Use only safe axis types for Broadcast.combine_axes

JuliaLang#30074 used the wrong notion of consistency since `OneTo(1)` is
consistent (wrt broadcasting) with any range, but `OneTo` cannot
handle `Slice(-1:1)`.

base/broadcast.jl

@@ -435,19 +435,16 @@ end
 _bcs1(a::Integer, b::Integer) = a == 1 ? b : (b == 1 ? a : (a == b ? a : throw(DimensionMismatch("arrays could not be broadcast to a common size"))))
 _bcs1(a::Integer, b) = a == 1 ? b : (first(b) == 1 && last(b) == a ? b : throw(DimensionMismatch("arrays could not be broadcast to a common size")))
 _bcs1(a, b::Integer) = _bcs1(b, a)
-_bcs1(a, b) = _bcsm(b, a) ? _sametype(b, a) : (_bcsm(a, b) ? _sametype(a, b) : throw(DimensionMismatch("arrays could not be broadcast to a common size")))
+_bcs1(a, b) = _bcsm(b, a) ? axistype(b, a) : (_bcsm(a, b) ? axistype(a, b) : throw(DimensionMismatch("arrays could not be broadcast to a common size")))
 # _bcsm tests whether the second index is consistent with the first
 _bcsm(a, b) = a == b || length(b) == 1
 _bcsm(a, b::Number) = b == 1
 _bcsm(a::Number, b::Number) = a == b || b == 1
 # Ensure inferrability when dealing with axes of different AbstractUnitRange types
 # (We may not want to define general promotion rules between, say, OneTo and Slice, but if
-# we get here we know the axes are at least consistent)
-_sametype(a::T, b::T) where T = a
-_sametype(a::OneTo, b::OneTo) = OneTo{Int}(a)
-_sametype(a::OneTo, b) = OneTo{Int}(a)
-_sametype(a, b::OneTo) = OneTo{Int}(a)
-_sametype(a, b) = UnitRange{Int}(a)
+# we get here we know the axes are at least consistent for the purposes of broadcasting)
+axistype(a::T, b::T) where T = a
+axistype(a, b) = UnitRange{Int}(a)
 
 ## Check that all arguments are broadcast compatible with shape
 # comparing one input against a shape

test/offsetarray.jl

@@ -470,6 +470,10 @@ A = OffsetArray(view(rand(4,4), 1:4, 4:-1:1), (-3,5))
 a = [1]
 b = OffsetArray(a, (0,))
 @test @inferred(a .+ b) == [2]
+a = OffsetArray([1, -2, 1], (-2,))
+@test a .* a' == OffsetArray([ 1 -2 1;
+ -2 4 -2;
+ 1 -2 1], (-2,-2))
 
 end # let