COMP: Pass /Zc:__cplusplus when compiling SWIG wrappers with MSVC

[hjmjohnson]

Problem

The Windows Python wrapping build fails with five C2280 errors, all from one root cause:

itkGradientImageFilterPython.cpp: error C2280:
  'std::unique_ptr<itk::ImageBoundaryCondition<TInputImage,TInputImage>,...>::unique_ptr(const unique_ptr &)':
  attempting to reference a deleted function

Root cause

Not the wrapping declarations. SWIG generates correct code:

std::unique_ptr< itkImageBoundaryConditionIF2 > arg2 ;
...
(&arg2)->reset((itkImageBoundaryConditionIF2 *)argp2);
(arg1)->SetBoundaryCondition(SWIG_STD_MOVE(arg2));

The %unique_ptr(...) typemap applies as intended and the one-argument mangled name resolves against
ImageBoundaryCondition<TInputImage, TInputImage> without trouble, since the default template argument
collapses for SWIG just as it does for C++.

The failure is in SWIG's move guard:

#if __cplusplus >= 201103L
# define SWIG_STD_MOVE(OBJ) std::move(OBJ)
#else
# define SWIG_STD_MOVE(OBJ) OBJ
#endif

MSVC reports __cplusplus as 199711L regardless of /std:c++NN unless /Zc:__cplusplus is given.
ITK passes /std:c++17 but never /Zc:__cplusplus. On MSVC the #else branch therefore wins,
SWIG_STD_MOVE(arg2) degrades from std::move(arg2) to plain arg2, and the by-value std::unique_ptr
parameter is copied.

Measured on the CI toolchain (MSVC 19.38.33145):

Flags	__cplusplus	SWIG emits
/std:c++17	199711L	a copy → C2280
/std:c++17 /Zc:__cplusplus	201703L	std::move

GCC and Clang report __cplusplus truthfully and emit std::move, which is why only the MSVC Python
wrapping build is affected. It is not that other platforms wrap a different instantiation set.

GradientImageFilter::SetBoundaryCondition (added in 6203371) is simply the first wrapped method to take
a std::unique_ptr by value, so it is the first to trip a pre-existing gap in the MSVC wrapper flags rather
than the cause of it.

Fix

Add /Zc:__cplusplus to the MSVC wrapper compile flags, alongside the /wd4244 /wd4996 already there. This
is scoped to the SWIG-generated wrapper sources, so it does not change how ITK itself is compiled, and it
covers any other SWIG feature gated on __cplusplus rather than only this call site.

No .wrap file or header change is required; the generated .cpp is unchanged by this fix.

Verification

Reproduced and fixed on Windows, MSVC 19.38.33145 (VS 2022, toolset 14.38), Ninja, Release x64, against
main at 2bdb783.

Before:

itkGradientImageFilterPython.cpp(4512): error C2280: ... attempting to reference a deleted function

After: ITKImageGradientPython builds and links clean — 0 errors, 0 C2280, producing
_ITKImageGradientPython.pyd. SWIG did not regenerate the .cpp; only the compile flags changed, confirming
this is purely a flag defect.

Scope of verification: the reproduction used a reduced wrapping configuration
(Module_ITKImageGradient only, ITK_WRAP_float=ON, ITK_WRAP_IMAGE_DIMS=2;3) to keep the turnaround short.
The flag applies to every wrapper target, so a full ITK_WRAP_PYTHON build is the appropriate regression
check before merging; that has not been run here.

AI assistance

• Tool: Claude Code (claude-opus-5)
• Role: root-cause analysis, hypothesis testing
• Contribution: traced the C2280 past the wrapping declarations to
  SWIG's SWIG_STD_MOVE guard, and measured __cplusplus with and
  without /Zc:__cplusplus on the CI toolchain to confirm
• Reproduced and verified locally on Windows/MSVC before committing

[greptile-apps]

Greptile Summary

The MSVC Python wrapper build flags now include /Zc:__cplusplus, allowing generated wrappers to report the configured C++ language version correctly while retaining the existing generated-code warning suppressions.

Confidence Score: 5/5

No blocking failure remains.

The focused CMake check exercised the MSVC wrapper-target configuration and confirmed that the generated target retains both existing warning suppressions and receives the new C++ conformance option.

T-Rex Logs

What T-Rex did

• Reviewed the focused MSVC wrapper target property harness used for both captures.
• Validated the baseline wrapper target flags before the added C++ conformance option, confirming exit code 0 and the baseline flags.
• Validated the PR-path wrapper target flags after the added C++ conformance option, confirming exit code 0 and all three expected flags.
• Confirmed the harness executable used for the captures was the exact focused harness and that no repository-tracked file was modified.

_{Ran code and verified through T-Rex}

_{Reviews (2): Last reviewed commit: "COMP: Pass /Zc:__cplusplus when compilin..." | Re-trigger Greptile}

[dzenanz]

Greptile is right. This comment does not need to be that long. The full explanation is in the commit message. A suggested solution is provided inline. Otherwise seems like a good fix.

[hjmjohnson]

@greptileai review