When promoting Diagonals with different element types, I’m encountering unexpected behavior:

```
using LinearAlgebra
diagInt = Diagonal([1])
diagFloat = Diagonal([1.0])
promote_type(typeof(diagInt),typeof(diagFloat)) #returns Diagonal
```

Instead of Diagonal, I expected Diagonal{Float64, Vector{Float64}}. The promotion should be handled by this promote_rule in the source code, but it seems that it is never called. In fact, if I copy and paste the method into the REPL, the promotion works as expected:

```
function Base.promote_rule(A::Type{<:Diagonal{<:Any,V}}, B::Type{<:Diagonal{<:Any,W}}) where {V,W}
X = promote_type(V, W)
T = eltype(X)
isconcretetype(T) && return Diagonal{T,X}
return typejoin(A, B)
end
promote_type(typeof(diagInt),typeof(diagFloat)) #now returns Diagonal{Float64, Vector{Float64}}
```

Why would overwriting the method with itself change the result?