I haven't been able to narrow down what the problem is, so this issue is quite verbose.
MethodError: Cannot `convert` an object of type RigidBodyDynamics.CustomCollections.UnsafeFastDict{RigidBodyDynamics.Graphs.vertex_index,RigidBodyDynamics.RigidBody{Float64},Array{Array{RigidBodyDynamics.Contact.SoftContactStateDeriv{Void,RigidBodyDynamics.Contact.ViscoelasticCoulombStateDeriv{SubArray{ForwardDiff.Dual{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},2},1,Array{ForwardDiff.Dual{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},ForwardDiff.Dual{_,Float64,2},2},1},Tuple{UnitRange{Int64}},true}}},1},1} where _<:ForwardDiff.Tag} to an object of type RigidBodyDynamics.CustomCollections.UnsafeFastDict{RigidBodyDynamics.Graphs.vertex_index,RigidBodyDynamics.RigidBody{Float64},Array{Array{RigidBodyDynamics.Contact.SoftContactStateDeriv{Void,RigidBodyDynamics.Contact.ViscoelasticCoulombStateDeriv{SubArray{ForwardDiff.Dual{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},2},1,Array{ForwardDiff.Dual{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},2},1},Tuple{UnitRange{Int64}},true}}},1},1}}
This may have arisen from a call to the constructor RigidBodyDynamics.CustomCollections.UnsafeFastDict{RigidBodyDynamics.Graphs.vertex_index,RigidBodyDynamics.RigidBody{Float64},Array{Array{RigidBodyDynamics.Contact.SoftContactStateDeriv{Void,RigidBodyDynamics.Contact.ViscoelasticCoulombStateDeriv{SubArray{ForwardDiff.Dual{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},2},1,Array{ForwardDiff.Dual{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},2},1},Tuple{UnitRange{Int64}},true}}},1},1}}(...),
since type constructors fall back to convert methods.
Stacktrace:
[1] RigidBodyDynamics.DynamicsResult{ForwardDiff.Dual{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},2},M} where M(::RigidBodyDynamics.Mechanism{Float64}) at /Users/goretkin/playgrounds/jump/packages/v0.6/RigidBodyDynamics/src/dynamics_result.jl:73
[2] evaluate_friction_kinetic_energy(::RigidBodyDynamics.MechanismState{Float64,Float64,Float64,TypeSortedCollections.TypeSortedCollection{Tuple{Array{RigidBodyDynamics.Joint{Float64,RigidBodyDynamics.Revolute{Float64}},1}},1}}, ::Float64, ::Float64, ::Array{ForwardDiff.Dual{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},2},1}) at /Users/goretkin/projects/julia_lqr/min_jump.jl:63
[3] vector_mode_gradient at /Users/goretkin/playgrounds/jump/packages/v0.6/ForwardDiff/src/gradient.jl:94 [inlined]
[4] gradient(::##2#3, ::Array{ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},1}, ::ForwardDiff.GradientConfig{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},2,Array{ForwardDiff.Dual{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},2},1}}) at /Users/goretkin/playgrounds/jump/packages/v0.6/ForwardDiff/src/gradient.jl:19
[5] vector_mode_jacobian(::ForwardDiff.##43#44{##2#3,ForwardDiff.HessianConfig{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},Float64,2,Array{ForwardDiff.Dual{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},2},1},0xb046287d533b082d,Array{ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},1}}}, ::Array{Float64,1}, ::ForwardDiff.JacobianConfig{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2,Array{ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},1}}) at /Users/goretkin/playgrounds/jump/packages/v0.6/ForwardDiff/src/jacobian.jl:132
[6] jacobian(::ForwardDiff.##43#44{##2#3,ForwardDiff.HessianConfig{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},Float64,2,Array{ForwardDiff.Dual{ForwardDiff.Tag{##2#3,0x096989b645fcecd8},ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},2},1},0xb046287d533b082d,Array{ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},1}}}, ::Array{Float64,1}, ::ForwardDiff.JacobianConfig{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2,Array{ForwardDiff.Dual{ForwardDiff.Tag{Void,0xb046287d533b082d},Float64,2},1}}) at /Users/goretkin/playgrounds/jump/packages/v0.6/ForwardDiff/src/jacobian.jl:21
[7] hessian(::##2#3, ::Array{Float64,1}) at /Users/goretkin/playgrounds/jump/packages/v0.6/ForwardDiff/src/hessian.jl:19
[8] include_from_node1(::String) at ./loading.jl:569
[9] include(::String) at ./sysimg.jl:14
RigidBodyDynamics.CustomCollections.UnsafeFastDict{
RigidBodyDynamics.Graphs.vertex_index,
RigidBodyDynamics.RigidBody{
Float64},
Array{
Array{
RigidBodyDynamics.Contact.SoftContactStateDeriv{
Void,
RigidBodyDynamics.Contact.ViscoelasticCoulombStateDeriv{
SubArray{
ForwardDiff.Dual{
ForwardDiff.Tag{
##2#3,
0x096989b645fcecd8},
ForwardDiff.Dual{
ForwardDiff.Tag{
Void,
0xb046287d533b082d},
Float64,
2},
2},
1,
Array{
ForwardDiff.Dual{
ForwardDiff.Tag{
##2#3,
0x096989b645fcecd8},
ForwardDiff.Dual{
_, # this line differs
Float64,
2},
2},
1},
Tuple{
UnitRange{
Int64}},
true}}},
1},
1}
where _<:ForwardDiff.Tag} # this line differs
RigidBodyDynamics.CustomCollections.UnsafeFastDict{
RigidBodyDynamics.Graphs.vertex_index,
RigidBodyDynamics.RigidBody{
Float64},
Array{
Array{
RigidBodyDynamics.Contact.SoftContactStateDeriv{
Void,
RigidBodyDynamics.Contact.ViscoelasticCoulombStateDeriv{
SubArray{
ForwardDiff.Dual{
ForwardDiff.Tag{
##2#3,
0x096989b645fcecd8},
ForwardDiff.Dual{
ForwardDiff.Tag{
Void,
0xb046287d533b082d},
Float64,
2},
2},
1,
Array{
ForwardDiff.Dual{
ForwardDiff.Tag{
##2#3,
0x096989b645fcecd8},
ForwardDiff.Dual{
ForwardDiff.Tag{ # there is a concrete tag
Void,
0xb046287d533b082d},
Float64,
2},
2},
1},
Tuple{
UnitRange{
Int64}},
true}}},
1},
1}}