[Clang] Add __builtin_is_within_lifetime to implement P2641R4's std::is_within_lifetime (#91895)

[P2641R4](https://wg21.link/P2641R4)

This new builtin function is declared `consteval`. Support for
`-fexperimental-new-constant-interpreter` will be added in a later
patch.

---------

Co-authored-by: cor3ntin <[email protected]>

Author: MitalAshok

clang/docs/ReleaseNotes.rst

@@ -122,6 +122,9 @@ C++2c Feature Support
 
 - Implemented `P2747R2 constexpr placement new <https://wg21.link/P2747R2>`_.
 
+- Added the ``__builtin_is_within_lifetime`` builtin, which supports
+  `P2641R4 Checking if a union alternative is active <https://wg21.link/p2641r4>`_
+
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 - Removed the restriction to literal types in constexpr functions in C++23 mode.

clang/include/clang/Basic/Builtins.td

@@ -934,6 +934,12 @@ def IsConstantEvaluated : LangBuiltin<"CXX_LANG"> {
   let Prototype = "bool()";
 }
 
+def IsWithinLifetime : LangBuiltin<"CXX_LANG"> {
+  let Spellings = ["__builtin_is_within_lifetime"];
+  let Attributes = [NoThrow, CustomTypeChecking, Consteval];
+  let Prototype = "bool(void*)";
+}
+
 // GCC exception builtins
 def EHReturn : Builtin {
   let Spellings = ["__builtin_eh_return"];

clang/include/clang/Basic/DiagnosticASTKinds.td

@@ -167,14 +167,14 @@ def note_constexpr_this : Note<
 def access_kind : TextSubstitution<
   "%select{read of|read of|assignment to|increment of|decrement of|"
   "member call on|dynamic_cast of|typeid applied to|construction of|"
-  "destruction of}0">;
+  "destruction of|read of}0">;
 def access_kind_subobject : TextSubstitution<
   "%select{read of|read of|assignment to|increment of|decrement of|"
   "member call on|dynamic_cast of|typeid applied to|"
-  "construction of subobject of|destruction of}0">;
+  "construction of subobject of|destruction of|read of}0">;
 def access_kind_volatile : TextSubstitution<
   "%select{read of|read of|assignment to|increment of|decrement of|"
-  "<ERROR>|<ERROR>|<ERROR>|<ERROR>|<ERROR>}0">;
+  "<ERROR>|<ERROR>|<ERROR>|<ERROR>|<ERROR>|<ERROR>}0">;
 def note_constexpr_lifetime_ended : Note<
   "%sub{access_kind}0 %select{temporary|variable}1 whose "
   "%plural{8:storage duration|:lifetime}0 has ended">;
@@ -407,6 +407,12 @@ def warn_is_constant_evaluated_always_true_constexpr : Warning<
   "'%0' will always evaluate to 'true' in a manifestly constant-evaluated expression">,
   InGroup<DiagGroup<"constant-evaluated">>;
 
+def err_invalid_is_within_lifetime : Note<
+  "'%0' cannot be called with "
+  "%select{a null pointer|a one-past-the-end pointer|"
+  "a pointer to an object whose lifetime has not yet begun}1"
+>;
+
 // inline asm related.
 let CategoryName = "Inline Assembly Issue" in {
   def err_asm_invalid_escape : Error<

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -12186,6 +12186,10 @@ def err_builtin_launder_invalid_arg : Error<
   "%select{non-pointer|function pointer|void pointer}0 argument to "
   "'__builtin_launder' is not allowed">;
 
+def err_builtin_is_within_lifetime_invalid_arg : Error<
+  "%select{non-|function }0pointer argument to '__builtin_is_within_lifetime' "
+  "is not allowed">;
+
 def err_builtin_invalid_arg_type: Error <
   "%ordinal0 argument must be "
   "%select{a vector, integer or floating point type|a matrix|"

clang/lib/AST/ByteCode/State.h

@@ -34,6 +34,7 @@ enum AccessKinds {
   AK_TypeId,
   AK_Construct,
   AK_Destroy,
+  AK_IsWithinLifetime,
 };
 
 /// The order of this enum is important for diagnostics.

clang/lib/AST/ExprConstant.cpp

@@ -1522,7 +1522,8 @@ CallStackFrame::~CallStackFrame() {
 }
 
 static bool isRead(AccessKinds AK) {
-  return AK == AK_Read || AK == AK_ReadObjectRepresentation;
+  return AK == AK_Read || AK == AK_ReadObjectRepresentation ||
+         AK == AK_IsWithinLifetime;
 }
 
 static bool isModification(AccessKinds AK) {
@@ -1532,6 +1533,7 @@ static bool isModification(AccessKinds AK) {
   case AK_MemberCall:
   case AK_DynamicCast:
   case AK_TypeId:
+  case AK_IsWithinLifetime:
     return false;
   case AK_Assign:
   case AK_Increment:
@@ -1549,7 +1551,8 @@ static bool isAnyAccess(AccessKinds AK) {
 
 /// Is this an access per the C++ definition?
 static bool isFormalAccess(AccessKinds AK) {
-  return isAnyAccess(AK) && AK != AK_Construct && AK != AK_Destroy;
+  return isAnyAccess(AK) && AK != AK_Construct && AK != AK_Destroy &&
+         AK != AK_IsWithinLifetime;
 }
 
 /// Is this kind of axcess valid on an indeterminate object value?
@@ -1561,6 +1564,7 @@ static bool isValidIndeterminateAccess(AccessKinds AK) {
     // These need the object's value.
     return false;
 
+  case AK_IsWithinLifetime:
   case AK_ReadObjectRepresentation:
   case AK_Assign:
   case AK_Construct:
@@ -3707,7 +3711,8 @@ struct CompleteObject {
     // In C++14 onwards, it is permitted to read a mutable member whose
     // lifetime began within the evaluation.
     // FIXME: Should we also allow this in C++11?
-    if (!Info.getLangOpts().CPlusPlus14)
+    if (!Info.getLangOpts().CPlusPlus14 &&
+        AK != AccessKinds::AK_IsWithinLifetime)
       return false;
     return lifetimeStartedInEvaluation(Info, Base, /*MutableSubobject*/true);
   }
@@ -3760,6 +3765,12 @@ findSubobject(EvalInfo &Info, const Expr *E, const CompleteObject &Obj,
     if ((O->isAbsent() && !(handler.AccessKind == AK_Construct && I == N)) ||
         (O->isIndeterminate() &&
          !isValidIndeterminateAccess(handler.AccessKind))) {
+      // Object has ended lifetime.
+      // If I is non-zero, some subobject (member or array element) of a
+      // complete object has ended its lifetime, so this is valid for
+      // IsWithinLifetime, resulting in false.
+      if (I != 0 && handler.AccessKind == AK_IsWithinLifetime)
+        return false;
       if (!Info.checkingPotentialConstantExpression())
         Info.FFDiag(E, diag::note_constexpr_access_uninit)
             << handler.AccessKind << O->isIndeterminate()
@@ -3927,6 +3938,9 @@ findSubobject(EvalInfo &Info, const Expr *E, const CompleteObject &Obj,
             // Placement new onto an inactive union member makes it active.
             O->setUnion(Field, APValue());
           } else {
+            // Pointer to/into inactive union member: Not within lifetime
+            if (handler.AccessKind == AK_IsWithinLifetime)
+              return false;
             // FIXME: If O->getUnionValue() is absent, report that there's no
             // active union member rather than reporting the prior active union
             // member. We'll need to fix nullptr_t to not use APValue() as its
@@ -11684,6 +11698,9 @@ class IntExprEvaluator
 
   bool ZeroInitialization(const Expr *E) { return Success(0, E); }
 
+  friend std::optional<bool> EvaluateBuiltinIsWithinLifetime(IntExprEvaluator &,
+                                                             const CallExpr *);
+
   //===--------------------------------------------------------------------===//
   //                            Visitor Methods
   //===--------------------------------------------------------------------===//
@@ -12743,6 +12760,11 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
     return Success(Info.InConstantContext, E);
   }
 
+  case Builtin::BI__builtin_is_within_lifetime:
+    if (auto result = EvaluateBuiltinIsWithinLifetime(*this, E))
+      return Success(*result, E);
+    return false;
+
   case Builtin::BI__builtin_ctz:
   case Builtin::BI__builtin_ctzl:
   case Builtin::BI__builtin_ctzll:
@@ -17322,3 +17344,84 @@ bool Expr::tryEvaluateStrLen(uint64_t &Result, ASTContext &Ctx) const {
   EvalInfo Info(Ctx, Status, EvalInfo::EM_ConstantFold);
   return EvaluateBuiltinStrLen(this, Result, Info);
 }
+
+namespace {
+struct IsWithinLifetimeHandler {
+  EvalInfo &Info;
+  static constexpr AccessKinds AccessKind = AccessKinds::AK_IsWithinLifetime;
+  using result_type = std::optional<bool>;
+  std::optional<bool> failed() { return std::nullopt; }
+  template <typename T>
+  std::optional<bool> found(T &Subobj, QualType SubobjType) {
+    return true;
+  }
+};
+
+std::optional<bool> EvaluateBuiltinIsWithinLifetime(IntExprEvaluator &IEE,
+                                                    const CallExpr *E) {
+  EvalInfo &Info = IEE.Info;
+  // Sometimes this is called during some sorts of constant folding / early
+  // evaluation. These are meant for non-constant expressions and are not
+  // necessary since this consteval builtin will never be evaluated at runtime.
+  // Just fail to evaluate when not in a constant context.
+  if (!Info.InConstantContext)
+    return std::nullopt;
+  assert(E->getBuiltinCallee() == Builtin::BI__builtin_is_within_lifetime);
+  const Expr *Arg = E->getArg(0);
+  if (Arg->isValueDependent())
+    return std::nullopt;
+  LValue Val;
+  if (!EvaluatePointer(Arg, Val, Info))
+    return std::nullopt;
+
+  auto Error = [&](int Diag) {
+    bool CalledFromStd = false;
+    const auto *Callee = Info.CurrentCall->getCallee();
+    if (Callee && Callee->isInStdNamespace()) {
+      const IdentifierInfo *Identifier = Callee->getIdentifier();
+      CalledFromStd = Identifier && Identifier->isStr("is_within_lifetime");
+    }
+    Info.CCEDiag(CalledFromStd ? Info.CurrentCall->getCallRange().getBegin()
+                               : E->getExprLoc(),
+                 diag::err_invalid_is_within_lifetime)
+        << (CalledFromStd ? "std::is_within_lifetime"
+                          : "__builtin_is_within_lifetime")
+        << Diag;
+    return std::nullopt;
+  };
+  // C++2c [meta.const.eval]p4:
+  //   During the evaluation of an expression E as a core constant expression, a
+  //   call to this function is ill-formed unless p points to an object that is
+  //   usable in constant expressions or whose complete object's lifetime began
+  //   within E.
+
+  // Make sure it points to an object
+  // nullptr does not point to an object
+  if (Val.isNullPointer() || Val.getLValueBase().isNull())
+    return Error(0);
+  QualType T = Val.getLValueBase().getType();
+  assert(!T->isFunctionType() &&
+         "Pointers to functions should have been typed as function pointers "
+         "which would have been rejected earlier");
+  assert(T->isObjectType());
+  // Hypothetical array element is not an object
+  if (Val.getLValueDesignator().isOnePastTheEnd())
+    return Error(1);
+  assert(Val.getLValueDesignator().isValidSubobject() &&
+         "Unchecked case for valid subobject");
+  // All other ill-formed values should have failed EvaluatePointer, so the
+  // object should be a pointer to an object that is usable in a constant
+  // expression or whose complete lifetime began within the expression
+  CompleteObject CO =
+      findCompleteObject(Info, E, AccessKinds::AK_IsWithinLifetime, Val, T);
+  // The lifetime hasn't begun yet if we are still evaluating the
+  // initializer ([basic.life]p(1.2))
+  if (Info.EvaluatingDeclValue && CO.Value == Info.EvaluatingDeclValue)
+    return Error(2);
+
+  if (!CO)
+    return false;
+  IsWithinLifetimeHandler handler{Info};
+  return findSubobject(Info, E, CO, Val.getLValueDesignator(), handler);
+}
+} // namespace

clang/lib/CodeGen/CGBuiltin.cpp

@@ -2538,6 +2538,9 @@ static RValue EmitHipStdParUnsupportedBuiltin(CodeGenFunction *CGF,
 RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
                                         const CallExpr *E,
                                         ReturnValueSlot ReturnValue) {
+  assert(!getContext().BuiltinInfo.isImmediate(BuiltinID) &&
+         "Should not codegen for consteval builtins");
+
   const FunctionDecl *FD = GD.getDecl()->getAsFunction();
   // See if we can constant fold this builtin.  If so, don't emit it at all.
   // TODO: Extend this handling to all builtin calls that we can constant-fold.

clang/lib/Sema/SemaChecking.cpp

@@ -1844,6 +1844,44 @@ static ExprResult BuiltinLaunder(Sema &S, CallExpr *TheCall) {
   return TheCall;
 }
 
+static ExprResult BuiltinIsWithinLifetime(Sema &S, CallExpr *TheCall) {
+  if (S.checkArgCount(TheCall, 1))
+    return ExprError();
+
+  ExprResult Arg = S.DefaultFunctionArrayLvalueConversion(TheCall->getArg(0));
+  if (Arg.isInvalid())
+    return ExprError();
+  QualType ParamTy = Arg.get()->getType();
+  TheCall->setArg(0, Arg.get());
+  TheCall->setType(S.Context.BoolTy);
+
+  // Only accept pointers to objects as arguments, which should have object
+  // pointer or void pointer types.
+  if (const auto *PT = ParamTy->getAs<PointerType>()) {
+    // LWG4138: Function pointer types not allowed
+    if (PT->getPointeeType()->isFunctionType()) {
+      S.Diag(TheCall->getArg(0)->getExprLoc(),
+             diag::err_builtin_is_within_lifetime_invalid_arg)
+          << 1;
+      return ExprError();
+    }
+    // Disallow VLAs too since those shouldn't be able to
+    // be a template parameter for `std::is_within_lifetime`
+    if (PT->getPointeeType()->isVariableArrayType()) {
+      S.Diag(TheCall->getArg(0)->getExprLoc(), diag::err_vla_unsupported)
+          << 1 << "__builtin_is_within_lifetime";
+      return ExprError();
+    }
+  } else {
+    S.Diag(TheCall->getArg(0)->getExprLoc(),
+           diag::err_builtin_is_within_lifetime_invalid_arg)
+        << 0;
+    return ExprError();
+  }
+
+  return TheCall;
+}
+
 // Emit an error and return true if the current object format type is in the
 // list of unsupported types.
 static bool CheckBuiltinTargetNotInUnsupported(
@@ -2276,6 +2314,8 @@ Sema::CheckBuiltinFunctionCall(FunctionDecl *FDecl, unsigned BuiltinID,
   }
   case Builtin::BI__builtin_launder:
     return BuiltinLaunder(*this, TheCall);
+  case Builtin::BI__builtin_is_within_lifetime:
+    return BuiltinIsWithinLifetime(*this, TheCall);
   case Builtin::BI__sync_fetch_and_add:
   case Builtin::BI__sync_fetch_and_add_1:
   case Builtin::BI__sync_fetch_and_add_2:

clang/lib/Sema/SemaExpr.cpp

@@ -17622,7 +17622,8 @@ HandleImmediateInvocations(Sema &SemaRef,
         (SemaRef.inTemplateInstantiation() && !ImmediateEscalating)) {
       SemaRef.Diag(DR->getBeginLoc(), diag::err_invalid_consteval_take_address)
           << ND << isa<CXXRecordDecl>(ND) << FD->isConsteval();
-      SemaRef.Diag(ND->getLocation(), diag::note_declared_at);
+      if (!FD->getBuiltinID())
+        SemaRef.Diag(ND->getLocation(), diag::note_declared_at);
       if (auto Context =
               SemaRef.InnermostDeclarationWithDelayedImmediateInvocations()) {
         SemaRef.Diag(Context->Loc, diag::note_invalid_consteval_initializer)