nicer errors when called on zero inputs

src/layers/basic.jl

@@ -572,7 +572,8 @@ end
 
 function _parallel_check(layers, xs)
   nl = length(layers)
-  nx = length(xs) 
+  @assert nl > 1  # dispatch handles nl==1 cases
+  nx = length(xs)
   if (nl != nx)
     throw(ArgumentError(lazy"Parallel with $nl > 1 sub-layers can take one input or $nl inputs, but got $nx inputs"))
   end
@@ -591,6 +592,8 @@ end
 (m::Parallel)(xs::Tuple) = m(xs...)  # tuple is always splatted
 (m::_ParallelONE)(xs::Tuple) = m(xs...)  # solves an ambiguity
 
+(m::Parallel)() = throw(ArgumentError("Parallel layer cannot take 0 inputs"))
+
 Base.getindex(m::Parallel, i) = m.layers[i]
 Base.getindex(m::Parallel, i::AbstractVector) = Parallel(m.connection, m.layers[i])
 Base.getindex(m::Parallel{<:Any, <:NamedTuple}, i::AbstractVector) =

test/layers/basic.jl

@@ -261,8 +261,15 @@ using Flux: activations
     end
 
     @testset "trivial cases" begin
+      # zero inputs, always an error
+      @test_throws ArgumentError Parallel(hcat)()
+      @test_throws ArgumentError Parallel(hcat, inv)()
+      @test_throws ArgumentError Parallel(hcat, inv, sqrt)()
+
+      # zero layers -- not useful... can we make this an error without a breaking change?
       @test Parallel(hcat) isa Parallel{typeof(hcat), Tuple{}}  # not a NamedTuple
       @test Parallel(hcat)(1) == hcat()
+
       @test Parallel(hcat, inv)(2) == hcat(1/2)  # still calls connection once.
     end