False positives in modernize-use-default-member-init in clang-tidy 21 when constant is in cpp file and class in a header

[arvid-norlander]

Another false positive (but distinct from #156295):

The release notes for clang-tidy 21 reads:

Improved modernize-use-default-member-init check by matching arithmetic operations, constexpr and static values, and detecting explicit casting of built-in types within member list initialization.

If you have:

test.h:

#include <cstdint>

class Example1
{
   explicit Example1(int32_t param);

   int32_t member;
};

test.cpp:

#include "test.h"

namespace {
constexpr double CONSTANT = 2.0;
}

Example1::Example1()
: member(CONSTANT)
{
}

clang-tidy-21 will incorrectly report:

/home/tmgarn/workspace/reproducer/test.h:7:12: error: use default member initializer for 'member' [modernize-use-default-member-init,-warnings-as-errors]
    7 |    int32_t member;
      |            ^     
      |                  {CONSTANT}
1 warning treated as error

If you use -fix, it will incorrectly apply this change, but since the constant is defined in the cpp file, the code will not build. Many times you would not want to move the constant to the header, so this should at least be a configurable part of the check.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Arvid Norlander (arvid-norlander)

Another false positive (but distinct from #156295):

The release notes for clang-tidy 21 reads:
> Improved modernize-use-default-member-init check by matching arithmetic operations, constexpr and static values, and detecting explicit casting of built-in types within member list initialization.

If you have:

test.h:

#include <cstdint>

class Example1
{
   explicit Example1(int32_t param);

   int32_t member;
};

test.cpp:

#include "test.h"

namespace {
constexpr double CONSTANT = 2.0;
}

Example1::Example1()
: member(CONSTANT)
{
}

clang-tidy-21 will incorrectly report:

/home/tmgarn/workspace/reproducer/test.h:7:12: error: use default member initializer for 'member' [modernize-use-default-member-init,-warnings-as-errors]
    7 |    int32_t member;
      |            ^     
      |                  {CONSTANT}
1 warning treated as error

If you use -fix, it will incorrectly apply this change, but since the constant is defined in the cpp file, the code will not build.

[arvid-norlander]

cc @RiverDave (since you were CCed on the other related issue, and absolutely nothing is happening over here).

[RiverDave]

cc @RiverDave (since you were CCed on the other related issue, and absolutely nothing is happening over here).

Hi, Unfortunately, I cannot work on these issues due to school-related stuff (I might on the winter). However, there's some effort to improve robustness on this check on: #159392