Prevent usage of distance functions on incompatible types

Types indicating differences or derivatives of unit vectors and unit
quaternions are needed to handle values with the same dimensionality as the
original types, but no constraints.

The two enum also need to be handled explicitly in a switch statement to
compile without warnings.  However, it is better if the corresponding code
raises an error, instead of computing something that has no consistent
definition.

src/colvarvalue.cpp

@@ -655,20 +655,23 @@ cvm::real colvarvalue::dist2(colvarvalue const &x2) const
     return (this->real_value - x2.real_value) * (this->real_value - x2.real_value);
   case colvarvalue::type_3vector:
     return (this->rvector_value - x2.rvector_value).norm2();
-  case colvarvalue::type_unit3vector:
-  case colvarvalue::type_unit3vectorderiv: {
+  case colvarvalue::type_unit3vector: {
     cvm::rvector const &v1 = this->rvector_value;
     cvm::rvector const &v2 = x2.rvector_value;
     cvm::real const theta = cvm::acos(v1 * v2);
     return theta * theta;
   }
   case colvarvalue::type_quaternion:
-  case colvarvalue::type_quaternionderiv:
     // angle between (*this) and x2 is the distance, the quaternion
     // object has it implemented internally
     return this->quaternion_value.dist2(x2.quaternion_value);
   case colvarvalue::type_vector:
     return (this->vector1d_value - x2.vector1d_value).norm2();
+  case colvarvalue::type_unit3vectorderiv:
+  case colvarvalue::type_quaternionderiv:
+    cvm::error("Error: computing a squared-distance between two variables of type \"" +
+                   type_desc(this->type()) + "\", for which it is not defined.\n",
+               COLVARS_BUG_ERROR);
   case colvarvalue::type_notset:
   default:
     this->undef_op();
@@ -688,20 +691,23 @@ colvarvalue colvarvalue::dist2_grad(colvarvalue const &x2) const
     return 2.0 * (this->real_value - x2.real_value);
   case colvarvalue::type_3vector:
     return 2.0 * (this->rvector_value - x2.rvector_value);
-  case colvarvalue::type_unit3vector:
-  case colvarvalue::type_unit3vectorderiv: {
+  case colvarvalue::type_unit3vector: {
     cvm::rvector const &v1 = this->rvector_value;
     cvm::rvector const &v2 = x2.rvector_value;
     cvm::real const cos_t = v1 * v2;
     return colvarvalue(2.0 * std::acos(cos_t) * -1.0 / cvm::sqrt(1.0 - cos_t * cos_t) * v2,
                        colvarvalue::type_unit3vectorderiv);
   }
   case colvarvalue::type_quaternion:
-  case colvarvalue::type_quaternionderiv:
     return this->quaternion_value.dist2_grad(x2.quaternion_value);
   case colvarvalue::type_vector:
     return colvarvalue(2.0 * (this->vector1d_value - x2.vector1d_value), colvarvalue::type_vector);
     break;
+  case colvarvalue::type_unit3vectorderiv:
+  case colvarvalue::type_quaternionderiv:
+    cvm::error("Error: computing a squared-distance gradient between two variables of type \"" +
+                   type_desc(this->type()) + "\", for which it is not defined.\n",
+               COLVARS_BUG_ERROR);
   case colvarvalue::type_notset:
   default:
     this->undef_op();