Driver: Don't look for libc++ headers in the install directory on Android.

The NDK uses a separate set of libc++ headers in the sysroot. Any headers
in the installation directory are not going to work on Android, not least
because they use a different name for the inline namespace (std::__1 instead
of std::__ndk1).

This effectively makes it impossible to produce a single toolchain that is
capable of targeting both Android and another platform that expects libc++
headers to be installed in the installation directory, such as Mac.

In order to allow this scenario to work, stop looking for headers in the
install directory on Android.

Differential Revision: https://reviews.llvm.org/D71154

Author: pcc

clang/lib/Driver/ToolChains/Linux.cpp

@@ -888,20 +888,25 @@ static std::string DetectLibcxxIncludePath(llvm::vfs::FileSystem &vfs,
 void Linux::addLibCxxIncludePaths(const llvm::opt::ArgList &DriverArgs,
                                   llvm::opt::ArgStringList &CC1Args) const {
   const std::string& SysRoot = computeSysRoot();
-  const std::string LibCXXIncludePathCandidates[] = {
-      DetectLibcxxIncludePath(getVFS(), getDriver().Dir + "/../include/c++"),
-      // If this is a development, non-installed, clang, libcxx will
-      // not be found at ../include/c++ but it likely to be found at
-      // one of the following two locations:
-      DetectLibcxxIncludePath(getVFS(), SysRoot + "/usr/local/include/c++"),
-      DetectLibcxxIncludePath(getVFS(), SysRoot + "/usr/include/c++") };
-  for (const auto &IncludePath : LibCXXIncludePathCandidates) {
+  auto AddIncludePath = [&](std::string Path) {
+    std::string IncludePath = DetectLibcxxIncludePath(getVFS(), Path);
     if (IncludePath.empty() || !getVFS().exists(IncludePath))
-      continue;
-    // Use the first candidate that exists.
+      return false;
     addSystemInclude(DriverArgs, CC1Args, IncludePath);
+    return true;
+  };
+  // Android never uses the libc++ headers installed alongside the toolchain,
+  // which are generally incompatible with the NDK libraries anyway.
+  if (!getTriple().isAndroid())
+    if (AddIncludePath(getDriver().Dir + "/../include/c++"))
+      return;
+  // If this is a development, non-installed, clang, libcxx will
+  // not be found at ../include/c++ but it likely to be found at
+  // one of the following two locations:
+  if (AddIncludePath(SysRoot + "/usr/local/include/c++"))
+    return;
+  if (AddIncludePath(SysRoot + "/usr/include/c++"))
     return;
-  }
 }
 
 void Linux::addLibStdCxxIncludePaths(const llvm::opt::ArgList &DriverArgs,

clang/test/Driver/android-no-installed-libcxx.cpp

@@ -0,0 +1,8 @@
+// Check that we don't find the libc++ in the installation directory when
+// targeting Android.
+
+// RUN: mkdir -p %t/bin
+// RUN: mkdir -p %t/include/c++/v1
+// RUN: %clang -target aarch64-linux-android -ccc-install-dir %t/bin \
+// RUN:   -stdlib=libc++ -fsyntax-only %s -### 2>&1 | FileCheck %s
+// CHECK-NOT: "-internal-isystem" "{{.*}}v1"

clang/test/Driver/stdlibxx-isystem.cpp

@@ -6,7 +6,7 @@
 // By default, we should search for libc++ next to the driver.
 // RUN: mkdir -p %t/bin
 // RUN: mkdir -p %t/include/c++/v1
-// RUN: %clang -target aarch64-linux-android -ccc-install-dir %t/bin \
+// RUN: %clang -target aarch64-linux-gnu -ccc-install-dir %t/bin \
 // RUN:   -stdlib=libc++ -fsyntax-only %s -### 2>&1 | \
 // RUN:   FileCheck -check-prefix=LIBCXX %s
 // RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %t/bin \
@@ -16,7 +16,7 @@
 // LIBCXX: "-internal-isystem" "[[INSTALLDIR]]/../include/c++/v1"
 
 // Passing -stdlib++-isystem should suppress the default search.
-// RUN: %clang -target aarch64-linux-android -ccc-install-dir %t/bin \
+// RUN: %clang -target aarch64-linux-gnu -ccc-install-dir %t/bin \
 // RUN:   -stdlib++-isystem /tmp/foo -stdlib++-isystem /tmp/bar -stdlib=libc++ \
 // RUN:   -fsyntax-only %s -### 2>&1 | FileCheck -check-prefix=NODEFAULT %s
 // RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %t/bin \
@@ -26,7 +26,7 @@
 // NODEFAULT-NOT: "-internal-isystem" "[[INSTALLDIR]]/../include/c++/v1"
 
 // And we should add it as an -internal-isystem.
-// RUN: %clang -target aarch64-linux-android -ccc-install-dir %t/bin \
+// RUN: %clang -target aarch64-linux-gnu -ccc-install-dir %t/bin \
 // RUN:   -stdlib++-isystem /tmp/foo -stdlib++-isystem /tmp/bar -stdlib=libc++ \
 // RUN:   -fsyntax-only %s -### 2>&1 | FileCheck -check-prefix=INCPATH %s
 // RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %t/bin \
@@ -35,7 +35,7 @@
 // INCPATH: "-internal-isystem" "/tmp/foo" "-internal-isystem" "/tmp/bar"
 
 // We shouldn't pass the -stdlib++-isystem to cc1.
-// RUN: %clang -target aarch64-linux-android -ccc-install-dir %t/bin \
+// RUN: %clang -target aarch64-linux-gnu -ccc-install-dir %t/bin \
 // RUN:   -stdlib++-isystem /tmp -stdlib=libc++ -fsyntax-only %s -### 2>&1 | \
 // RUN:   FileCheck -check-prefix=NOCC1 %s
 // RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %t/bin \
@@ -44,7 +44,7 @@
 // NOCC1-NOT: "-stdlib++-isystem" "/tmp"
 
 // It should respect -nostdinc++.
-// RUN: %clang -target aarch64-linux-android -ccc-install-dir %t/bin \
+// RUN: %clang -target aarch64-linux-gnu -ccc-install-dir %t/bin \
 // RUN:   -stdlib++-isystem /tmp/foo -stdlib++-isystem /tmp/bar -nostdinc++ \
 // RUN:   -fsyntax-only %s -### 2>&1 | FileCheck -check-prefix=NOSTDINCXX %s
 // RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %t/bin \