[clang] [clang][PAC] Add __builtin_get_vtable_pointer (PR #139790)

[Oliver Hunt via cfe-commits]

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@@ -1825,6 +1825,37 @@ static ExprResult PointerAuthStringDiscriminator(Sema &S, CallExpr *Call) {
   return Call;
 }
 
+static ExprResult GetVTablePointer(Sema &S, CallExpr *Call) {
+  if (S.checkArgCount(Call, 1))
+    return ExprError();
+  ExprResult ThisArg = S.DefaultFunctionArrayLvalueConversion(Call->getArg(0));
+  if (ThisArg.isInvalid())
+    return ExprError();
+  Call->setArg(0, ThisArg.get());
+  const Expr *Subject = Call->getArg(0);
+  QualType SubjectType = Subject->getType();
+  const CXXRecordDecl *SubjectRecord = SubjectType->getPointeeCXXRecordDecl();
+  if (!SubjectType->isPointerType() || !SubjectRecord) {
+    S.Diag(Subject->getBeginLoc(), diag::err_get_vtable_pointer_incorrect_type)
+        << 0 << SubjectType;
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ojhunt wrote:

or we could make every diag encode the type of the diag as a function type, and have things fail at compile time when possible. This would be super great as it would provide a great stress test for clangs template code :D :D :D 

https://github.com/llvm/llvm-project/pull/139790