Fastmath screws up return types

[moble]

julia> 1.0 * imz  # This returns a QuatVec, as expected
 + 0.0𝐢 + 0.0𝐣 + 1.0𝐤

julia> @fastmath 1.0 * imz  # This returns a Quaternion!
0.0 + 0.0𝐢 + 0.0𝐣 + 1.0𝐤

Under the covers, the second version is calling

Base.FastMath.mul_fast(s::Float64, q::QuatVecF64)

which returns a QuaternionF64. I guess this has to do with the fact that I explicitly delineate the various combinations for Base.:*, but don't do so for mul_fast.