ENH: Add class template argument deduction (CTAD) support to ImageRegion

Simplified the definition of `IndexType` and `SizeType` in order to add CTAD
support to `ImageRegion`. Added a deduction guide, to avoid compiler warnings.

Supports declaring a region, without explicitly specifying its dimension, e.g.:

    itk::ImageRegion region(index, size);

Modules/Core/Common/include/itkImageRegion.h

@@ -113,15 +113,15 @@ class ITK_TEMPLATE_EXPORT ImageRegion final
   }
 
   /** Index type alias support An index is used to access pixel values. */
-  using IndexType = Index<Self::ImageDimension>;
+  using IndexType = Index<VImageDimension>;
   using IndexValueType = typename IndexType::IndexValueType;
   using OffsetType = typename IndexType::OffsetType;
   using OffsetValueType = typename OffsetType::OffsetValueType;
   using IndexValueArrayType = IndexValueType[ImageDimension];
   using OffsetTableType = OffsetValueType[ImageDimension + 1];
 
   /** Size type alias support A size is used to define region bounds. */
-  using SizeType = Size<Self::ImageDimension>;
+  using SizeType = Size<VImageDimension>;
   using SizeValueType = typename SizeType::SizeValueType;
 
   /** Slice region type alias. SliceRegion is one dimension less than Self. */
@@ -152,7 +152,8 @@ class ITK_TEMPLATE_EXPORT ImageRegion final
   ImageRegion(const Self &) noexcept = default;
 
   /** Constructor that takes an index and size. ImageRegion is a lightweight
-   * object that is not reference counted, so this constructor is public. */
+   * object that is not reference counted, so this constructor is public.
+   * \note This constructor supports class template argument deduction (CTAD). */
   ImageRegion(const IndexType & index, const SizeType & size) noexcept
     : // Note: Use parentheses instead of curly braces to initialize data members,
       // to avoid AppleClang 6.0.0.6000056 compile errors, "no viable conversion..."
@@ -162,7 +163,8 @@ class ITK_TEMPLATE_EXPORT ImageRegion final
 
   /** Constructor that takes a size and assumes an index of zeros. ImageRegion
    * is lightweight object that is not reference counted so this constructor
-   * is public. */
+   * is public.
+   * \note This constructor supports class template argument deduction (CTAD). */
   ImageRegion(const SizeType & size) noexcept
     : m_Size(size)
   {
@@ -408,6 +410,12 @@ class ITK_TEMPLATE_EXPORT ImageRegion final
   friend class ImageBase<VImageDimension>;
 };
 
+
+// Deduction guide to avoid compiler warnings (-wctad-maybe-unsupported) when using class template argument deduction.
+template <unsigned int VImageDimension>
+ImageRegion(const Index<VImageDimension> &, const Size<VImageDimension> &)->ImageRegion<VImageDimension>;
+
+
 template <unsigned int VImageDimension>
 std::ostream &
 operator<<(std::ostream & os, const ImageRegion<VImageDimension> & region);

Modules/Core/Common/test/itkImageRegionGTest.cxx

@@ -40,11 +40,30 @@ CheckTrivialCopyabilityOfImageRegion()
   return !isImageRegionTriviallyCopyable;
 #endif
 }
+
+
+// Checks that `ImageRegion` supports class template argument deduction (CTAD).
+template <unsigned int VDimension>
+constexpr bool
+CheckClassTemplateArgumentDeduction()
+{
+  using ExpectedType = itk::ImageRegion<VDimension>;
+
+  static_assert(
+    std::is_same_v<decltype(itk::ImageRegion(itk::Index<VDimension>{}, itk::Size<VDimension>{})), ExpectedType>,
+    "The `ImageRegion(Index, Size)` constructor should support CTAD!");
+  static_assert(std::is_same_v<decltype(itk::ImageRegion(itk::Size<VDimension>{})), ExpectedType>,
+                "The `ImageRegion(Size)` constructor should support CTAD!");
+  return true;
+}
+
+
 } // namespace
 
 static_assert(CheckTrivialCopyabilityOfImageRegion<2>() && CheckTrivialCopyabilityOfImageRegion<3>(),
               "ImageRegion<VDimension> should be trivially copyable when legacy support is removed.");
 
+static_assert(CheckClassTemplateArgumentDeduction<2>() && CheckClassTemplateArgumentDeduction<3>());
 
 // Tests that a zero-sized region is not considered to be inside of another region.
 TEST(ImageRegion, ZeroSizedRegionIsNotInside)