Revert "[clang] Don't spuriously pass -stdlib=libc++ to CC1 on Darwin"

This broke the instrprof-darwin-exports.c test on mac, see e.g.
https://green.lab.llvm.org/green/job/clang-stage1-RA/32351/

> Previously, we would be passing down -stdlib=libc++ from the Driver
> to CC1 whenever the default standard library on the platform was libc++,
> even if -stdlib= had not been passed to the Driver. This meant that we
> would pass -stdlib=libc++ in nonsensical circumstances, such as when
> compiling C code.
>
> This logic had been added in b534ce4 to make sure that header
> search paths were set up properly. However, since libc++ is now the
> default Standard Library on Darwin, passing this explicitly is not
> required anymore. Indeed, if no -stdlib= is specified, CC1 will end
> up using libc++ if it queries which standard library to use, without
> having to be told.
>
> Not passing -stdlib= at all to CC1 on Darwin should become possible
> once CC1 stops relying on it to set up framework search paths.
>
> Furthermore, this commit also removes a diagnostic checking whether the
> deployment target is too old to support libc++. Nowadays, all supported
> deployment targets use libc++ and compiling with libstdc++ is not
> supported anymore. The Driver was the wrong place to issue this
> diagnostic since it doesn't know whether libc++ will actually be linked
> against (e.g. C vs C++), which would lead to spurious diagnostics.
> Given that these targets are not supported anymore, we simply drop
> the diagnostic instead of trying to refactor it into CC1.
>
> rdar://103198514
>
> Differential Revision: https://reviews.llvm.org/D139938

This reverts commit 6540f32.

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -203,6 +203,8 @@ def warn_drv_missing_plugin_name : Warning<
 def warn_drv_missing_plugin_arg : Warning<
   "missing plugin argument for plugin %0 in %1">,
   InGroup<InvalidCommandLineArgument>;
+def err_drv_invalid_libcxx_deployment : Error<
+  "invalid deployment target for -stdlib=libc++ (requires %0 or later)">;
 def err_drv_invalid_argument_to_option : Error<
   "invalid argument '%0' to -%1">;
 def err_drv_missing_sanitizer_ignorelist : Error<

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -2869,6 +2869,7 @@ Darwin::TranslateArgs(const DerivedArgList &Args, StringRef BoundArch,
   // First get the generic Apple args, before moving onto Darwin-specific ones.
   DerivedArgList *DAL =
       MachO::TranslateArgs(Args, BoundArch, DeviceOffloadKind);
+  const OptTable &Opts = getDriver().getOpts();
 
   // If no architecture is bound, none of the translations here are relevant.
   if (BoundArch.empty())
@@ -2900,6 +2901,26 @@ Darwin::TranslateArgs(const DerivedArgList &Args, StringRef BoundArch,
     }
   }
 
+  if (!Args.getLastArg(options::OPT_stdlib_EQ) &&
+      GetCXXStdlibType(Args) == ToolChain::CST_Libcxx)
+    DAL->AddJoinedArg(nullptr, Opts.getOption(options::OPT_stdlib_EQ),
+                      "libc++");
+
+  // Validate the C++ standard library choice.
+  CXXStdlibType Type = GetCXXStdlibType(*DAL);
+  if (Type == ToolChain::CST_Libcxx) {
+    // Check whether the target provides libc++.
+    StringRef where;
+
+    // Complain about targeting iOS < 5.0 in any way.
+    if (isTargetIOSBased() && isIPhoneOSVersionLT(5, 0))
+      where = "iOS 5.0";
+
+    if (where != StringRef()) {
+      getDriver().Diag(clang::diag::err_drv_invalid_libcxx_deployment) << where;
+    }
+  }
+
   auto Arch = tools::darwin::getArchTypeForMachOArchName(BoundArch);
   if ((Arch == llvm::Triple::arm || Arch == llvm::Triple::thumb)) {
     if (Args.hasFlag(options::OPT_fomit_frame_pointer,

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

@@ -159,16 +159,3 @@
 // 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"
-
-// Make sure that on Darwin, we use libc++ header search paths by default even when
-// -stdlib= isn't passed.
-//
-// RUN: %clang -### %s -fsyntax-only 2>&1 \
-// RUN:     --target=x86_64-apple-darwin \
-// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
-// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
-// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
-// RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_no_libcxx \
-// RUN:               --check-prefix=CHECK-LIBCXX-STDLIB-UNSPECIFIED %s
-// CHECK-LIBCXX-STDLIB-UNSPECIFIED: "-cc1"
-// CHECK-LIBCXX-STDLIB-UNSPECIFIED: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"

clang/test/Driver/darwin-stdlib.cpp

@@ -1,29 +1,10 @@
-// Make sure that the Driver passes down -stdlib=libc++ to CC1 when specified explicitly.
-//
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch arm64  -miphoneos-version-min=7.0 -stdlib=libc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBCXX %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/              -mmacosx-version-min=10.9  -stdlib=libc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBCXX %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7s -miphoneos-version-min=7.0 -stdlib=libc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBCXX %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7k                            -stdlib=libc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBCXX %s
-// CHECK-LIBCXX: "-stdlib=libc++"
-// CHECK-LIBCXX-NOT: "-stdlib=libstdc++"
+// This test will fail if CLANG_DEFAULT_CXX_STDLIB is set to libstdc++.
+// XFAIL: default-cxx-stdlib=libstdc++
 
-// Make sure that the Driver passes down -stdlib=libstdc++ to CC1 when specified explicitly. Note that this
-// shouldn't really be used on Darwin because libstdc++ is not supported anymore, but we still pin down the
-// Driver behavior for now.
-//
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch arm64  -miphoneos-version-min=7.0 -stdlib=libstdc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBSTDCXX %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/              -mmacosx-version-min=10.9  -stdlib=libstdc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBSTDCXX %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7s -miphoneos-version-min=7.0 -stdlib=libstdc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBSTDCXX %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7k                            -stdlib=libstdc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBSTDCXX %s
-// CHECK-LIBSTDCXX: "-stdlib=libstdc++"
-// CHECK-LIBSTDCXX-NOT: "-stdlib=libc++"
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch arm64 -miphoneos-version-min=7.0 %s -### 2>&1 | FileCheck %s
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -mmacosx-version-min=10.9 %s -### 2>&1 | FileCheck %s
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7s -miphoneos-version-min=7.0 %s -### 2>&1 | FileCheck %s
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7k %s -### 2>&1 | FileCheck %s
 
-// Make sure that the Driver does not spuriously pass down any -stdlib=<...> option to CC1 if none is
-// specified on the command-line. In that case, CC1 should use the default standard library, which is
-// going to be libc++.
-//
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch arm64  -miphoneos-version-min=7.0 %s -### 2>&1 | FileCheck --check-prefix=CHECK-NONE %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/              -mmacosx-version-min=10.9  %s -### 2>&1 | FileCheck --check-prefix=CHECK-NONE %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7s -miphoneos-version-min=7.0 %s -### 2>&1 | FileCheck --check-prefix=CHECK-NONE %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7k                            %s -### 2>&1 | FileCheck --check-prefix=CHECK-NONE %s
-// CHECK-NONE-NOT: "-stdlib="
+// CHECK: "-stdlib=libc++"
+// CHECK-NOT: "-stdlib=libstdc++"