libpisp: Switch to C++20 standard

Switch the default build to use C++ 20 standard. There is a compile
failure regarding and "ambiguous operator", fix this:

In file included from /usr/include/boost/assert.hpp:64,
                 from /usr/include/boost/log/detail/attachable_sstream_buf.hpp:24,
                 from /usr/include/boost/log/utility/formatting_ostream.hpp:33,
                 from /usr/include/boost/log/sources/record_ostream.hpp:36,
                 from /usr/include/boost/log/trivial.hpp:24,
                 from ../subprojects/libpisp/src/libpisp/common/logging.hpp:17,
                 from ../subprojects/libpisp/src/libpisp/backend/tiling/input_stage.cpp:9:
../subprojects/libpisp/src/libpisp/backend/tiling/input_stage.cpp: In member function ‘virtual void tiling::InputStage::PushCropDown(tiling::Interval, tiling::Dir)’:
../subprojects/libpisp/src/libpisp/backend/tiling/input_stage.cpp:95:33: warning: C++20 says that these are ambiguous, even though the second is reversed:
   95 |         PISP_ASSERT(interval == input_interval_);
      |                                 ^~~~~~~~~~~~~~~
../subprojects/libpisp/src/libpisp/backend/tiling/input_stage.cpp:95:9: note: in expansion of macro ‘PISP_ASSERT’
   95 |         PISP_ASSERT(interval == input_interval_);
      |         ^~~~~~~~~~~
In file included from ../subprojects/libpisp/src/libpisp/backend/tiling/stages.hpp:11,
                 from ../subprojects/libpisp/src/libpisp/backend/tiling/input_stage.hpp:9,
                 from ../subprojects/libpisp/src/libpisp/backend/tiling/input_stage.cpp:7:
../subprojects/libpisp/src/libpisp/backend/tiling/types.hpp:64:14: note: candidate 1: ‘bool tiling::Interval::operator==(const tiling::Interval&)’
   64 |         bool operator==(Interval const &other) { return offset == other.offset && length == other.length; }
      |              ^~~~~~~~
../subprojects/libpisp/src/libpisp/backend/tiling/types.hpp:64:14: note: candidate 2: ‘bool tiling::Interval::operator==(const tiling::Interval&)’ (reversed)
../subprojects/libpisp/src/libpisp/backend/tiling/types.hpp:64:14: note: try making the operator a ‘const’ member function

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>

Author: naushir

meson.build

@@ -7,7 +7,7 @@ project('libpisp', 'c', 'cpp',
         default_options : [
             'werror=true',
             'warning_level=2',
-            'cpp_std=c++17',
+            'cpp_std=c++20',
             'buildtype=release',
         ],
         license : 'BSD-2-Clause')

src/libpisp/backend/tiling/types.hpp

@@ -29,8 +29,8 @@ struct Length2
 	explicit Length2(int len) : Length2(len, len) {}
 	int dx, dy;
 	int operator[](Dir dir) const { return dir == Dir::Y ? dy : dx; }
-	Length2 operator-(Length2 const &other) { return Length2(dx - other.dx, dy - other.dy); }
-	Length2 operator/(int num) { return Length2(dx / num, dy / num); }
+	Length2 operator-(Length2 const &other) const { return Length2(dx - other.dx, dy - other.dy); }
+	Length2 operator/(int num) const { return Length2(dx / num, dy / num); }
 };
 
 inline std::ostream &operator<<(std::ostream &os, Length2 const &l)
@@ -44,7 +44,7 @@ struct Crop
 	Crop(int _start, int _end) : start(_start), end(_end) {}
 	explicit Crop(int crop) : Crop(crop, crop) {}
 	int start, end;
-	Crop operator+(Crop const &other) { return Crop(start + other.start, end + other.end); }
+	Crop operator+(Crop const &other) const { return Crop(start + other.start, end + other.end); }
 };
 
 inline std::ostream &operator<<(std::ostream &os, Crop const &c)
@@ -61,11 +61,11 @@ struct Interval
 	int End() const { return offset + length; }
 	void SetStart(int start) { length += (offset - start), offset = start; } // leave End unchanged
 	void SetEnd(int end) { length = end - offset; }
-	bool operator==(Interval const &other) { return offset == other.offset && length == other.length; }
+	bool operator==(Interval const &other) const { return offset == other.offset && length == other.length; }
 	Interval operator-(Crop const &crop) const { return Interval(offset - crop.start, length - crop.start - crop.end); }
-	Crop operator-(Interval const &other) { return Crop(other.offset - offset, End() - other.End()); }
-	bool operator>(Interval const &other) { return offset <= other.offset && End() >= other.End(); } // contains
-	bool operator<(Interval const &other) { return offset >= other.offset && End() <= other.End(); } // is contained
+	Crop operator-(Interval const &other) const { return Crop(other.offset - offset, End() - other.End()); }
+	bool operator>(Interval const &other) const { return offset <= other.offset && End() >= other.End(); } // contains
+	bool operator<(Interval const &other) const { return offset >= other.offset && End() <= other.End(); } // is contained
 	Interval &operator|=(int off)
 	{
 		if (off < offset)
@@ -88,7 +88,7 @@ struct Crop2
 	Crop x, y;
 	Crop operator[](Dir dir) const { return dir == Dir::Y ? y : x; }
 	Crop &operator[](Dir dir) { return dir == Dir::Y ? y : x; }
-	Crop2 operator+(Crop2 const &other) { return Crop2(x + other.x, y + other.y); }
+	Crop2 operator+(Crop2 const &other) const { return Crop2(x + other.x, y + other.y); }
 };
 
 inline std::ostream &operator<<(std::ostream &os, Crop2 const &c)
@@ -104,8 +104,8 @@ struct Interval2
 	Interval x, y;
 	Interval operator[](Dir dir) const { return dir == Dir::Y ? y : x; }
 	Interval &operator[](Dir dir) { return dir == Dir::Y ? y : x; }
-	bool operator==(Interval2 const &other) { return x == other.x && y == other.y; }
-	bool operator!=(Interval2 const &other) { return !(*this == other); }
+	bool operator==(Interval2 const &other) const { return x == other.x && y == other.y; }
+	bool operator!=(Interval2 const &other) const { return !(*this == other); }
 };
 
 inline std::ostream &operator<<(std::ostream &os, Interval2 const &i)