diff --git a/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp b/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
--- a/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
+++ b/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
@@ -18,7 +18,9 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/FileUtilities.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/Program.h"
 #include <string>
 #include <tuple>
 #include <vector>
@@ -27,6 +29,113 @@
 namespace clangd {
 namespace {
 
+// Query apple's `xcrun` launcher, which is the source of truth for "how should"
+// clang be invoked on this system.
+llvm::Optional<std::string> queryXcrun(llvm::ArrayRef<llvm::StringRef> Argv) {
+  auto Xcrun = llvm::sys::findProgramByName("xcrun");
+  if (!Xcrun) {
+    log("Couldn't find xcrun. Hopefully you have a non-apple toolchain...");
+    return llvm::None;
+  }
+  llvm::SmallString<64> OutFile;
+  llvm::sys::fs::createTemporaryFile("clangd-xcrun", "", OutFile);
+  llvm::FileRemover OutRemover(OutFile);
+  llvm::Optional<llvm::StringRef> Redirects[3] = {
+      /*stdin=*/{""}, /*stdout=*/{OutFile}, /*stderr=*/{""}};
+  vlog("Invoking {0} to find clang installation", *Xcrun);
+  int Ret = llvm::sys::ExecuteAndWait(*Xcrun, Argv,
+                                      /*Env=*/llvm::None, Redirects,
+                                      /*SecondsToWait=*/10);
+  if (Ret != 0) {
+    log("xcrun exists but failed with code {0}. "
+        "If you have a non-apple toolchain, this is OK. "
+        "Otherwise, try xcode-select --install.",
+        Ret);
+    return llvm::None;
+  }
+
+  auto Buf = llvm::MemoryBuffer::getFile(OutFile);
+  if (!Buf) {
+    log("Can't read xcrun output: {0}", Buf.getError().message());
+    return llvm::None;
+  }
+  StringRef Path = Buf->get()->getBuffer().trim();
+  if (Path.empty()) {
+    log("xcrun produced no output");
+    return llvm::None;
+  }
+  return Path.str();
+}
+
+// On Mac, `which clang` is /usr/bin/clang. It runs `xcrun clang`, which knows
+// where the real clang is kept. We need to do the same thing,
+// because cc1 (not the driver!) will find libc++ relative to argv[0].
+llvm::Optional<std::string> queryMacClangPath() {
+#ifndef __APPLE__
+  return llvm::None;
+#endif
+
+  return queryXcrun({"xcrun", "--find", "clang"});
+}
+
+// Resolve symlinks if possible.
+std::string resolve(std::string Path) {
+  llvm::SmallString<128> Resolved;
+  if (llvm::sys::fs::real_path(Path, Resolved))
+    return Path; // On error;
+  return Resolved.str();
+}
+
+// Get a plausible full `clang` path.
+// This is used in the fallback compile command, or when the CDB returns a
+// generic driver with no path.
+llvm::StringRef getFallbackClangPath() {
+  static const std::string &MemoizedFallbackPath = [&]() -> std::string {
+    // The driver and/or cc1 sometimes depend on the binary name to compute
+    // useful things like the standard library location.
+    // We need to emulate what clang on this system is likely to see.
+    // cc1 in particular looks at the "real path" of the running process, and
+    // so if /usr/bin/clang is a symlink, it sees the resolved path.
+    // clangd doesn't have that luxury, so we resolve symlinks ourselves.
+
+    // /usr/bin/clang on a mac is a program that redirects to the right clang.
+    // We resolve it as if it were a symlink.
+    if (auto MacClang = queryMacClangPath())
+      return resolve(std::move(*MacClang));
+    // On other platforms, just look for compilers on the PATH.
+    for (const char* Name : {"clang", "gcc", "cc"})
+      if (auto PathCC = llvm::sys::findProgramByName(Name))
+        return resolve(std::move(*PathCC));
+    // Fallback: a nonexistent 'clang' binary next to clangd.
+    static int Dummy;
+    std::string ClangdExecutable =
+        llvm::sys::fs::getMainExecutable("clangd", (void *)&Dummy);
+    SmallString<128> ClangPath;
+    ClangPath = llvm::sys::path::parent_path(ClangdExecutable);
+    llvm::sys::path::append(ClangPath, "clang");
+    return ClangPath.str();
+  }();
+  return MemoizedFallbackPath;
+}
+
+// On mac, /usr/bin/clang sets SDKROOT and then invokes the real clang.
+// The effect of this is to set -isysroot correctly. We do the same.
+const std::string *getMacSysroot() {
+#ifndef __APPLE__
+  return nullptr;
+#endif
+
+  // SDKROOT overridden in environment, respect it. Driver will set isysroot.
+  if (::getenv("SDKROOT"))
+    return nullptr;
+  static const llvm::Optional<std::string> &Sysroot =
+      queryXcrun({"xcrun", "--show-sdk-path"});
+  return Sysroot ? Sysroot.getPointer() : nullptr;
+}
+
+// Transform a command into the form we want to send to the driver.
+// The command was originally either from the CDB or is the fallback command,
+// and may have been modified by OverlayCDB.
 void adjustArguments(tooling::CompileCommand &Cmd,
                      llvm::StringRef ResourceDir) {
   tooling::ArgumentsAdjuster ArgsAdjuster = tooling::combineAdjusters(
@@ -40,10 +149,35 @@
                                 tooling::getClangSyntaxOnlyAdjuster()));
 
   Cmd.CommandLine = ArgsAdjuster(Cmd.CommandLine, Cmd.Filename);
+  // Check whether the flag exists, either as -flag or -flag=*
+  auto Has = [&](llvm::StringRef Flag) {
+    for (llvm::StringRef Arg : Cmd.CommandLine) {
+      if (Arg.consume_front(Flag) && (Arg.empty() || Arg[0] == '='))
+        return true;
+    }
+    return false;
+  };
   // Inject the resource dir.
-  // FIXME: Don't overwrite it if it's already there.
-  if (!ResourceDir.empty())
+  if (!ResourceDir.empty() && !Has("-resource-dir"))
     Cmd.CommandLine.push_back(("-resource-dir=" + ResourceDir).str());
+  if (!Has("-isysroot"))
+    if (const std::string *MacSysroot = getMacSysroot()) {
+      Cmd.CommandLine.push_back("-isysroot");
+      Cmd.CommandLine.push_back(*MacSysroot);
+    }
+
+  // If the driver is a generic name like "g++" with no path, add a clang path.
+  // This makes it easier for us to find the standard libraries on mac.
+  if (!Cmd.CommandLine.empty()) {
+    std::string &Driver = Cmd.CommandLine.front();
+    if (Driver == "clang" || Driver == "clang++" || Driver == "gcc" ||
+        Driver == "g++" || Driver == "cc" || Driver == "c++") {
+      llvm::SmallString<128> QualifiedDriver =
+          llvm::sys::path::parent_path(getFallbackClangPath());
+      llvm::sys::path::append(QualifiedDriver, Driver);
+      Driver = QualifiedDriver.str();
+    }
+  }
 }
 
 std::string getStandardResourceDir() {
@@ -63,19 +197,9 @@
 
 } // namespace
 
-static std::string getFallbackClangPath() {
-  static int Dummy;
-  std::string ClangdExecutable =
-      llvm::sys::fs::getMainExecutable("clangd", (void *)&Dummy);
-  SmallString<128> ClangPath;
-  ClangPath = llvm::sys::path::parent_path(ClangdExecutable);
-  llvm::sys::path::append(ClangPath, "clang");
-  return ClangPath.str();
-}
-
 tooling::CompileCommand
 GlobalCompilationDatabase::getFallbackCommand(PathRef File) const {
-  std::vector<std::string> Argv = {getFallbackClangPath()};
+  std::vector<std::string> Argv = {"clang"};
   // Clang treats .h files as C by default and files without extension as linker
   // input, resulting in unhelpful diagnostics.
   // Parsing as Objective C++ is friendly to more cases.
diff --git a/clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp b/clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp
--- a/clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp
+++ b/clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp
@@ -542,7 +542,7 @@
   FS.Files[testPath("A.h")] = "";
   Cmd.Filename = "../A.cc";
   Cmd.Directory = testPath("build");
-  Cmd.CommandLine = {"clang++", "../A.cc", "-fsyntax-only"};
+  Cmd.CommandLine = {"/bin/clang++", "../A.cc", "-fsyntax-only"};
   CDB.setCompileCommand(testPath("build/../A.cc"), Cmd);
   ASSERT_TRUE(Idx.blockUntilIdleForTest());
 
@@ -558,7 +558,7 @@
 
   // FIXME: Changing compile commands should be enough to invalidate the cache.
   FS.Files[testPath("A.cc")] = " ";
-  Cmd.CommandLine = {"clang++", "../A.cc", "-Dfoo", "-fsyntax-only"};
+  Cmd.CommandLine = {"/bin/clang++", "../A.cc", "-Dfoo", "-fsyntax-only"};
   CDB.setCompileCommand(testPath("build/../A.cc"), Cmd);
   ASSERT_TRUE(Idx.blockUntilIdleForTest());
 
diff --git a/clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp b/clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp
--- a/clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp
+++ b/clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp
@@ -105,8 +105,9 @@
 TEST_F(OverlayCDBTest, GetFallbackCommand) {
   OverlayCDB CDB(Base.get(), {"-DA=4"});
   EXPECT_THAT(CDB.getFallbackCommand(testPath("bar.cc")).CommandLine,
-              ElementsAre("clang", "-DA=2", testPath("bar.cc"), "-DA=4",
-                          "-fsyntax-only", StartsWith("-resource-dir")));
+              ElementsAre(EndsWith("clang"), "-DA=2", testPath("bar.cc"),
+                          "-DA=4", "-fsyntax-only",
+                          StartsWith("-resource-dir")));
 }
 
 TEST_F(OverlayCDBTest, NoBase) {