[Clang] Implement P0848 (Conditionally Trivial Special Member Functions)

This patch implements P0848 in Clang.

During the instantiation of a C++ class, in `Sema::ActOnFields`, we evaluate constraints for all the SMFs and compare the constraints to compute the eligibility. We defer the computation of the type's [copy-]trivial bits from addedMember to the eligibility computation, like we did for destructors in D126194. `canPassInRegisters` is modified as well to better respect the ineligibility of functions.

Note: Because of the non-implementation of DR1734 and DR1496, I treat deleted member functions as 'eligible' for the purpose of [copy-]triviallity. This is unfortunate, but I couldn't think of a way to make this make sense otherwise.

Reviewed By: #clang-language-wg, cor3ntin, aaron.ballman

Differential Revision: https://reviews.llvm.org/D128619

1 files changed, 5 insertions, 1 deletions

diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index 20bfbf1..162cc26 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -673,7 +673,11 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
 
   // C++20 features.
   if (LangOpts.CPlusPlus20) {
-    //Builder.defineMacro("__cpp_aggregate_paren_init", "201902L");
+    // Builder.defineMacro("__cpp_aggregate_paren_init", "201902L");
+
+    // P0848 is implemented, but we're still waiting for other concepts
+    // issues to be addressed before bumping __cpp_concepts up to 202002L.
+    // Refer to the discussion of this at https://reviews.llvm.org/D128619.
     Builder.defineMacro("__cpp_concepts", "201907L");
     Builder.defineMacro("__cpp_conditional_explicit", "201806L");
     //Builder.defineMacro("__cpp_consteval", "201811L");