promotion: symmetrize promotion rules recursively with promote_type.

If promote_rule(T,S) and promote_rule(S,T) disagree, call promote_type
again on the results to get a single common answer. This can resolve
cases where there is a conflict between two such rules, making the
promotion system a bit more robust.

base/promotion.jl

@@ -112,7 +112,7 @@ promote_type{T,S}(::Type{T}, ::Type{S}) =
 
 promote_rule(T, S) = None
 
-promote_result(t,s,T,S) = typejoin(T,S)
+promote_result(t,s,T,S) = promote_type(T,S)
 # If no promote_rule is defined, both directions give None. In that
 # case use typejoin on the original types instead.
 promote_result{T,S}(::Type{T},::Type{S},::Type{None},::Type{None}) = typejoin(T, S)

test/numbers.jl

@@ -1742,3 +1742,8 @@ for T = (Uint8,Int8,Uint16,Int16,Uint32,Int32,Uint64,Int64,Uint128,Int128)
         @test convert(T,n*(n^typemax(T))) == one(T)
     end
 end
+
+@test is(false*pi, 0.0)
+@test is(true*pi, float64(pi))
+@test is(pi*false, 0.0)
+@test is(pi*true, float64(pi))