nothing to do with alias:
julia> convert((SMatrix{3, 3, T} where T<:AbstractFloat), rand(3,3))
3×3 SMatrix{3, 3, Float64, 9} with indices SOneTo(3)×SOneTo(3):
0.60079 0.0957968 0.160429
0.0510449 0.0892437 0.435343
0.897627 0.389427 0.964163
julia> convert((UpperTriangular{T,SMatrix{3,3,T}} where T<:AbstractFloat), rand(3,3))
ERROR: MethodError: Cannot `convert` an object of type
Matrix{Float64} to an object of type
UpperTriangular{T, SMatrix{3, 3, T}} where T<:AbstractFloat
there’s no equivalent for UpperTriangular