Test marking single-argument constructor as constexpr

[scpeters]

This adds an additional commit (09f921e) on top of #3315 to test a suggestion in #3315 (comment).

This fails to compile on my local machine:

OgreMain/src/OgreVector.cpp:34:19: error: constexpr variable 'gVector2Zero' must be initialized by a constant expression
   34 | constexpr Vector2 gVector2Zero( 0 );
      |                   ^~~~~~~~~~~~~~~~~
OgreMain/include/OgreVector.h:304:28: note: non-constexpr constructor 'VectorBase' cannot be used in a constant expression
  304 |         explicit constexpr Vector(T s)
      |                            ^
OgreMain/src/OgreVector.cpp:34:19: note: in call to 'Vector(0.000000e+00)'
   34 | constexpr Vector2 gVector2Zero( 0 );
      |                   ^
OgreMain/include/OgreVector.h:70:9: note: declared here
   70 |         VectorBase() {}
      |         ^
OgreMain/src/OgreVector.cpp:39:19: error: constexpr variable 'gVector2UnitScale' must be initialized by a constant expression
   39 | constexpr Vector2 gVector2UnitScale( 1 );
      |                   ^~~~~~~~~~~~~~~~~~~~~~
OgreMain/include/OgreVector.h:304:28: note: non-constexpr constructor 'VectorBase' cannot be used in a constant expression
  304 |         explicit constexpr Vector(T s)
      |                            ^
OgreMain/src/OgreVector.cpp:39:19: note: in call to 'Vector(1.000000e+00)'
   39 | constexpr Vector2 gVector2UnitScale( 1 );
      |                   ^
OgreMain/include/OgreVector.h:70:9: note: declared here
   70 |         VectorBase() {}
      |         ^
OgreMain/src/OgreVector.cpp:41:19: error: constexpr variable 'gVector3Zero' must be initialized by a constant expression
   41 | constexpr Vector3 gV[ 94%] Building CXX object OgreMain/CMakeFiles/OgreMain.dir/src/OgreVertexIndexData.cpp.o
ector3Zero( 0 );
      |                   ^~~~~~~~~~~~~~~~~
OgreMain/include/OgreVector.h:304:28: note: non-constexpr constructor 'VectorBase' cannot be used in a constant expression
  304 |         explicit constexpr Vector(T s)
      |                            ^
OgreMain/src/OgreVector.cpp:41:19: note: in call to 'Vector(0.000000e+00)'
   41 | constexpr Vector3 gVector3Zero( 0 );
      |                   ^
OgreMain/include/OgreVector.h:130:9: note: declared here
  130 |         VectorBase() {}
      |         ^
OgreMain/src/OgreVector.cpp:48:19: error: constexpr variable 'gVector3UnitScale' must be initialized by a constant expression
   48 | constexpr Vector3 gVector3UnitScale( 1 );
      |                   ^~~~~~~~~~~~~~~~~~~~~~
OgreMain/include/OgreVector.h:304:28: note: non-constexpr constructor 'VectorBase' cannot be used in a constant expression
  304 |         explicit constexpr Vector(T s)
      |                            ^
OgreMain/src/OgreVector.cpp:48:19: note: in call to 'Vector(1.000000e+00)'
   48 | constexpr Vector3 gVector3UnitScale( 1 );
      |                   ^
OgreMain/include/OgreVector.h:130:9: note: declared here
  130 |         VectorBase() {}
      |         ^
OgreMain/src/OgreVector.cpp:50:19: error: constexpr variable 'gVector4Zero' must be initialized by a constant expression
   50 | constexpr Vector4 gVector4Zero( 0 );
      |                   ^~~~~~~~~~~~~~~~~
OgreMain/include/OgreVector.h:304:28: note: non-constexpr constructor 'VectorBase' cannot be used in a constant expression
  304 |         explicit constexpr Vector(T s)
      |                            ^
OgreMain/src/OgreVector.cpp:50:19: note: in call to 'Vector(0.000000e+00)'
   50 | constexpr Vector4 gVector4Zero( 0 );
      |                   ^
OgreMain/include/OgreVector.h:259:9: note: declared here
  259 |         VectorBase() {}
      |         ^

[scpeters]

09f921e causes build failures, so I tried marking more constructors as constexpr in 57d2bbf, which also fails

[paroj]

so I tried marking more constructors as constexpr in 57d2bbf, which also fails

yeah.. apparently we need C++20 for this