use Base.promote_op in promote_operation_fallback

Author: kalmarek

src/interface.jl

@@ -11,16 +11,23 @@ function promote_operation_fallback(op::Function, x::Type{<:AbstractArray}, y::T
     # We replace it by a more helpful error here.
     error("`promote_operation($op, $x, $y)` not implemented yet, please report this.")
 end
-function promote_operation_fallback(::typeof(/), ::Type{S}, ::Type{T}) where {S,T}
-    return typeof(zero(S) / oneunit(T))
+
+function promote_operation_fallback(
+    op::Function,
+    ::Type{S},
+    ::Type{T},
+) where {S,T}
+    U = Base.promote_op(op, S, T)
+    return return U == Union{} ? typeof(op(oneunit(S), oneunit(T))) : U
 end
 # Julia v1.0.x has trouble with inference with the `Vararg` method, see
 # https://travis-ci.org/jump-dev/JuMP.jl/jobs/617606373
-function promote_operation_fallback(op::F, ::Type{S}, ::Type{T}) where {F<:Function,S,T}
-    return typeof(op(zero(S), zero(T)))
-end
-function promote_operation_fallback(op::F, args::Vararg{Type,N}) where {F<:Function,N}
-    return typeof(op(zero.(args)...))
+function promote_operation_fallback(
+    op::F,
+    args::Vararg{Type,N},
+) where {F<:Function,N}
+    U = Base.promote_op(op, args...)
+    return return U == Union{} ? typeof(op(oneunit.(args)...)) : U
 end
 promote_operation_fallback(::typeof(*), ::Type{T}) where {T} = T
 function promote_operation_fallback(