Add unary promote_op() for Bool

Fixes a small inconsistency between broadcast(+, ::Bool) and +(::Bool).
Also useful to implement comparison operators for Nullables.

Author: nalimilan

base/bool.jl

@@ -60,6 +60,8 @@ cld(x::Bool, y::Bool) = div(x,y)
 rem(x::Bool, y::Bool) = y ? false : throw(DivideError())
 mod(x::Bool, y::Bool) = rem(x,y)
 
+promote_op{T}(op::Type{T}, ::Type{Bool}) = T # to fix ambiguity
+promote_op(op, ::Type{Bool}) = typeof(op(true))
 promote_op(op, ::Type{Bool}, ::Type{Bool}) = typeof(op(true, true))
 promote_op(::typeof(^), ::Type{Bool}, ::Type{Bool}) = Bool
 promote_op{T<:Integer}(::typeof(^), ::Type{Bool}, ::Type{T}) = Bool

test/broadcast.jl

@@ -196,3 +196,8 @@ end
 let a = broadcast(Float32, [3, 4, 5])
     @test eltype(a) == Float32
 end
+
+# PR 16988
+@test Base.promote_op(+, Bool) === Int
+@test isa(broadcast(+, true), Array{Int,0})
+@test Base.promote_op(Float64, Bool) === Float64