[Driver] Fix detection of libc++ with empty sysroot. (#66947)

b1e3cd1 dropped the leading slash here,
presumably unintentionally.

(I don't understand Driver well enough to know how/where this is
supposed
to be tested, but it broke clangd on any project that uses the
system-installed -stdlib=libc++ on a standard debian install)

Author: sam-mccall

clang/lib/Driver/ToolChains/Gnu.cpp

@@ -3100,6 +3100,8 @@ Generic_GCC::addLibCxxIncludePaths(const llvm::opt::ArgList &DriverArgs,
                                    llvm::opt::ArgStringList &CC1Args) const {
   const Driver &D = getDriver();
   std::string SysRoot = computeSysRoot();
+  if (SysRoot.empty())
+    SysRoot = llvm::sys::path::get_separator();
 
   auto AddIncludePath = [&](StringRef Path, bool TargetDirRequired = false) {
     std::string Version = detectLibcxxVersion(Path);

clang/unittests/Driver/ToolChainTest.cpp

@@ -19,10 +19,12 @@
 #include "clang/Driver/Driver.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/StringExtras.h"
 #include "llvm/MC/TargetRegistry.h"
 #include "llvm/Support/TargetSelect.h"
 #include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/Support/raw_ostream.h"
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include <memory>
 
@@ -316,6 +318,52 @@ TEST(ToolChainTest, VFSSolarisMultiGCCInstallation) {
   }
 }
 
+MATCHER_P(jobHasArgs, Substr, "") {
+  const driver::Command &C = arg;
+  std::string Args = "";
+  llvm::ListSeparator Sep(" ");
+  for (const char *Arg : C.getArguments()) {
+    Args += Sep;
+    Args += Arg;
+  }
+  if (is_style_windows(llvm::sys::path::Style::native))
+    std::replace(Args.begin(), Args.end(), '\\', '/');
+  if (llvm::StringRef(Args).contains(Substr))
+    return true;
+  *result_listener << "whose args are '" << Args << "'";
+  return false;
+}
+
+TEST(ToolChainTest, VFSGnuLibcxxPathNoSysroot) {
+  IntrusiveRefCntPtr<DiagnosticOptions> DiagOpts = new DiagnosticOptions();
+
+  IntrusiveRefCntPtr<DiagnosticIDs> DiagID(new DiagnosticIDs());
+  struct TestDiagnosticConsumer : public DiagnosticConsumer {};
+  IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> InMemoryFileSystem(
+      new llvm::vfs::InMemoryFileSystem);
+
+  const char *EmptyFiles[] = {
+      "foo.cpp",
+      "/bin/clang",
+      "/usr/include/c++/v1/cstdio",
+  };
+
+  for (const char *Path : EmptyFiles)
+    InMemoryFileSystem->addFile(Path, 0,
+                                llvm::MemoryBuffer::getMemBuffer("\n"));
+
+  {
+    DiagnosticsEngine Diags(DiagID, &*DiagOpts, new TestDiagnosticConsumer);
+    Driver TheDriver("/bin/clang", "x86_64-unknown-linux-gnu", Diags,
+                     "clang LLVM compiler", InMemoryFileSystem);
+    std::unique_ptr<Compilation> C(TheDriver.BuildCompilation(
+        {"/bin/clang", "-fsyntax-only", "-stdlib=libc++", "foo.cpp"}));
+    ASSERT_TRUE(C);
+    EXPECT_THAT(C->getJobs(), testing::ElementsAre(jobHasArgs(
+                                  "-internal-isystem /usr/include/c++/v1")));
+  }
+}
+
 TEST(ToolChainTest, DefaultDriverMode) {
   IntrusiveRefCntPtr<DiagnosticOptions> DiagOpts = new DiagnosticOptions();