStdlibIncludeArgs(DriverArgs, CC1Args);
 
-  if (DriverArgs.hasArg(options::OPT_nostdinc, options::OPT_nostdlibinc,
-                        options::OPT_nostdincxx))
+  if (DriverArgs.hasArg(options::OPT_nostdlibinc, options::OPT_nostdincxx))
     return;
 
   llvm::SmallString<128> Sysroot = GetEffectiveSysroot(DriverArgs);
 
   switch (GetCXXStdlibType(DriverArgs)) {
   case ToolChain::CST_Libcxx: {
     // On Darwin, libc++ can be installed in one of the following two places:
-    // 1. Alongside the compiler in         <install>/include/c++/v1
-    // 2. In a SDK (or a custom sysroot) in <sysroot>/usr/include/c++/v1
+    // 1. In a SDK (or a custom sysroot) in <sysroot>/usr/include/c++/v1
+    // 2. Alongside the compiler in         <install>/include/c++/v1
     //
     // The precendence of paths is as listed above, i.e. we take the first path
     // that exists. Also note that we never include libc++ twice -- we take the
     // first path that exists and don't send the other paths to CC1 (otherwise
     // include_next could break).
 
     // Check for (1)
+    llvm::SmallString<128> SysrootUsr = Sysroot;
+    llvm::sys::path::append(SysrootUsr, "usr", "include", "c++", "v1");
+    if (getVFS().exists(SysrootUsr)) {
+      addSystemInclude(DriverArgs, CC1Args, SysrootUsr);
+      return;
+    } else if (DriverArgs.hasArg(options::OPT_v)) {
+      llvm::errs() << "ignoring nonexistent directory \"" << SysrootUsr
+                   << "\"\n";
+    }
+
+    // Otherwise, check for (2)
     // Get from '<install>/bin' to '<install>/include/c++/v1'.
     // Note that InstallBin can be relative, so we use '..' instead of
     // parent_path.
@@ -2497,17 +2507,6 @@ void DarwinClang::AddClangCXXStdlibIncludeArgs(
                    << "\"\n";
     }
 
-    // Otherwise, check for (2)
-    llvm::SmallString<128> SysrootUsr = Sysroot;
-    llvm::sys::path::append(SysrootUsr, "usr", "include", "c++", "v1");
-    if (getVFS().exists(SysrootUsr)) {
-      addSystemInclude(DriverArgs, CC1Args, SysrootUsr);
-      return;
-    } else if (DriverArgs.hasArg(options::OPT_v)) {
-      llvm::errs() << "ignoring nonexistent directory \"" << SysrootUsr
-                   << "\"\n";
-    }
-
     // Otherwise, don't add any path.
     break;
   }

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

@@ -53,7 +53,7 @@
 // CHECK-LIBCXX-SYSROOT-1-NOT: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 
 // Check with both headers in the sysroot and headers alongside the installation
-// (the headers in the toolchain should be preferred over the <sysroot> headers).
+// (the <sysroot> headers should be preferred over the headers in the toolchain).
 // Ensure that both -isysroot and --sysroot work, and that isysroot has precedence
 // over --sysroot.
 //
@@ -89,10 +89,10 @@
 // RUN:               --check-prefix=CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1 %s
 //
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "-cc1"
-// 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"
+// CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
+// CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1-NOT: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 
-// Make sure that using -nostdinc, -nostdinc++ or -nostdlib will drop both the toolchain
+// Make sure that using -nostdinc++ or -nostdlib will drop both the toolchain
 // C++ include path and the sysroot one.
 //
 // RUN: %clang -### %s -fsyntax-only 2>&1 \
@@ -104,9 +104,9 @@
 // RUN:     -nostdinc \
 // RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
 // RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
-// RUN:               --check-prefix=CHECK-LIBCXX-NOSTDLIBINC %s
+// RUN:               --check-prefix=CHECK-LIBCXX-NOSTDINC %s
 // CHECK-LIBCXX-NOSTDINC: "-cc1"
-// CHECK-LIBCXX-NOSTDINC-NOT: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
+// CHECK-LIBCXX-NOSTDINC: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 // CHECK-LIBCXX-NOSTDINC-NOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
 //
 // RUN: %clang -### %s -fsyntax-only 2>&1 \
@@ -157,8 +157,8 @@
 // RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_no_libcxx \
 // RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_no_libcxx \
 // RUN:               --check-prefix=CHECK-LIBCXX-MISSING-BOTH %s
-// CHECK-LIBCXX-MISSING-BOTH: ignoring nonexistent directory "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 // CHECK-LIBCXX-MISSING-BOTH: ignoring nonexistent directory "[[SYSROOT]]/usr/include/c++/v1"
+// CHECK-LIBCXX-MISSING-BOTH: ignoring nonexistent directory "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 
 // Make sure that on Darwin, we use libc++ header search paths by default even when
 // -stdlib= isn't passed.