`Array` conversion from banded matrices might be inconsistent

[jishnub]

Using the example from the special tests:

struct TypeWithoutZero end
struct TypeWithZero end
Base.promote_rule(::Type{TypeWithoutZero}, ::Type{TypeWithZero}) = TypeWithZero
Base.convert(::Type{TypeWithZero}, ::TypeWithoutZero) = TypeWithZero()
Base.zero(x::Union{TypeWithoutZero, TypeWithZero}) = zero(typeof(x))
Base.zero(::Type{<:Union{TypeWithoutZero, TypeWithZero}}) = TypeWithZero()
LinearAlgebra.symmetric(::TypeWithoutZero, ::Symbol) = TypeWithoutZero()
LinearAlgebra.symmetric_type(::Type{TypeWithoutZero}) = TypeWithoutZero
Base.copy(A::TypeWithoutZero) = A
Base.transpose(::TypeWithoutZero) = TypeWithoutZero()
d  = fill(TypeWithoutZero(), 3)
D  = Diagonal(d)

With this,

julia> D
3×3 Diagonal{TypeWithoutZero, Vector{TypeWithoutZero}}:
 TypeWithoutZero()        ⋅                  ⋅       
       ⋅            TypeWithoutZero()        ⋅       
       ⋅                  ⋅            TypeWithoutZero()

julia> Array(D) # doesn't preserve diagonal elements
3×3 Matrix{TypeWithZero}:
 TypeWithZero()  TypeWithZero()  TypeWithZero()
 TypeWithZero()  TypeWithZero()  TypeWithZero()
 TypeWithZero()  TypeWithZero()  TypeWithZero()

julia> [x for x in D] # preserves diagonal elements
3×3 Matrix{Any}:
 TypeWithoutZero()  TypeWithZero()     TypeWithZero()
 TypeWithZero()     TypeWithoutZero()  TypeWithZero()
 TypeWithZero()     TypeWithZero()     TypeWithoutZero()

julia> collect(D) # perhaps a missing method
ERROR: MethodError: Cannot `convert` an object of type TypeWithZero to an object of type TypeWithoutZero
The function `convert` exists, but no method is defined for this combination of argument types.

The Array constructor was added in JuliaLang/julia#44707 to address a breakage in JuMP, but these should probably be consistent? In particular, it seems odd that the diagonal elements are not preserved by the Array constructor, and that

julia> Array(D) == D
false

Perhaps the eltype of the Array should be a Union in this case, instead of being concrete?