[clang] [CIR] Upstream support for null and virtual method pointers (PR #176522)

[Sirui Mu via cfe-commits]

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@@ -202,7 +202,29 @@ mlir::TypedAttr LowerItaniumCXXABI::lowerMethodConstant(
         loweredMethodTy, mlir::ArrayAttr::get(attr.getContext(), {zero, zero}));
   }
 
-  assert(!cir::MissingFeatures::virtualMethodAttr());
+  if (attr.isVirtual()) {
+    if (useARMMethodPtrABI) {
+      // ARM C++ ABI 3.2.1:
+      //   This ABI specifies that adj contains twice the this
+      //   adjustment, plus 1 if the member function is virtual. The
+      //   least significant bit of adj then makes exactly the same
+      //   discrimination as the least significant bit of ptr does for
+      //   Itanium.
+      llvm_unreachable("ARM method ptr abi NYI");
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Lancern wrote:

Change this to a MissingFeature check.

https://github.com/llvm/llvm-project/pull/176522