[C++2a] Add __builtin_bit_cast, used to implement std::bit_cast

This commit adds a new builtin, __builtin_bit_cast(T, v), which performs a
bit_cast from a value v to a type T. This expression can be evaluated at
compile time under specific circumstances.

The compile time evaluation currently doesn't support bit-fields, but I'm
planning on fixing this in a follow up (some of the logic for figuring this out
is in CodeGen). I'm also planning follow-ups for supporting some more esoteric
types that the constexpr evaluator supports, as well as extending
__builtin_memcpy constexpr evaluation to use the same infrastructure.

rdar://44987528

Differential revision: https://reviews.llvm.org/D62825

llvm-svn: 364954

1 files changed, 1 insertions, 0 deletions

diff --git a/clang/lib/AST/ExprClassification.cpp b/clang/lib/AST/ExprClassification.cpp
index 757a17660a1..c61ee703aca 100644
--- a/clang/lib/AST/ExprClassification.cpp
+++ b/clang/lib/AST/ExprClassification.cpp
@@ -343,6 +343,7 @@ static Cl::Kinds ClassifyInternal(ASTContext &Ctx, const Expr *E) {
   case Expr::CXXReinterpretCastExprClass:
   case Expr::CXXConstCastExprClass:
   case Expr::ObjCBridgedCastExprClass:
+  case Expr::BuiltinBitCastExprClass:
     // Only in C++ can casts be interesting at all.
     if (!Lang.CPlusPlus) return Cl::CL_PRValue;
     return ClassifyUnnamed(Ctx, cast<ExplicitCastExpr>(E)->getTypeAsWritten());