C++/ObjC++: switch to gnu++17 as the default standard

Clang's default C++ standard is now `gnu++17` instead of `gnu++14`:
https://discourse.llvm.org/t/c-objc-switch-to-gnu-17-as-the-default-dialect/64360

* CUDA/HIP are unchanged: C++14 from D103221.
* Sony PS4/PS5 are unchanged: https://discourse.llvm.org/t/c-objc-switch-to-gnu-17-as-the-default-dialect/64360/6
* lit feature `default-std-cxx` is added to keep CLANG_DEFAULT_STD_CXX=xxx tests working.
  Whether the cmake variable should be retained is disccused in D133375.

Depends on D131464

Close llvm#56946

Reviewed By: #clang-language-wg, aaron.ballman

Differential Revision: https://reviews.llvm.org/D131465

Author: MaskRay

clang/docs/ReleaseNotes.rst

@@ -200,6 +200,10 @@ C++ Language Changes in Clang
 
 - Implemented DR692, DR1395 and DR1432. Use the ``-fclang-abi-compat=15`` option
   to get the old partial ordering behavior regarding packs.
+- Clang's default C++/ObjC++ standard is now ``gnu++17`` instead of ``gnu++14``.
+  This means Clang will by default accept code using features from C++17 and
+  conforming GNU extensions. Projects incompatible with C++17 can add
+  ``-std=gnu++14`` to their build settings to restore the previous behaviour.
 
 C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^

clang/lib/Basic/LangStandards.cpp

@@ -75,10 +75,9 @@ LangStandard::Kind clang::getDefaultLanguageStandard(clang::Language Lang,
     if (CLANG_DEFAULT_STD_CXX != LangStandard::lang_unspecified)
       return CLANG_DEFAULT_STD_CXX;
 
-    if (T.isDriverKit())
-      return LangStandard::lang_gnucxx17;
-    else
+    if (T.isPS())
       return LangStandard::lang_gnucxx14;
+    return LangStandard::lang_gnucxx17;
   case Language::RenderScript:
     return LangStandard::lang_c99;
   case Language::HIP:

clang/test/Preprocessor/lang-std.cpp

@@ -0,0 +1,14 @@
+// UNSUPPORTED: default-std-cxx, ps4, ps5
+/// Test default standards when CLANG_DEFAULT_STD_CXX is unspecified.
+/// PS4/PS5 default to gnu++14.
+
+// RUN: %clang_cc1 -dM -E %s | FileCheck --check-prefix=CXX17 %s
+// RUN: %clang_cc1 -dM -E -x cuda %s | FileCheck --check-prefix=CXX14 %s
+// RUN: %clang_cc1 -dM -E -x hip %s | FileCheck --check-prefix=CXX14 %s
+
+// RUN: %clang_cc1 -dM -E -x cuda -std=c++14 %s | FileCheck --check-prefix=CXX14 %s
+// RUN: %clang_cc1 -dM -E -x hip -std=c++98 %s | FileCheck --check-prefix=CXX98 %s
+
+// CXX98: #define __cplusplus 199711L
+// CXX14: #define __cplusplus 201402L
+// CXX17: #define __cplusplus 201703L

clang/test/Preprocessor/lang-std.cu

@@ -131,6 +131,9 @@ def have_host_jit_feature_support(feature_name):
 if config.clang_enable_opaque_pointers:
     config.available_features.add('enable-opaque-pointers')
 
+if config.clang_default_std_cxx != '':
+    config.available_features.add('default-std-cxx')
+
 # Set available features we allow tests to conditionalize on.
 #
 if config.clang_default_cxx_stdlib != '':

clang/test/lit.cfg.py

@@ -24,6 +24,7 @@ config.have_zlib = @LLVM_ENABLE_ZLIB@
 config.clang_arcmt = @CLANG_ENABLE_ARCMT@
 config.clang_default_pie_on_linux = @CLANG_DEFAULT_PIE_ON_LINUX@
 config.clang_enable_opaque_pointers = @CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL@
+config.clang_default_std_cxx = "@CLANG_DEFAULT_STD_CXX@"
 config.clang_default_cxx_stdlib = "@CLANG_DEFAULT_CXX_STDLIB@"
 config.clang_staticanalyzer = @CLANG_ENABLE_STATIC_ANALYZER@
 config.clang_staticanalyzer_z3 = @LLVM_WITH_Z3@