[clang-scan-deps] IntegerLiteralSeparator breaks preprocessing conditionals with digit separators

[MorganCaron]

I'm using clang-format with the following IntegerLiteralSeparator settings:

IntegerLiteralSeparator:
  Binary: 4
  Decimal: 3
  Hex: 2

When formatting code like this:

#if __cpp_lib_smart_ptr_owner_equality >= 202306L

clang-format transforms it into:

#if __cpp_lib_smart_ptr_owner_equality >= 202'306L

which causes a compilation error because the preprocessor does not accept digit separators ' in numeric literals inside directives.

I’m aware that since C++20, there is an alternative to test feature macros using:

#if __has_cpp_attribute(__cpp_lib_smart_ptr_owner_equality)

However, it would still be useful to have either:

• an option to disable IntegerLiteralSeparator only inside preprocessor directives, or
• for the behavior to be changed so that IntegerLiteralSeparator is not applied at all to preprocessor lines or specific preprocessor instructions such as #if

This suggestion is intended to make formatting smoother and avoid breaking code when numeric literals appear in preprocessor conditionals

[llvmbot]

@llvm/issue-subscribers-clang-format

Author: Morgan (MorganCaron)

I'm using clang-format with the following ``IntegerLiteralSeparator`` settings:

IntegerLiteralSeparator:
  Binary: 4
  Decimal: 3
  Hex: 2

When formatting code like this:

#if __cpp_lib_smart_ptr_owner_equality >= 202306L

clang-format transforms it into:

#if __cpp_lib_smart_ptr_owner_equality >= 202'306L

which causes a compilation error because the preprocessor does not accept digit separators ' in numeric literals inside directives.

I’m aware that since C++20, there is an alternative to test feature macros using:

#if __has_cpp_attribute(__cpp_lib_smart_ptr_owner_equality)

However, it would still be useful to have either:

• an option to disable IntegerLiteralSeparator only inside preprocessor directives, or
• for the behavior to be changed so that IntegerLiteralSeparator is not applied at all to preprocessor lines or specific preprocessor instructions such as #if

This suggestion is intended to make formatting smoother and avoid breaking code when numeric literals appear in preprocessor conditionals

[owenca]

#if __cpp_lib_smart_ptr_owner_equality >= 202'306L
which causes a compilation error because the preprocessor does not accept digit separators ' in numeric literals inside directives.

Looks okay on Compiler Explorer.

[MorganCaron]

[  0%]: <CppUtils-UnitTests> generating.module.deps modules/Thread/SharedPtr.mpp
/opt/llvm-git/bin/clang-scan-deps --format=p1689 -- /opt/llvm-git/bin/clang -x c++ -c modules/Thread/SharedPtr.mpp -o build/.objs/CppUtils-UnitTests/linux/x86_64/release/modules/Thread/SharedPtr.mpp.o -Qunused-arguments -m64 -fvisibility=hidden -fvisibility-inlines-hidden -Wall -Wextra -Wpedantic -Werror -O3 -std=c++26 -stdlib=libc++ -cxx-isystem/opt/llvm-git/include/c++/v1 -Ibuild/.gens/CppUtils-UnitTests/linux/x86_64/release/platform/windows/idl -Iinclude -Ibuild/.gens/CppUtils/linux/x86_64/release/platform/windows/idl -fexperimental-library -Wconversion -Wfatal-errors -Wno-deprecated-declarations -Wno-unknown-attributes -fcolor-diagnostics -fansi-escape-codes -DNDEBUG
error: Error while scanning dependencies for modules/Thread/SharedPtr.mpp:
modules/Thread/SharedPtr.mpp:10:46: error: token is not a valid binary operator in a preprocessor subexpression

stack traceback:
        [C]: in function 'error'
        @programdir/core/base/os.lua:1079: in function 'os.raiselevel'
        (...tail calls...)
        @programdir/core/main.lua:329: in upvalue 'cotask'
        @programdir/core/base/scheduler.lua:406: in function <@programdir/core/base/scheduler.lua:399>

modules/Thread/SharedPtr.mpp :

export module CppUtils.Thread.SharedPtr;

import std;

export namespace CppUtils::Thread::SharedPtr
{
	[[nodiscard]] inline auto ownerEqual(const auto& lhs, const auto& rhs) -> bool
	{
		// C++26: std::shared_ptr<T>::owner_equal & std::weak_ptr<T>::owner_equal
#if __cpp_lib_smart_ptr_owner_equality >= 202'306L
		return lhs.owner_equal(rhs);
#else
		return not lhs.owner_before(rhs) and not rhs.owner_before(lhs);
#endif
	}
}

The project compiles successfully when I remove the '

I've tried to reproduce it on Compiler Explorer, without success so far. I'm still trying.

[MorganCaron]

Hi,

I found the root cause of the issue: the problem happens only with clang-scan-deps, not with the regular clang compiler.

Minimal example with a digit separator in a preprocessor directive:

#if 1'000 > 0
int main()
{
    return 0;
}
#else
#    error "Preprocessor failed"
#endif

When removing the digit separator ('), the same code works fine with clang-scan-deps, confirming that it's the separator causing the issue.

This shows clang-scan-deps doesn’t support digit separators in preprocessor conditionals.

I’m using clang and clang-scan-deps version 21.0.0.

This issue should be re-categorized to clang-scan-deps.