r360464 - Fix handling of objects under construction during constant expression
Richard Smith via cfe-commits
cfe-commits at lists.llvm.org
Fri May 10 13:05:33 PDT 2019
Author: rsmith
Date: Fri May 10 13:05:32 2019
New Revision: 360464
URL: http://llvm.org/viewvc/llvm-project?rev=360464&view=rev
Log:
Fix handling of objects under construction during constant expression
evaluation.
It's not enough to just track the LValueBase that we're evaluating, we
need to also track the path to the objects whose constructors are
running.
Modified:
cfe/trunk/include/clang/AST/APValue.h
cfe/trunk/include/clang/Basic/DiagnosticASTKinds.td
cfe/trunk/lib/AST/APValue.cpp
cfe/trunk/lib/AST/ExprConstant.cpp
cfe/trunk/test/SemaCXX/constant-expression-cxx1y.cpp
Modified: cfe/trunk/include/clang/AST/APValue.h
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/AST/APValue.h?rev=360464&r1=360463&r2=360464&view=diff
==============================================================================
--- cfe/trunk/include/clang/AST/APValue.h (original)
+++ cfe/trunk/include/clang/AST/APValue.h Fri May 10 13:05:32 2019
@@ -96,10 +96,14 @@ public:
return Version;
}
- bool operator==(const LValueBase &Other) const {
- return Ptr == Other.Ptr && CallIndex == Other.CallIndex &&
- Version == Other.Version;
+ friend bool operator==(const LValueBase &LHS, const LValueBase &RHS) {
+ return LHS.Ptr == RHS.Ptr && LHS.CallIndex == RHS.CallIndex &&
+ LHS.Version == RHS.Version;
}
+ friend bool operator!=(const LValueBase &LHS, const LValueBase &RHS) {
+ return !(LHS == RHS);
+ }
+ friend llvm::hash_code hash_value(const LValueBase &Base);
private:
PtrTy Ptr;
Modified: cfe/trunk/include/clang/Basic/DiagnosticASTKinds.td
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticASTKinds.td?rev=360464&r1=360463&r2=360464&view=diff
==============================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticASTKinds.td (original)
+++ cfe/trunk/include/clang/Basic/DiagnosticASTKinds.td Fri May 10 13:05:32 2019
@@ -114,6 +114,8 @@ def note_constexpr_access_volatile_obj :
"%select{read of|assignment to|increment of|decrement of}0 volatile "
"%select{temporary|object %2|member %2}1 is not allowed in "
"a constant expression">;
+def note_constexpr_volatile_here : Note<
+ "volatile %select{temporary created|object declared|member declared}0 here">;
def note_constexpr_ltor_mutable : Note<
"read of mutable member %0 is not allowed in a constant expression">;
def note_constexpr_ltor_non_const_int : Note<
Modified: cfe/trunk/lib/AST/APValue.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/AST/APValue.cpp?rev=360464&r1=360463&r2=360464&view=diff
==============================================================================
--- cfe/trunk/lib/AST/APValue.cpp (original)
+++ cfe/trunk/lib/AST/APValue.cpp Fri May 10 13:05:32 2019
@@ -58,13 +58,16 @@ llvm::DenseMapInfo<clang::APValue::LValu
DenseMapInfo<unsigned>::getTombstoneKey());
}
+namespace clang {
+llvm::hash_code hash_value(const APValue::LValueBase &Base) {
+ return llvm::hash_combine(Base.getOpaqueValue(), Base.getCallIndex(),
+ Base.getVersion());
+}
+}
+
unsigned llvm::DenseMapInfo<clang::APValue::LValueBase>::getHashValue(
const clang::APValue::LValueBase &Base) {
- llvm::FoldingSetNodeID ID;
- ID.AddPointer(Base.getOpaqueValue());
- ID.AddInteger(Base.getCallIndex());
- ID.AddInteger(Base.getVersion());
- return ID.ComputeHash();
+ return hash_value(Base);
}
bool llvm::DenseMapInfo<clang::APValue::LValueBase>::isEqual(
Modified: cfe/trunk/lib/AST/ExprConstant.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/AST/ExprConstant.cpp?rev=360464&r1=360463&r2=360464&view=diff
==============================================================================
--- cfe/trunk/lib/AST/ExprConstant.cpp (original)
+++ cfe/trunk/lib/AST/ExprConstant.cpp Fri May 10 13:05:32 2019
@@ -622,6 +622,40 @@ namespace {
}
};
+ /// A reference to an object whose construction we are currently evaluating.
+ struct ObjectUnderConstruction {
+ APValue::LValueBase Base;
+ ArrayRef<APValue::LValuePathEntry> Path;
+ friend bool operator==(const ObjectUnderConstruction &LHS,
+ const ObjectUnderConstruction &RHS) {
+ return LHS.Base == RHS.Base && LHS.Path == RHS.Path;
+ }
+ friend llvm::hash_code hash_value(const ObjectUnderConstruction &Obj) {
+ return llvm::hash_combine(Obj.Base, Obj.Path);
+ }
+ };
+ enum class ConstructionPhase { None, Bases, AfterBases };
+}
+
+namespace llvm {
+template<> struct DenseMapInfo<ObjectUnderConstruction> {
+ using Base = DenseMapInfo<APValue::LValueBase>;
+ static ObjectUnderConstruction getEmptyKey() {
+ return {Base::getEmptyKey(), {}}; }
+ static ObjectUnderConstruction getTombstoneKey() {
+ return {Base::getTombstoneKey(), {}};
+ }
+ static unsigned getHashValue(const ObjectUnderConstruction &Object) {
+ return hash_value(Object);
+ }
+ static bool isEqual(const ObjectUnderConstruction &LHS,
+ const ObjectUnderConstruction &RHS) {
+ return LHS == RHS;
+ }
+};
+}
+
+namespace {
/// EvalInfo - This is a private struct used by the evaluator to capture
/// information about a subexpression as it is folded. It retains information
/// about the AST context, but also maintains information about the folded
@@ -672,32 +706,35 @@ namespace {
/// declaration whose initializer is being evaluated, if any.
APValue *EvaluatingDeclValue;
- /// EvaluatingObject - Pair of the AST node that an lvalue represents and
- /// the call index that that lvalue was allocated in.
- typedef std::pair<APValue::LValueBase, std::pair<unsigned, unsigned>>
- EvaluatingObject;
-
- /// EvaluatingConstructors - Set of objects that are currently being
- /// constructed.
- llvm::DenseSet<EvaluatingObject> EvaluatingConstructors;
+ /// Set of objects that are currently being constructed.
+ llvm::DenseMap<ObjectUnderConstruction, ConstructionPhase>
+ ObjectsUnderConstruction;
struct EvaluatingConstructorRAII {
EvalInfo &EI;
- EvaluatingObject Object;
+ ObjectUnderConstruction Object;
bool DidInsert;
- EvaluatingConstructorRAII(EvalInfo &EI, EvaluatingObject Object)
+ EvaluatingConstructorRAII(EvalInfo &EI, ObjectUnderConstruction Object,
+ bool HasBases)
: EI(EI), Object(Object) {
- DidInsert = EI.EvaluatingConstructors.insert(Object).second;
+ DidInsert =
+ EI.ObjectsUnderConstruction
+ .insert({Object, HasBases ? ConstructionPhase::Bases
+ : ConstructionPhase::AfterBases})
+ .second;
+ }
+ void finishedConstructingBases() {
+ EI.ObjectsUnderConstruction[Object] = ConstructionPhase::AfterBases;
}
~EvaluatingConstructorRAII() {
- if (DidInsert) EI.EvaluatingConstructors.erase(Object);
+ if (DidInsert) EI.ObjectsUnderConstruction.erase(Object);
}
};
- bool isEvaluatingConstructor(APValue::LValueBase Decl, unsigned CallIndex,
- unsigned Version) {
- return EvaluatingConstructors.count(
- EvaluatingObject(Decl, {CallIndex, Version}));
+ ConstructionPhase
+ isEvaluatingConstructor(APValue::LValueBase Base,
+ ArrayRef<APValue::LValuePathEntry> Path) {
+ return ObjectsUnderConstruction.lookup({Base, Path});
}
/// If we're currently speculatively evaluating, the outermost call stack
@@ -786,7 +823,6 @@ namespace {
void setEvaluatingDecl(APValue::LValueBase Base, APValue &Value) {
EvaluatingDecl = Base;
EvaluatingDeclValue = &Value;
- EvaluatingConstructors.insert({Base, {0, 0}});
}
const LangOptions &getLangOpts() const { return Ctx.getLangOpts(); }
@@ -2788,6 +2824,8 @@ namespace {
/// A handle to a complete object (an object that is not a subobject of
/// another object).
struct CompleteObject {
+ /// The identity of the object.
+ APValue::LValueBase Base;
/// The value of the complete object.
APValue *Value;
/// The type of the complete object.
@@ -2795,9 +2833,9 @@ struct CompleteObject {
bool LifetimeStartedInEvaluation;
CompleteObject() : Value(nullptr) {}
- CompleteObject(APValue *Value, QualType Type,
+ CompleteObject(APValue::LValueBase Base, APValue *Value, QualType Type,
bool LifetimeStartedInEvaluation)
- : Value(Value), Type(Type),
+ : Base(Base), Value(Value), Type(Type),
LifetimeStartedInEvaluation(LifetimeStartedInEvaluation) {
assert(Value && "missing value for complete object");
}
@@ -2806,6 +2844,20 @@ struct CompleteObject {
};
} // end anonymous namespace
+static QualType getSubobjectType(QualType ObjType, QualType SubobjType,
+ bool IsMutable = false) {
+ // C++ [basic.type.qualifier]p1:
+ // - A const object is an object of type const T or a non-mutable subobject
+ // of a const object.
+ if (ObjType.isConstQualified() && !IsMutable)
+ SubobjType.addConst();
+ // - A volatile object is an object of type const T or a subobject of a
+ // volatile object.
+ if (ObjType.isVolatileQualified())
+ SubobjType.addVolatile();
+ return SubobjType;
+}
+
/// Find the designated sub-object of an rvalue.
template<typename SubobjectHandler>
typename SubobjectHandler::result_type
@@ -2830,16 +2882,61 @@ findSubobject(EvalInfo &Info, const Expr
const FieldDecl *LastField = nullptr;
const bool MayReadMutableMembers =
Obj.LifetimeStartedInEvaluation && Info.getLangOpts().CPlusPlus14;
+ const FieldDecl *VolatileField = nullptr;
// Walk the designator's path to find the subobject.
for (unsigned I = 0, N = Sub.Entries.size(); /**/; ++I) {
if (O->isUninit()) {
if (!Info.checkingPotentialConstantExpression())
- Info.FFDiag(E, diag::note_constexpr_access_uninit) << handler.AccessKind;
+ Info.FFDiag(E, diag::note_constexpr_access_uninit)
+ << handler.AccessKind;
return handler.failed();
}
- if (I == N) {
+ // C++ [class.ctor]p5:
+ // const and volatile semantics are not applied on an object under
+ // construction.
+ if ((ObjType.isConstQualified() || ObjType.isVolatileQualified()) &&
+ ObjType->isRecordType() &&
+ Info.isEvaluatingConstructor(
+ Obj.Base, llvm::makeArrayRef(Sub.Entries.begin(),
+ Sub.Entries.begin() + I)) !=
+ ConstructionPhase::None) {
+ ObjType = Info.Ctx.getCanonicalType(ObjType);
+ ObjType.removeLocalConst();
+ ObjType.removeLocalVolatile();
+ }
+
+ // If this is our last pass, check that the final object type is OK.
+ if (I == N || (I == N - 1 && ObjType->isAnyComplexType())) {
+ // Accesses to volatile objects are prohibited.
+ if (ObjType.isVolatileQualified()) {
+ if (Info.getLangOpts().CPlusPlus) {
+ int DiagKind;
+ SourceLocation Loc;
+ const NamedDecl *Decl = nullptr;
+ if (VolatileField) {
+ DiagKind = 2;
+ Loc = VolatileField->getLocation();
+ Decl = VolatileField;
+ } else if (auto *VD = Obj.Base.dyn_cast<const ValueDecl*>()) {
+ DiagKind = 1;
+ Loc = VD->getLocation();
+ Decl = VD;
+ } else {
+ DiagKind = 0;
+ if (auto *E = Obj.Base.dyn_cast<const Expr *>())
+ Loc = E->getExprLoc();
+ }
+ Info.FFDiag(E, diag::note_constexpr_access_volatile_obj, 1)
+ << handler.AccessKind << DiagKind << Decl;
+ Info.Note(Loc, diag::note_constexpr_volatile_here) << DiagKind;
+ } else {
+ Info.FFDiag(E, diag::note_invalid_subexpr_in_const_expr);
+ }
+ return handler.failed();
+ }
+
// If we are reading an object of class type, there may still be more
// things we need to check: if there are any mutable subobjects, we
// cannot perform this read. (This only happens when performing a trivial
@@ -2847,7 +2944,9 @@ findSubobject(EvalInfo &Info, const Expr
if (ObjType->isRecordType() && handler.AccessKind == AK_Read &&
!MayReadMutableMembers && diagnoseUnreadableFields(Info, E, ObjType))
return handler.failed();
+ }
+ if (I == N) {
if (!handler.found(*O, ObjType))
return false;
@@ -2898,10 +2997,8 @@ findSubobject(EvalInfo &Info, const Expr
return handler.failed();
}
- bool WasConstQualified = ObjType.isConstQualified();
- ObjType = ObjType->castAs<ComplexType>()->getElementType();
- if (WasConstQualified)
- ObjType.addConst();
+ ObjType = getSubobjectType(
+ ObjType, ObjType->castAs<ComplexType>()->getElementType());
assert(I == N - 1 && "extracting subobject of scalar?");
if (O->isComplexInt()) {
@@ -2938,34 +3035,17 @@ findSubobject(EvalInfo &Info, const Expr
} else
O = &O->getStructField(Field->getFieldIndex());
- bool WasConstQualified = ObjType.isConstQualified();
- ObjType = Field->getType();
- if (WasConstQualified && !Field->isMutable())
- ObjType.addConst();
-
- if (ObjType.isVolatileQualified()) {
- if (Info.getLangOpts().CPlusPlus) {
- // FIXME: Include a description of the path to the volatile subobject.
- Info.FFDiag(E, diag::note_constexpr_access_volatile_obj, 1)
- << handler.AccessKind << 2 << Field;
- Info.Note(Field->getLocation(), diag::note_declared_at);
- } else {
- Info.FFDiag(E, diag::note_invalid_subexpr_in_const_expr);
- }
- return handler.failed();
- }
-
+ ObjType = getSubobjectType(ObjType, Field->getType(), Field->isMutable());
LastField = Field;
+ if (Field->getType().isVolatileQualified())
+ VolatileField = Field;
} else {
// Next subobject is a base class.
const CXXRecordDecl *Derived = ObjType->getAsCXXRecordDecl();
const CXXRecordDecl *Base = getAsBaseClass(Sub.Entries[I]);
O = &O->getStructBase(getBaseIndex(Derived, Base));
- bool WasConstQualified = ObjType.isConstQualified();
- ObjType = Info.Ctx.getRecordType(Base);
- if (WasConstQualified)
- ObjType.addConst();
+ ObjType = getSubobjectType(ObjType, Info.Ctx.getRecordType(Base));
}
}
}
@@ -3178,18 +3258,6 @@ static CompleteObject findCompleteObject
return CompleteObject();
}
- // Accesses of volatile-qualified objects are not allowed.
- if (BaseType.isVolatileQualified()) {
- if (Info.getLangOpts().CPlusPlus) {
- Info.FFDiag(E, diag::note_constexpr_access_volatile_obj, 1)
- << AK << 1 << VD;
- Info.Note(VD->getLocation(), diag::note_declared_at);
- } else {
- Info.FFDiag(E);
- }
- return CompleteObject();
- }
-
// Unless we're looking at a local variable or argument in a constexpr call,
// the variable we're reading must be const.
if (!Frame) {
@@ -3198,6 +3266,7 @@ static CompleteObject findCompleteObject
// OK, we can read and modify an object if we're in the process of
// evaluating its initializer, because its lifetime began in this
// evaluation.
+ LifetimeStartedInEvaluation = true;
} else if (AK != AK_Read) {
// All the remaining cases only permit reading.
Info.FFDiag(E, diag::note_constexpr_modify_global);
@@ -3292,29 +3361,6 @@ static CompleteObject findCompleteObject
BaseVal = Frame->getTemporary(Base, LVal.Base.getVersion());
assert(BaseVal && "missing value for temporary");
}
-
- // Volatile temporary objects cannot be accessed in constant expressions.
- if (BaseType.isVolatileQualified()) {
- if (Info.getLangOpts().CPlusPlus) {
- Info.FFDiag(E, diag::note_constexpr_access_volatile_obj, 1)
- << AK << 0;
- Info.Note(Base->getExprLoc(), diag::note_constexpr_temporary_here);
- } else {
- Info.FFDiag(E);
- }
- return CompleteObject();
- }
- }
-
- // During the construction of an object, it is not yet 'const'.
- // FIXME: This doesn't do quite the right thing for const subobjects of the
- // object under construction.
- if (Info.isEvaluatingConstructor(LVal.getLValueBase(),
- LVal.getLValueCallIndex(),
- LVal.getLValueVersion())) {
- BaseType = Info.Ctx.getCanonicalType(BaseType);
- BaseType.removeLocalConst();
- LifetimeStartedInEvaluation = true;
}
// In C++14, we can't safely access any mutable state when we might be
@@ -3327,7 +3373,8 @@ static CompleteObject findCompleteObject
(AK != AK_Read && Depth < Info.SpeculativeEvaluationDepth))
return CompleteObject();
- return CompleteObject(BaseVal, BaseType, LifetimeStartedInEvaluation);
+ return CompleteObject(LVal.getLValueBase(), BaseVal, BaseType,
+ LifetimeStartedInEvaluation);
}
/// Perform an lvalue-to-rvalue conversion on the given glvalue. This
@@ -3361,7 +3408,7 @@ static bool handleLValueToRValueConversi
APValue Lit;
if (!Evaluate(Lit, Info, CLE->getInitializer()))
return false;
- CompleteObject LitObj(&Lit, Base->getType(), false);
+ CompleteObject LitObj(LVal.Base, &Lit, Base->getType(), false);
return extractSubobject(Info, Conv, LitObj, LVal.Designator, RVal);
} else if (isa<StringLiteral>(Base) || isa<PredefinedExpr>(Base)) {
// Special-case character extraction so we don't have to construct an
@@ -4520,8 +4567,9 @@ static bool HandleConstructorCall(const
}
EvalInfo::EvaluatingConstructorRAII EvalObj(
- Info, {This.getLValueBase(),
- {This.getLValueCallIndex(), This.getLValueVersion()}});
+ Info,
+ ObjectUnderConstruction{This.getLValueBase(), This.Designator.Entries},
+ RD->getNumBases());
CallStackFrame Frame(Info, CallLoc, Definition, &This, ArgValues);
// FIXME: Creating an APValue just to hold a nonexistent return value is
@@ -4595,6 +4643,11 @@ static bool HandleConstructorCall(const
BaseType->getAsCXXRecordDecl(), &Layout))
return false;
Value = &Result.getStructBase(BasesSeen++);
+
+ // This is the point at which the dynamic type of the object becomes this
+ // class type.
+ if (BasesSeen == RD->getNumBases())
+ EvalObj.finishedConstructingBases();
} else if ((FD = I->getMember())) {
if (!HandleLValueMember(Info, I->getInit(), Subobject, FD, &Layout))
return false;
@@ -5021,6 +5074,8 @@ public:
/// A member expression where the object is a prvalue is itself a prvalue.
bool VisitMemberExpr(const MemberExpr *E) {
+ assert(!Info.Ctx.getLangOpts().CPlusPlus11 &&
+ "missing temporary materialization conversion");
assert(!E->isArrow() && "missing call to bound member function?");
APValue Val;
@@ -5035,7 +5090,10 @@ public:
assert(BaseTy->castAs<RecordType>()->getDecl()->getCanonicalDecl() ==
FD->getParent()->getCanonicalDecl() && "record / field mismatch");
- CompleteObject Obj(&Val, BaseTy, true);
+ // Note: there is no lvalue base here. But this case should only ever
+ // happen in C or in C++98, where we cannot be evaluating a constexpr
+ // constructor, which is the only case the base matters.
+ CompleteObject Obj(APValue::LValueBase(), &Val, BaseTy, true);
SubobjectDesignator Designator(BaseTy);
Designator.addDeclUnchecked(FD);
@@ -6629,6 +6687,12 @@ bool RecordExprEvaluator::VisitInitListE
const RecordDecl *RD = E->getType()->castAs<RecordType>()->getDecl();
if (RD->isInvalidDecl()) return false;
const ASTRecordLayout &Layout = Info.Ctx.getASTRecordLayout(RD);
+ auto *CXXRD = dyn_cast<CXXRecordDecl>(RD);
+
+ EvalInfo::EvaluatingConstructorRAII EvalObj(
+ Info,
+ ObjectUnderConstruction{This.getLValueBase(), This.Designator.Entries},
+ CXXRD && CXXRD->getNumBases());
if (RD->isUnion()) {
const FieldDecl *Field = E->getInitializedFieldInUnion();
@@ -6655,7 +6719,6 @@ bool RecordExprEvaluator::VisitInitListE
return EvaluateInPlace(Result.getUnionValue(), Info, Subobject, InitExpr);
}
- auto *CXXRD = dyn_cast<CXXRecordDecl>(RD);
if (Result.isUninit())
Result = APValue(APValue::UninitStruct(), CXXRD ? CXXRD->getNumBases() : 0,
std::distance(RD->field_begin(), RD->field_end()));
@@ -6663,7 +6726,7 @@ bool RecordExprEvaluator::VisitInitListE
bool Success = true;
// Initialize base classes.
- if (CXXRD) {
+ if (CXXRD && CXXRD->getNumBases()) {
for (const auto &Base : CXXRD->bases()) {
assert(ElementNo < E->getNumInits() && "missing init for base class");
const Expr *Init = E->getInit(ElementNo);
@@ -6680,6 +6743,8 @@ bool RecordExprEvaluator::VisitInitListE
}
++ElementNo;
}
+
+ EvalObj.finishedConstructingBases();
}
// Initialize members.
Modified: cfe/trunk/test/SemaCXX/constant-expression-cxx1y.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/constant-expression-cxx1y.cpp?rev=360464&r1=360463&r2=360464&view=diff
==============================================================================
--- cfe/trunk/test/SemaCXX/constant-expression-cxx1y.cpp (original)
+++ cfe/trunk/test/SemaCXX/constant-expression-cxx1y.cpp Fri May 10 13:05:32 2019
@@ -1159,3 +1159,49 @@ enum InEnum2 : int {
enum class InEnum3 {
THREE = indirect_builtin_constant_p("abc")
};
+
+// [class.ctor]p4:
+// A constructor can be invoked for a const, volatile or const volatile
+// object. const and volatile semantics are not applied on an object under
+// construction. They come into effect when the constructor for the most
+// derived object ends.
+namespace ObjectsUnderConstruction {
+ struct A {
+ int n;
+ constexpr A() : n(1) { n = 2; }
+ };
+ struct B {
+ const A a;
+ constexpr B(bool mutate) {
+ if (mutate)
+ const_cast<A &>(a).n = 3; // expected-note {{modification of object of const-qualified type 'const int'}}
+ }
+ };
+ constexpr B b(false);
+ static_assert(b.a.n == 2, "");
+ constexpr B bad(true); // expected-error {{must be initialized by a constant expression}} expected-note {{in call to 'B(true)'}}
+
+ struct C {
+ int n;
+ constexpr C() : n(1) { n = 2; }
+ };
+ constexpr int f(bool get) {
+ volatile C c; // expected-note {{here}}
+ return get ? const_cast<int&>(c.n) : 0; // expected-note {{read of volatile object 'c'}}
+ }
+ static_assert(f(false) == 0, ""); // ok, can modify volatile c.n during c's initialization: it's not volatile then
+ static_assert(f(true) == 2, ""); // expected-error {{constant}} expected-note {{in call}}
+
+ struct Aggregate {
+ int x = 0;
+ int y = ++x;
+ };
+ constexpr Aggregate aggr1;
+ static_assert(aggr1.x == 1 && aggr1.y == 1, "");
+ // FIXME: This is not specified by the standard, but sanity requires it.
+ constexpr Aggregate aggr2 = {};
+ static_assert(aggr2.x == 1 && aggr2.y == 1, "");
+
+ // The lifetime of 'n' begins at the initialization, not before.
+ constexpr int n = ++const_cast<int&>(n); // expected-error {{constant expression}} expected-note {{modification}}
+}
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