Add promote_eltype primitive to ArrayInterfaceCore

The description is pretty self-explanatory. I found that this is a generally missing part of the interface which is required in order to compute types generated when doing type promotions on the element types of arrays.

Author: ChrisRackauckas

docs/src/api.md

@@ -33,6 +33,7 @@ ArrayInterfaceCore.lu_instance
 ArrayInterfaceCore.matrix_colors
 ArrayInterfaceCore.issingular
 ArrayInterfaceCore.parent_type
+ArrayInterfaceCore.promote_eltype
 ArrayInterfaceCore.restructure
 ArrayInterfaceCore.safevec
 ArrayInterfaceCore.zeromatrix

lib/ArrayInterfaceCore/Project.toml

@@ -1,6 +1,6 @@
 name = "ArrayInterfaceCore"
 uuid = "30b0a656-2188-435a-8636-2ec0e6a096e2"
-version = "0.1.18"
+version = "0.1.19"
 
 [deps]
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"

lib/ArrayInterfaceCore/src/ArrayInterfaceCore.jl

@@ -131,6 +131,17 @@ parent_type(@nospecialize T::Type{<:Union{Base.Slice,Base.IdentityUnitRange}}) =
 parent_type(::Type{Diagonal{T,V}}) where {T,V} = V
 parent_type(T::Type) = T
 
+"""
+    promote_eltype(::Type{<:AbstractArray{T,N}}, ::Type{T2})
+
+Computes the type of the `AbstractArray` that results from the element
+type changing to `promote_type(T,T2)`.
+
+Note that no generic fallback is given.
+"""
+function promote_eltype end
+promote_eltype(::Type{Array{T,N}}, ::Type{T2}) where {T,T2,N} = Array{promote_type(T,T2),N}
+
 """
     buffer(x)