fix AbstractArrayStyle dimension promotion rule

Author: marius311

base/broadcast.jl

@@ -137,11 +137,12 @@ BroadcastStyle(a::AbstractArrayStyle, ::Style{Tuple})    = a
 BroadcastStyle(::A, ::A) where A<:ArrayStyle             = A()
 BroadcastStyle(::ArrayStyle, ::ArrayStyle)               = Unknown()
 BroadcastStyle(::A, ::A) where A<:AbstractArrayStyle     = A()
-Base.@pure function BroadcastStyle(a::A, b::B) where {A<:AbstractArrayStyle{M},B<:AbstractArrayStyle{N}} where {M,N}
-    if Base.typename(A) === Base.typename(B)
-        return A(Val(max(M, N)))
+function BroadcastStyle(::A, ::B) where {M,N,A<:AbstractArrayStyle{M},B<:AbstractArrayStyle{N}}
+    if M!=N && M!=Any && N!=Any && A(Val(max(M,N)))==B(Val(max(M,N)))
+        A(Val(max(M,N)))
+    else
+        Unknown()
     end
-    return Unknown()
 end
 # Any specific array type beats DefaultArrayStyle
 BroadcastStyle(a::AbstractArrayStyle{Any}, ::DefaultArrayStyle) = a

test/broadcast.jl

@@ -990,3 +990,19 @@ p0 = copy(p)
     @test isequal(identity.(Vector{<:Union{Int, Missing}}[[1, 2],[missing, 1]]),
                   [[1, 2],[missing, 1]])
 end
+
+# test the dimension promotion provided by the generic
+# BroadcastStyle(::AbstractArrayStyle, ::AbstractArrayStyle)
+struct CustomStyle{T,N} <: Broadcast.AbstractArrayStyle{N} end
+Broadcast.BroadcastStyle(::CustomStyle{:A,N}, ::CustomStyle{:B,N}) where {N} = CustomStyle{:A,N}()
+CustomStyle{T,N}(::Val{M}) where {T,N,M} = CustomStyle{T,M}()
+# test this isn't spoiled:
+@test @inferred(Broadcast.result_style(CustomStyle{:A,1}(), CustomStyle{:B,1}())) ==
+      @inferred(Broadcast.result_style(CustomStyle{:B,1}(), CustomStyle{:A,1}())) == CustomStyle{:A,1}()
+# test dimension promotion works here:
+@test @inferred(Broadcast.result_style(CustomStyle{:A,1}(), CustomStyle{:A,2}())) == CustomStyle{:A,2}()
+# here the user would need to specify a custom rule:
+@test @inferred(Broadcast.result_style(CustomStyle{:A,1}(), CustomStyle{:B,2}())) ==
+      @inferred(Broadcast.result_style(CustomStyle{:B,2}(), CustomStyle{:A,1}())) ==
+      @inferred(Broadcast.result_style(CustomStyle{:A,2}(), CustomStyle{:B,1}())) ==
+      @inferred(Broadcast.result_style(CustomStyle{:B,1}(), CustomStyle{:A,2}())) == Broadcast.ArrayConflict()