Type-inference failure in matmul

[baggepinnen]

The eltype of the result infers to Any instead of Num

julia> @variables x
1-element Vector{Num}:
 x

julia> a = [x x]
1×2 Matrix{Num}:
 x  x

julia> a * a'
1×1 Matrix{Any}:
 2(x^2)

This is bad, since it leads to things like

ERROR: MethodError: no method matching zero(::Type{Any})

when trying to use the result

[baggepinnen]

fixed on master