Adds a warning for aligned new in MSVC before C++17

[luken-google]

Summary

MSVC requires C++17 to support aligned new and defining the macro
__cpp_aligned_new. Clang does not, which could result in incompatibility
between clang-cl and msvc-built binaries. Issue a warning when this
use case is detected.

BUG=https://github.com/llvm/llvm-project/issues/55389

[luken-google]

Probably better to address this in the Driver. See also https://reviews.llvm.org/D127641. I'm going to close this for now and see if we want to revisit after that lands or as part of that change.

clang/include/clang/Basic/DiagnosticFrontendKinds.td

 def err_fe_unable_to_create_target : Error<
     "unable to create target: '%0'">;
 def err_fe_unable_to_interface_with_target : Error<
     "unable to interface with target machine">;
 def err_fe_unable_to_open_output : Error<
     "unable to open output file '%0': '%1'">;
 def warn_fe_macro_contains_embedded_newline : Warning<
     "macro '%0' contains embedded newline; text after the newline is ignored">;
+def warn_fe_msvc_aligned_new_before_cpp17 : Warning<
+    "possible incompatibility with MSVC, it does not support alignedNew before C++17">,
+    InGroup<MicrosoftCppMacro>;
 def warn_fe_cc_print_header_failure : Warning<
     "unable to open CC_PRINT_HEADERS file: %0 (using stderr)">;
 def warn_fe_cc_log_diagnostics_failure : Warning<
     "unable to open CC_LOG_DIAGNOSTICS file: %0 (using stderr)">;
 def warn_fe_unable_to_open_stats_file : Warning<
     "unable to open statistics output file '%0': '%1'">,
     InGroup<DiagGroup<"unable-to-open-stats-file">>;
 def err_fe_no_pch_in_dir : Error<

clang/lib/Frontend/InitPreprocessor.cpp

if (LangOpts.GPUDefaultStream ==
         LangOptions::GPUDefaultStreamKind::PerThread)
       Builder.defineMacro("HIP_API_PER_THREAD_DEFAULT_STREAM");
   }
 }
 
 /// Initialize the predefined C++ language feature test macros defined in
 /// ISO/IEC JTC1/SC22/WG21 (C++) SD-6: "SG10 Feature Test Recommendations".
 static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
-                                                 MacroBuilder &Builder) {
+                                                 MacroBuilder &Builder,
+                                                 DiagnosticsEngine &Diags) {
   // C++98 features.
   if (LangOpts.RTTI)
     Builder.defineMacro("__cpp_rtti", "199711L");
   if (LangOpts.CXXExceptions)
     Builder.defineMacro("__cpp_exceptions", "199711L");
 
   // C++11 features.
   if (LangOpts.CPlusPlus11) {
if (LangOpts.CPlusPlus17) {
     Builder.defineMacro("__cpp_aggregate_bases", "201603L");
     Builder.defineMacro("__cpp_structured_bindings", "201606L");
     Builder.defineMacro("__cpp_nontype_template_args",
                         "201411L"); // (not latest)
     Builder.defineMacro("__cpp_fold_expressions", "201603L");
     Builder.defineMacro("__cpp_guaranteed_copy_elision", "201606L");
     Builder.defineMacro("__cpp_nontype_template_parameter_auto", "201606L");
   }
-  if (LangOpts.AlignedAllocation && !LangOpts.AlignedAllocationUnavailable)
+  if (LangOpts.AlignedAllocation && !LangOpts.AlignedAllocationUnavailable) {
+    // LLVM supports __cpp_aligned_new in C++ version older than C++17, but MSVC
+    // does not. Warn about the difference in support.
+    if (LangOpts.MSVCCompat && LangOpts.CPlusPlus && !LangOpts.CPlusPlus17)
+      Diags.Report(diag::warn_fe_msvc_aligned_new_before_cpp17);
     Builder.defineMacro("__cpp_aligned_new", "201606L");
+  }
   if (LangOpts.RelaxedTemplateTemplateArgs)
     Builder.defineMacro("__cpp_template_template_args", "201611L");
 
   // C++20 features.
   if (LangOpts.CPlusPlus20) {
     //Builder.defineMacro("__cpp_aggregate_paren_init", "201902L");
     Builder.defineMacro("__cpp_concepts", "201907L");
     Builder.defineMacro("__cpp_conditional_explicit", "201806L");
#include "clang/Basic/OpenCLExtensions.def"
   // Assume compiling for FULL profile
   Builder.defineMacro("__opencl_c_int64");
 }
 
 static void InitializePredefinedMacros(const TargetInfo &TI,
                                        const LangOptions &LangOpts,
                                        const FrontendOptions &FEOpts,
                                        const PreprocessorOptions &PPOpts,
-                                       MacroBuilder &Builder) {
+                                       MacroBuilder &Builder,
+                                       DiagnosticsEngine &Diags) {
   // Compiler version introspection macros.
   Builder.defineMacro("__llvm__");  // LLVM Backend
   Builder.defineMacro("__clang__"); // Clang Frontend
 #define TOSTR2(X) #X
 #define TOSTR(X) TOSTR2(X)
   Builder.defineMacro("__clang_major__", TOSTR(CLANG_VERSION_MAJOR));
   Builder.defineMacro("__clang_minor__", TOSTR(CLANG_VERSION_MINOR));
   Builder.defineMacro("__clang_patchlevel__", TOSTR(CLANG_VERSION_PATCHLEVEL));
#undef TOSTR2
   }
 
   // Define a macro that describes the Objective-C boolean type even for C
   // and C++ since BOOL can be used from non Objective-C code.
   Builder.defineMacro("__OBJC_BOOL_IS_BOOL",
                       Twine(TI.useSignedCharForObjCBool() ? "0" : "1"));
 
   if (LangOpts.CPlusPlus)
-    InitializeCPlusPlusFeatureTestMacros(LangOpts, Builder);
+    InitializeCPlusPlusFeatureTestMacros(LangOpts, Builder, Diags);
 
   // darwin_constant_cfstrings controls this. This is also dependent
   // on other things like the runtime I believe.  This is set even for C code.
   if (!LangOpts.NoConstantCFStrings)
       Builder.defineMacro("__CONSTANT_CFSTRINGS__");
 
   if (LangOpts.ObjC)
     Builder.defineMacro("OBJC_NEW_PROPERTIES");
void clang::InitializePreprocessor(
 
   // Install things like __POWERPC__, __GNUC__, etc into the macro table.
   if (InitOpts.UsePredefines) {
     // FIXME: This will create multiple definitions for most of the predefined
     // macros. This is not the right way to handle this.
     if ((LangOpts.CUDA || LangOpts.OpenMPIsDevice || LangOpts.SYCLIsDevice) &&
         PP.getAuxTargetInfo())
       InitializePredefinedMacros(*PP.getAuxTargetInfo(), LangOpts, FEOpts,
-                                 PP.getPreprocessorOpts(), Builder);
+                                 PP.getPreprocessorOpts(), Builder, PP.getDiagnostics());
 
     InitializePredefinedMacros(PP.getTargetInfo(), LangOpts, FEOpts,
-                               PP.getPreprocessorOpts(), Builder);
+                               PP.getPreprocessorOpts(), Builder, PP.getDiagnostics());
 
     // Install definitions to make Objective-C++ ARC work well with various
     // C++ Standard Library implementations.
     if (LangOpts.ObjC && LangOpts.CPlusPlus &&
         (LangOpts.ObjCAutoRefCount || LangOpts.ObjCWeak)) {
       switch (InitOpts.ObjCXXARCStandardLibrary) {
       case ARCXX_nolib:
       case ARCXX_libcxx:

clang/test/Frontend/aligned-new-before-cpp17.cpp

+// RUN: %clang_cc1 -fms-compatibility -faligned-allocation -std=c++03 %s
+// CHECK: warning: possible incompatibility with MSVC, it does not support alignedNew before C++17
+// RUN: %clang_cc1 -fms-compatibility -faligned-allocation -std=c++11 %s
+// CHECK: warning: possible incompatibility with MSVC, it does not support alignedNew before C++17
+// RUN: %clang_cc1 -fms-compatibility -faligned-allocation -std=c++14 %s
+// CHECK: warning: possible incompatibility with MSVC, it does not support alignedNew before C++17
+// RUN: %clang_cc1 -fms-compatibility -faligned-allocation -std=c++17 %s
+// expected-no-diagnostics