SolidReference behavior

[ShuffleWire]

Hi everyone !

I'm playing with SolidReference, and notice the following behavior : This world work fine : (a "robot" made of a body and a wheel, linked with a hinge, bump on the floor and rest)

world file

#VRML_SIM R2021b utf8
WorldInfo {
  coordinateSystem "NUE"
  lineScale 10
}
Viewpoint {
  orientation -0.8522758578957162 0.49184059024082777 0.17809743355723795 0.767813751873505
  position 1.1327057410454138 1.9767842570412657 2.1912937179224135
}
TexturedBackground {
}
TexturedBackgroundLight {
}
RectangleArena {
}
Robot {
  translation 0 0.17 0
  rotation 0 0 1 0.261799
  children [
    Shape {
      geometry DEF Body Box {
        size 0.2 0.1 0.4
      }
    }
    HingeJoint {
      jointParameters HingeJointParameters {
        anchor 0 0 0.1
      }
      endPoint Solid {
        translation 0.19 0 0.1
        rotation 0 0 1 -1.5707953071795862
        children [
          Shape {
            geometry DEF Wheel Cylinder {
              height 0.1
              radius 0.09
            }
          }
        ]
        boundingObject USE Wheel
        physics Physics {
        }
      }
    }
  ]
  boundingObject USE Body
  physics Physics {
  }
}

But if I move the solid "Wheel" as a children of the Robot node, and use a solidreference on the hinge, the simulation doesn't work (or at least the behavior is not the same)

world file

#VRML_SIM R2021b utf8
WorldInfo {
  coordinateSystem "NUE"
  lineScale 10
}
Viewpoint {
  orientation -0.8522758578957162 0.49184059024082777 0.17809743355723795 0.767813751873505
  position 1.1327057410454138 1.9767842570412657 2.1912937179224135
}
TexturedBackground {
}
TexturedBackgroundLight {
}
RectangleArena {
}
Robot {
  translation 0 0.17 0
  rotation 0 0 1 0.261799
  children [
    Solid {
      translation 0.19 0 0.1
      rotation 0 0 1 -1.5707953071795862
      children [
        Shape {
          geometry DEF Wheel Cylinder {
            height 0.1
            radius 0.09
          }
        }
      ]
      boundingObject USE Wheel
      physics Physics {
      }
    }
    Shape {
      geometry DEF Body Box {
        size 0.2 0.1 0.4
      }
    }
    HingeJoint {
      jointParameters HingeJointParameters {
        anchor 0 0 0.1
      }
      endPoint SolidReference {
        solidName "solid"
      }
    }
  ]
  boundingObject USE Body
  physics Physics {
  }
}

I expect that both way yield the same results, is that correct ?

The doc about SolidReference state that : "The only constraint when referring to a Solid is that both Solid and Joint must be descendants of a common upper Solid." In my case, They are both descendants of a Robot which itself inheriting from Solid, so that should be ok.
EDIT : I've transformed the Robot node into a Solid Node with the same result, the inheritance shouldn't be the issue, then. Moreover, using a SolidReference to a node that is already inside a Joint work fine (as showcased in the SolidReference doc). It's seems that it's only because of the fact that the SolidReference I'm using is pointing to Solid that are not inside a Joint. It look like the target of the SolidReference default in that case to the root of the world or something like that.)
EDIT2 : I've changed the target of the solidReference to "static environnemnent" and got the same result than setting it to "solid(1)", which seems to indicate than there is some sort of issue with the SolidReference target

I've experimented with adding another Solid node just between the Robot node and it's children, that help a bit, but cause me others trouble on others use case, and I don't like putting Solid inside Solid...

world file

#VRML_SIM R2021b utf8
WorldInfo {
  coordinateSystem "NUE"
  lineScale 10
}
Viewpoint {
  orientation -0.8522758578957162 0.49184059024082777 0.17809743355723795 0.767813751873505
  position 1.1327057410454138 1.9767842570412657 2.1912937179224135
}
TexturedBackground {
}
TexturedBackgroundLight {
}
RectangleArena {
}
Robot {
  translation 0 0.17 0
  rotation 0 0 1 0.261799
  children [
    Solid {
      children [
        Solid {
          translation 0.19 0 0.1
          rotation 0 0 1 -1.5707953071795862
          children [
            Shape {
              geometry DEF Wheel Cylinder {
                height 0.1
                radius 0.09
              }
            }
          ]
          boundingObject USE Wheel
          physics Physics {
          }
        }
        HingeJoint {
          jointParameters HingeJointParameters {
            anchor 0 0 0.1
          }
          endPoint SolidReference {
            solidName "solid"
          }
        }
      ]
      name "solid(1)"
    }
    Shape {
      geometry DEF Body Box {
        size 0.2 0.1 0.4
      }
    }
  ]
  boundingObject USE Body
  physics Physics {
  }
}

Finally, I've managed to get that thing working by putting my wheel Solid as a children of the Robot, and the Hinge joint in a Solid (solid(1)).
But I don't like this solution (solid(1) is kinda useless and hacky, I feel), and that don't match the doc statement, (solid is not a child of a Solid)

world file

#VRML_SIM R2021b utf8
WorldInfo {
  coordinateSystem "NUE"
  lineScale 10
}
Viewpoint {
  orientation -0.8522758578957162 0.49184059024082777 0.17809743355723795 0.767813751873505
  position 1.1327057410454138 1.9767842570412657 2.1912937179224135
}
TexturedBackground {
}
TexturedBackgroundLight {
}
RectangleArena {
}
Robot {
  translation 0 0.17 0
  rotation 0 0 1 0.261799
  children [
    Solid {
      translation 0.19 0 0.1
      rotation 0 0 1 -1.5707953071795862
      children [
        Shape {
          geometry DEF Wheel Cylinder {
            height 0.1
            radius 0.09
          }
        }
      ]
      boundingObject USE Wheel
      physics Physics {
      }
    }
    Solid {
      children [
        HingeJoint {
          jointParameters HingeJointParameters {
            anchor 0 0 0.1
          }
          endPoint SolidReference {
            solidName "solid"
          }
        }
      ]
      name "solid(1)"
    }
    Shape {
      geometry DEF Body Box {
        size 0.2 0.1 0.4
      }
    }
  ]
  boundingObject USE Body
  physics Physics {
  }
}

Any idea what could cause the issue ?

EDIT : Sorry for the messy post, I'm adding info along the way :)

Thanks !

[ShuffleWire]

Ok my bad, as long as I understand it correctly, there is just some "implicit solid merging" going on !