[Clang][Sema] Add __builtin_infer_alloc_token() declaration and semantic checks

clang/include/clang/Basic/Builtins.td

@@ -4727,6 +4727,13 @@ def PtrauthStringDiscriminator : Builtin {
   let Prototype = "size_t(char const*)";
 }
 
+// AllocToken builtins.
+def InferAllocToken : Builtin {
+  let Spellings = ["__builtin_infer_alloc_token"];
+  let Attributes = [NoThrow, Const, Pure, CustomTypeChecking, Constexpr, UnevaluatedArguments];
+  let Prototype = "size_t(...)";
+}
+
 // OpenCL v2.0 s6.13.16, s9.17.3.5 - Pipe functions.
 // We need the generic prototype, since the packet type could be anything.
 def ReadPipe : OCLPipeLangBuiltin {

clang/lib/Sema/SemaChecking.cpp

@@ -1498,6 +1498,24 @@ static void builtinAllocaAddrSpace(Sema &S, CallExpr *TheCall) {
   TheCall->setType(S.Context.getPointerType(RT));
 }
 
+static bool checkBuiltinInferAllocToken(Sema &S, CallExpr *TheCall) {
+  if (S.checkArgCountAtLeast(TheCall, 1))
+    return true;
+
+  for (Expr *Arg : TheCall->arguments()) {
+    // If argument is dependent on a template parameter, we can't resolve now.
+    if (Arg->isTypeDependent() || Arg->isValueDependent())
+      continue;
+    // Reject void types.
+    QualType ArgTy = Arg->IgnoreParenImpCasts()->getType();
+    if (ArgTy->isVoidType())
+      return S.Diag(Arg->getBeginLoc(), diag::err_param_with_void_type);
+  }
+
+  TheCall->setType(S.Context.UnsignedLongLongTy);
+  return false;
+}
+
 namespace {
 enum PointerAuthOpKind {
   PAO_Strip,
@@ -2779,6 +2797,10 @@ Sema::CheckBuiltinFunctionCall(FunctionDecl *FDecl, unsigned BuiltinID,
       builtinAllocaAddrSpace(*this, TheCall);
     }
     break;
+  case Builtin::BI__builtin_infer_alloc_token:
+    if (checkBuiltinInferAllocToken(*this, TheCall))
+      return ExprError();
+    break;
   case Builtin::BI__arithmetic_fence:
     if (BuiltinArithmeticFence(TheCall))
       return ExprError();

clang/test/SemaCXX/alloc-token.cpp

@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++23 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++23 -fsyntax-only -verify %s -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++23 -fsyntax-only -verify %s -falloc-token-mode=typehash -DMODE_TYPEHASH
+
+#if !__has_builtin(__builtin_infer_alloc_token)
+#error "missing __builtin_infer_alloc_token"
+#endif
+
+template <typename T = void>
+void template_test() {
+  __builtin_infer_alloc_token(T()); // no error if not instantiated
+}
+
+template <typename T>
+void negative_template_test() {
+  __builtin_infer_alloc_token(T()); // expected-error {{argument may not have 'void' type}}
+}
+
+void negative_tests() {
+  __builtin_infer_alloc_token(); // expected-error {{too few arguments to function call}}
+  __builtin_infer_alloc_token((void)0); // expected-error {{argument may not have 'void' type}}
+  negative_template_test<void>(); // expected-note {{in instantiation of function template specialization 'negative_template_test<void>' requested here}}
+}