[CodeGen] Correctly model std::byte's aliasing properties

std::byte, when defined as an enum, needs to be given special treatment
with regards to its aliasing properties. An array of std::byte is
allowed to be used as storage for other types.

This fixes PR33916.

Differential Revision: https://reviews.llvm.org/D35824

llvm-svn: 309058

1 files changed, 6 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenTBAA.cpp b/clang/lib/CodeGen/CodeGenTBAA.cpp
index 04224e72679..8a75a552d9f 100644
--- a/clang/lib/CodeGen/CodeGenTBAA.cpp
+++ b/clang/lib/CodeGen/CodeGenTBAA.cpp
@@ -139,6 +139,12 @@ CodeGenTBAA::getTBAAInfo(QualType QTy) {
     }
   }
 
+  // C++1z [basic.lval]p10: "If a program attempts to access the stored value of
+  // an object through a glvalue of other than one of the following types the
+  // behavior is undefined: [...] a char, unsigned char, or std::byte type."
+  if (Ty->isStdByteType())
+    return MetadataCache[Ty] = getChar();
+
   // Handle pointers.
   // TODO: Implement C++'s type "similarity" and consider dis-"similar"
   // pointers distinct.