Summing SingleVariable throws bad error

[odow]

This is bad:

julia> x = MOI.SingleVariable(MOI.VariableIndex(1))
MathOptInterface.SingleVariable(MathOptInterface.VariableIndex(1))

julia> x + x
ERROR: UndefVarError: T not defined
Stacktrace:
 [1] +(arg::MathOptInterface.SingleVariable, args::MathOptInterface.SingleVariable)
   @ MathOptInterface.Utilities ~/.julia/dev/MathOptInterface/src/Utilities/functions.jl:1746
 [2] top-level scope
   @ REPL[15]:1