[clang] [Clang] Add __builtin_get_counted_by builtin (PR #102549)

[via cfe-commits]

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@@ -6590,8 +6590,34 @@ ExprResult Sema::BuildCallExpr(Scope *Scope, Expr *Fn, SourceLocation LParenLoc,
     return CallExpr::Create(Context, Fn, ArgExprs, Context.DependentTy,
                             VK_PRValue, RParenLoc, CurFPFeatureOverrides());
   }
-  return BuildResolvedCallExpr(Fn, NDecl, LParenLoc, ArgExprs, RParenLoc,
-                               ExecConfig, IsExecConfig);
+
+  Result = BuildResolvedCallExpr(Fn, NDecl, LParenLoc, ArgExprs, RParenLoc,
+                                 ExecConfig, IsExecConfig);
+
+  if (FunctionDecl *FDecl = dyn_cast_or_null<FunctionDecl>(NDecl);
+      FDecl && FDecl->getBuiltinID() == Builtin::BI__builtin_get_counted_by) {
+    if (const MemberExpr *ME = ArgExprs[0]->getMemberExpr()) {
+      bool IsFlexibleArrayMember = ME->isFlexibleArrayMemberLike(
+          Context, getLangOpts().getStrictFlexArraysLevel());
+
+      if (!ME->HasSideEffects(Context) && IsFlexibleArrayMember &&
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Sirraide wrote:

Oh, also, this should issue a diagnostic (probably honestly just straight-up an error) if the expression isn’t a member access of any sort. Or is there a use case where this might be applied to something that isn’t even a member expression?

https://github.com/llvm/llvm-project/pull/102549