From 9ee2c69f27dce129019903a420f4709cdb0b20eb Mon Sep 17 00:00:00 2001
From: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
Date: Wed, 13 Mar 2024 11:55:12 -0400
Subject: [PATCH] disallow zero layers, let's try this out

---
 src/layers/basic.jl  | 3 ++-
 test/layers/basic.jl | 7 ++-----
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/src/layers/basic.jl b/src/layers/basic.jl
index 3f57d628ca..a240a089f9 100644
--- a/src/layers/basic.jl
+++ b/src/layers/basic.jl
@@ -558,9 +558,10 @@ function Parallel(connection; kw...)
   if :layers in keys(layers) || :connection in keys(layers)
     throw(ArgumentError("a Parallel layer cannot have a named sub-layer called `connection` or `layers`"))
   end
-  isempty(layers) && return Parallel(connection, ())
   Parallel(connection, layers)
 end
+Parallel(connection, layers::Union{Tuple{}, @NamedTuple{}}) =
+    throw(ArgumentError("cannot construct a Parallel layer with no sub-layers"))
 
 @layer :expand Parallel
 
diff --git a/test/layers/basic.jl b/test/layers/basic.jl
index d09579c257..8e33340611 100644
--- a/test/layers/basic.jl
+++ b/test/layers/basic.jl
@@ -266,11 +266,8 @@ using Flux: activations
       @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.
+      # zero layers -- not useful... now made an error
+      @test_throws ArgumentError Parallel(hcat)
     end
 
     @testset "connection is called once" begin