BUG: Propagate test helper status and mark the helpers [[nodiscard]]

[hjmjohnson]

Test helpers that return EXIT_SUCCESS/EXIT_FAILURE are now [[nodiscard]], so a caller that drops the status no longer compiles. That plus the six original call-site fixes turns 13 explicit failure returns and 31 ITK_TEST_* assertions back into real assertions, and exposed a wrong superclass assertion that only MSVC evaluates.

The defect

switch (std::stoi(argv[1]))
{
  case 2:
    PerformBSplineExpImageRegistration<2>(argc, argv);   // return value ignored
    break;
  ...
}
return EXIT_SUCCESS;

The helpers do run — each one's argument-count guard matches its registered CMake invocation — so the defect is latent: the assertions execute but their verdict is discarded.

File	Swallowed inside the helper
itkLabelOverlapMeasuresImageFilterTest.cxx	6 return EXIT_FAILURE
itkBSplineExponentialImageRegistrationTest.cxx	2 + 2 ITK_TEST_*
itkBSplineImageRegistrationTest.cxx	2 + 2
itkExponentialImageRegistrationTest.cxx	3 + 7
itkTimeVaryingBSplineVelocityFieldImageRegistrationTest.cxx	13 ITK_TEST_*
itkTimeVaryingVelocityFieldImageRegistrationTest.cxx	7 ITK_TEST_*

Commit 2: the Windows failure this unmasked

itkLabelOverlapMeasuresImageFilterTest's helper asserts

ITK_EXERCISE_BASIC_OBJECT_METHODS(filter, LabelOverlapMeasuresImageFilter, ImageToImageFilter);

but the filter derives from ImageSink (itkLabelOverlapMeasuresImageFilter.h:43), and the enclosing test asserts ImageSink correctly 140 lines later.

The assertion is compiler-dependent. itkTestingMacros.h:54-83 defines two variants: under __GNUC__ the macro only prints the class name, while everywhere else it also compares Superclass::GetNameOfClass() and returns EXIT_FAILURE on mismatch. GCC and Clang both define __GNUC__, so only MSVC evaluates the check — and only once commit 1 stopped discarding the helper's status did that reach the test result. Pixi-Cxx (windows-2022) was the sole red check before this fix.

Commit 3: [[nodiscard]] across the test tree

290 declarations in 211 files. Selection is mechanical: a file-scope, non-entry-point function returning int whose body contains EXIT_SUCCESS or EXIT_FAILURE.

The attribute goes on every declaration, not only the definition. [[nodiscard]] governs calls that follow the declaration carrying it, so a bare forward declaration leaves earlier call sites unchecked — annotating the 14 forward declarations is what exposed the CoherenceEnhancingDiffusionTest site below.

Deliberately not annotated:

• 38 int-returning helpers whose return is a value, not a status (lookup in itkExceptionObjectTest, the dynamic_castDownCast* family, the test*Spline helpers). Annotating these would only force (void) casts at legitimate discard sites.
• TestKernelTransform, which uses a bare 0/1 convention and needs a semantics change first — handled separately.
• itkSyNImageRegistrationTest and itkBSplineSyNImageRegistrationTest, whose discarded call sites are fixed by BUG: Make the SyN registration tests actually run a registration #6703. Annotating them without that PR's argc fix makes itkBSplineSyNImageRegistrationTest fail. They can be annotated once BUG: Make the SyN registration tests actually run a registration #6703 lands.

The attribute exposed 30 discarded call sites in 8 tests, all fixed here:

File	Sites
itkCentralDifferenceImageFunctionOnVectorSpeedTest.cxx	10
itkObjectToObjectMultiMetricv4RegistrationTest.cxx	5
itkDiscreteGaussianImageFilterTest2.cxx	4
itkSLICImageFilterTest.cxx	4
itkFFTDiscreteGaussianImageFilterTest.cxx	2
itkFancyStringTest.cxx	2
itkMeanSquaresImageToImageMetricv4OnVectorTest2.cxx	2
CoherenceEnhancingDiffusionTest.cxx	1

Where a helper is called several times in one scope the status accumulates with testStatus |= (the existing idiom in itkColorTableTest.cxx); plain = is used only where the branches are mutually exclusive.

Three of these were invisible to a source-level audit: in itkFancyStringTest and CoherenceEnhancingDiffusionTest the discard happens inside ITK_TRY_EXPECT_NO_EXCEPTION(...), where the call is not lexically a statement. The compiler sees it after expansion; a grep does not.

Test plan

Release, Ninja, AppleClang, macOS arm64, in a build tree configured against this branch alone (Module_ITKReview=ON added so no annotated file goes uncompiled).

• Full build of 8452 targets plus incremental passes: zero -Wunused-result diagnostics, zero errors, no new warnings.
• ctest -j8: 100% passed, 0 failed out of 6207.
• pre-commit run --all-files exits 0.

Not exercised locally: the annotated files under Video/BridgeOpenCV, Video/BridgeVXL and the GPU modules, which need OpenCV / VXL / OpenCL. Every helper and call site in those 20 files was inspected by hand and all consume their return value. CI covers the compile.

[hjmjohnson]

@greptileai review