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[clang] Add support for consteval null terminated strings #173140

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13 changes: 10 additions & 3 deletions clang/include/clang/Basic/DiagnosticSemaKinds.td
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Original file line number Diff line number Diff line change
Expand Up @@ -1746,15 +1746,22 @@ def subst_user_defined_msg : TextSubstitution<
"%select{the message|the expression}0 in "
"%select{a static assertion|this asm operand}0">;

def err_user_defined_msg_invalid : Error<
"%sub{subst_user_defined_msg}0 must be a string literal or an "
"object with 'data()' and 'size()' member functions">;
def err_user_defined_msg_invalid
: Error<"%sub{subst_user_defined_msg}0 must be a null terminated constant "
"string or an "
"object with 'data()' and 'size()' member functions">;
def err_user_defined_msg_missing_member_function : Error<
"the %select{message|string}0 object in "
"%select{this static assertion|this asm operand}0 is missing %select{"
"a 'size()' member function|"
"a 'data()' member function|"
"'data()' and 'size()' member functions}1">;
def err_user_defined_msg_not_null_terminated_string
: Error<"%sub{subst_user_defined_msg}0 is not null terminated">;
def ext_consteval_string_constants
: Extension<"consteval string constants are an extension">,
DefaultWarn,
InGroup<DiagGroup<"consteval-string-constants-extension">>;
def err_user_defined_msg_invalid_mem_fn_ret_ty : Error<
"%sub{subst_user_defined_msg}0 must have a '%select{size|data}1()' member "
"function returning an object convertible to '%select{std::size_t|const char *}1'">;
Expand Down
9 changes: 5 additions & 4 deletions clang/include/clang/Sema/Sema.h
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Expand Up @@ -831,10 +831,11 @@ enum class CCEKind {
ArrayBound, ///< Array bound in array declarator or new-expression.
ExplicitBool, ///< Condition in an explicit(bool) specifier.
Noexcept, ///< Condition in a noexcept(bool) specifier.
StaticAssertMessageSize, ///< Call to size() in a static assert
///< message.
StaticAssertMessageData, ///< Call to data() in a static assert
///< message.
StaticAssertMessageSize, ///< Call to size() in a static assert
///< message.
StaticAssertMessageData, ///< Call to data() in a static assert
///< message.
StaticAssertNullTerminatedString, ///< tryEvaluateStrLen
};

/// Enums for the diagnostics of target, target_version and target_clones.
Expand Down
8 changes: 5 additions & 3 deletions clang/lib/AST/ByteCode/Context.cpp
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Expand Up @@ -294,13 +294,15 @@ bool Context::evaluateStrlen(State &Parent, const Expr *E, uint64_t &Result) {
if (!FieldDesc->isPrimitiveArray())
return false;

if (Ptr.isDummy() || Ptr.isUnknownSizeArray())
if (Ptr.isDummy() || Ptr.isUnknownSizeArray() || Ptr.isPastEnd())
return false;

unsigned N = Ptr.getNumElems();
if (Ptr.elemSize() == 1) {
Result = strnlen(reinterpret_cast<const char *>(Ptr.getRawAddress()), N);
return Result != N;
unsigned Size = N - Ptr.getIndex();
Comment on lines -297 to +302
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@ojhunt ojhunt Dec 20, 2025

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It is not possible to hit this in the new interpreter prior to this change, the existing code fails to handle out of bounds pointer into a string literal (or presumably any array), or an in bounds offset.

It feels like it should be in a different PR, but because it can't currently be hit it can't be tested?

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@ojhunt ojhunt Dec 21, 2025

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@tbaederr maybe has ideas on testing it on it's own?

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@tbaederr tbaederr Dec 21, 2025

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Adding it to this PR is fine IMO.

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@tbaederr tbaederr Dec 21, 2025

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Actually, this might fix #173175, which got filed just 10 minutes ago.

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@ojhunt ojhunt Dec 21, 2025

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@tbaederr ah ha - I was trying to find a string function that would actually pass that expression to this code but I couldn't work out the correct way to make the call, I think I was trying to get the crash via the first expression, but huzzah we have a test case - will pull this bit out in a moment

Result =
strnlen(reinterpret_cast<const char *>(Ptr.getRawAddress()), Size);
return Result != Size;
}

PrimType ElemT = FieldDesc->getPrimType();
Expand Down
42 changes: 42 additions & 0 deletions clang/lib/Sema/SemaDeclCXX.cpp
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Expand Up @@ -17693,6 +17693,44 @@ void Sema::DiagnoseStaticAssertDetails(const Expr *E) {
}
}

template <typename ResultType>
static bool EvaluateAsNullTerminatedCharBuffer(
Sema &SemaRef, Expr *Message, ResultType &Result, ASTContext &Ctx,
Sema::StringEvaluationContext EvalContext, bool ErrorOnInvalidMessage) {
SourceLocation Loc = Message->getBeginLoc();
QualType SizeT = Ctx.getSizeType();
QualType ConstCharPtr = Ctx.getPointerType(Ctx.getConstType(Ctx.CharTy));
Expr::EvalResult Status;
SmallVector<PartialDiagnosticAt, 8> Notes;
Status.Diag = &Notes;

auto DiagnoseInvalidConstantString = [&]() {
SemaRef.Diag(Loc, diag::err_user_defined_msg_not_null_terminated_string)
<< EvalContext;
for (const auto &Note : Notes)
SemaRef.Diag(Note.first, Note.second);
return !ErrorOnInvalidMessage;
};
ExprResult EvaluatedData = SemaRef.BuildConvertedConstantExpression(
Message, ConstCharPtr, CCEKind::StaticAssertNullTerminatedString);
if (EvaluatedData.isInvalid())
return DiagnoseInvalidConstantString();

uint64_t Length = 0;
if (!EvaluatedData.get()->tryEvaluateStrLen(Length, Ctx))
return DiagnoseInvalidConstantString();

llvm::APInt SizeVal(Ctx.getIntWidth(SizeT), Length);
Expr *SizeExpr = IntegerLiteral::Create(Ctx, SizeVal, SizeT, Loc);

bool EvalResult = Message->EvaluateCharRangeAsString(
Result, SizeExpr, EvaluatedData.get(), Ctx, Status);
if (!EvalResult || !Notes.empty())
return DiagnoseInvalidConstantString();
SemaRef.Diag(Loc, diag::ext_consteval_string_constants);
return true;
Comment on lines +17730 to +17731
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@cor3ntin cor3ntin Dec 20, 2025

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Oh, you put that here! I would put it in the caller

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@ojhunt ojhunt Dec 21, 2025

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if we make evaluating a null terminated expression a flag or mode then we can remove this condition - maybe I should then just make the format string or similar routines use the "evaluate null terminated expressions" flag? it seems like such a change would be a minor change (he says with unfounded optimism)

}

template <typename ResultType>
static bool EvaluateAsStringImpl(Sema &SemaRef, Expr *Message,
ResultType &Result, ASTContext &Ctx,
Expand Down Expand Up @@ -17726,6 +17764,10 @@ static bool EvaluateAsStringImpl(Sema &SemaRef, Expr *Message,

SourceLocation Loc = Message->getBeginLoc();
QualType T = Message->getType().getNonReferenceType();
if (T->isPointerType() && T->getPointeeType()->isCharType())
return EvaluateAsNullTerminatedCharBuffer(
SemaRef, Message, Result, Ctx, EvalContext, ErrorOnInvalidMessage);

Comment on lines +17767 to +17770
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@cor3ntin cor3ntin Dec 20, 2025

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The problem of doing that here is that it is a non-standard behavior.
So at the minimum we'd need extension warnings. But I'm a bit uncomfortable adding that extension to begin with.

I think the functionality is best left to format string / ptrauth for now.

Which brings the question... how do we test this?

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@ojhunt ojhunt Dec 20, 2025

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A gross option would be to have a langopt + "enable this" cli flag, but that would imply people be able to rely on being able to do this.

auto *RD = T->getAsCXXRecordDecl();
if (!RD) {
SemaRef.Diag(Loc, diag::err_user_defined_msg_invalid) << EvalContext;
Expand Down
29 changes: 28 additions & 1 deletion clang/test/Parser/asm.cpp
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Expand Up @@ -35,6 +35,9 @@ struct string_view {
int foo1 asm ((string_view("test"))); // expected-error {{expected string literal in 'asm'}}
int func() asm ((string_view("test"))); // expected-error {{expected string literal in 'asm'}}

constexpr const char* getConstantString(const char* s) {
return s;
}

void f2() {
asm(string_view("")); // expected-error {{expected string literal or parenthesized constant expression in 'asm'}}
Expand All @@ -44,6 +47,13 @@ void f2() {
asm("" :: string_view("")); // expected-error {{expected string literal or parenthesized constant expression in 'asm'}}
asm(::string_view("")); // expected-error {{expected string literal or parenthesized constant expression in 'asm'}}

asm(getConstantString("")); // expected-error {{expected string literal or parenthesized constant expression in 'asm'}}
asm("" : getConstantString("")); // expected-error {{expected string literal or parenthesized constant expression in 'asm'}}
asm("" : : getConstantString("")); // expected-error {{expected string literal or parenthesized constant expression in 'asm'}}
asm("" : : : getConstantString("")); // expected-error {{expected ')'}}
asm("" :: getConstantString("")); // expected-error {{expected string literal or parenthesized constant expression in 'asm'}}
asm(::getConstantString("")); // expected-error {{expected string literal or parenthesized constant expression in 'asm'}}

int i;

asm((string_view("")));
Expand All @@ -55,5 +65,22 @@ void f2() {
asm("" : (::string_view("+g")) (i) : (::string_view("g")) (0) : (string_view("memory")));


asm((0)); // expected-error {{the expression in this asm operand must be a string literal or an object with 'data()' and 'size()' member functions}}
asm((getConstantString("")));
// expected-warning@-1 {{consteval string constants are an extension}}
asm((::getConstantString("")));
// expected-warning@-1 {{consteval string constants are an extension}}
asm("" : (::getConstantString("+g")) (i));
// expected-warning@-1 {{consteval string constants are an extension}}
asm("" : (::getConstantString("+g"))); // expected-error {{expected '(' after 'asm operand'}}
// expected-warning@-1 {{consteval string constants are an extension}}
asm("" : (::getConstantString("+g")) (i) : (::getConstantString("g")) (0));
// expected-warning@-1 2 {{consteval string constants are an extension}}
asm("" : (::getConstantString("+g")) (i) : (::getConstantString("g"))); // expected-error {{expected '(' after 'asm operand'}}
// expected-warning@-1 2 {{consteval string constants are an extension}}
asm("" : (::getConstantString("+g")) (i) : (::getConstantString("g")) (0) : (getConstantString("memory")));
// expected-warning@-1 3 {{consteval string constants are an extension}}



asm((0)); // expected-error {{the expression in this asm operand must be a null terminated constant string or an object with 'data()' and 'size()' member functions}}
}
4 changes: 2 additions & 2 deletions clang/test/SemaCXX/gnu-asm-constexpr.cpp
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Expand Up @@ -77,7 +77,7 @@ struct string_view {


void f() {
asm(("")); // expected-error {{the expression in this asm operand must be a string literal or an object with 'data()' and 'size()' member functions}}
asm(("")); // expected-error {{the expression in this asm operand must be a null terminated constant string or an object with 'data()' and 'size()' member functions}}
asm((NotAString{})); // expected-error {{the string object in this asm operand is missing 'data()' and 'size()' member functions}}
asm((MessageInvalidData{})); // expected-error {{the expression in this asm operand must have a 'data()' member function returning an object convertible to 'const char *'}} \
// expected-error {{too few arguments to function call, expected 1, have 0}}
Expand Down Expand Up @@ -106,7 +106,7 @@ void test_dependent1(int i) {

template void test_dependent1<int>(int);
// expected-note@-1 {{in instantiation of function template specialization}}
// expected-error@#err-int {{the expression in this asm operand must be a string literal or an object with 'data()' and 'size()' member functions}}
// expected-error@#err-int {{the expression in this asm operand must be a null terminated constant string or an object with 'data()' and 'size()' member functions}}
// expected-error@#err-int2 {{cannot initialize a value of type 'int' with an lvalue of type 'const char[3]'}}
// expected-error@#err-int3 {{cannot initialize a value of type 'int' with an lvalue of type 'const char[2]'}}
// expected-error@#err-int4 {{cannot initialize a value of type 'int' with an lvalue of type 'const char[7]'}}
Expand Down
51 changes: 49 additions & 2 deletions clang/test/SemaCXX/static-assert-cxx26.cpp
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Expand Up @@ -2,7 +2,7 @@
// RUN: %clang_cc1 -std=c++2c -triple=x86_64-linux -fsyntax-only %s -verify -fexperimental-new-constant-interpreter

static_assert(true, "");
static_assert(true, 0); // expected-error {{the message in a static assertion must be a string literal or an object with 'data()' and 'size()' member functions}}
static_assert(true, 0); // expected-error {{the message in a static assertion must be a null terminated constant string or an object with 'data()' and 'size()' member functions}}
struct Empty{};
static_assert(true, Empty{}); // expected-error {{the message object in this static assertion is missing 'data()' and 'size()' member functions}}
struct NoData {
Expand Down Expand Up @@ -288,7 +288,7 @@ struct Good {

template <typename Ty>
struct Bad {
static_assert(false, Ty{}); // expected-error {{the message in a static assertion must be a string literal or an object with 'data()' and 'size()' member functions}} \
static_assert(false, Ty{}); // expected-error {{the message in a static assertion must be a null terminated constant string or an object with 'data()' and 'size()' member functions}} \
// expected-error {{static assertion failed}}
};

Expand Down Expand Up @@ -416,3 +416,50 @@ static_assert(
// expected-note@-1 {{read of dereferenced one-past-the-end pointer is not allowed in a constant expression}}
);
}

static_assert(false, &(" basic test"[1]));
// expected-error@-1 {{static assertion failed: basic test}}
// expected-warning@-2 {{consteval string constants are an extension}}

constexpr const char *constexpr_global = "global_constexpr";
constexpr const char null_terminated_buffer[] = { 'n', 'u', 'l', 'l', 't', 'e', 'r', 'm', 0 };
constexpr const char no_null_buffer[] = { 'n', 'o', 'n', 'u', 'l', 'l', 't', 'e', 'r', 'm' };

constexpr const char *selector(int i) {
constexpr const char * a_constant = "a_constant";
const char *non_constexpr = "non-constexpr string";
switch (i) {
case 0: return "case 0";
case 1: return a_constant;
case 2: return constexpr_global;
case 3: return null_terminated_buffer;
case 4: return &(""[1]); // point to after the null terminator
case 5: return nullptr;
case 6: return no_null_buffer;
}
};

static_assert(false, selector(0));
// expected-error@-1 {{static assertion failed: case 0}}
// expected-warning@-2 {{consteval string constants are an extension}}
static_assert(false, selector(1));
// expected-error@-1 {{static assertion failed: a_constant}}
// expected-warning@-2 {{consteval string constants are an extension}}
static_assert(false, selector(2));
// expected-error@-1 {{static assertion failed: global_constexpr}}
// expected-warning@-2 {{consteval string constants are an extension}}
static_assert(false, selector(3));
// expected-error@-1 {{static assertion failed: nullterm}}
// expected-warning@-2 {{consteval string constants are an extension}}
static_assert(false, selector(4));
// expected-error@-1 {{the message in a static assertion is not null terminated}}
// expected-error@-2 {{static assertion failed}}
static_assert(false, selector(5));
// expected-error@-1 {{the message in a static assertion is not null terminated}}
// expected-error@-2 {{static assertion failed}}
static_assert(false, selector(6));
// expected-error@-1 {{the message in a static assertion is not null terminated}}
// expected-error@-2 {{static assertion failed}}
static_assert(false, selector(7));
// expected-error@-1 {{the message in a static assertion is not null terminated}}
// expected-error@-2 {{static assertion failed}}