Wct/fix repeats euclidean (#1307)

* Add failing test

* Fix repeated inputs

* Remove comment

* Undo change

* Uncomment tests

* Bump patch version

* Defensive choice of zero

Co-authored-by: Miha Zgubic <mzgubic@users.noreply.github.com>

Co-authored-by: Miha Zgubic <mzgubic@users.noreply.github.com>

Project.toml

@@ -1,6 +1,6 @@
 name = "Zygote"
 uuid = "e88e6eb3-aa80-5325-afca-941959d7151f"
-version = "0.6.47"
+version = "0.6.48"
 
 [deps]
 AbstractFFTs = "621f4979-c628-5d54-868e-fcf4e3e8185c"

src/lib/distances.jl

@@ -79,11 +79,16 @@ end
 end
 
 @adjoint function pairwise(::Euclidean, X::AbstractMatrix; dims=2)
-  D, back = pullback(X -> pairwise(SqEuclidean(), X; dims = dims), X)
-  D .= sqrt.(D)
-  return D, function(Δ)
-    Δ = Δ ./ (2 .* max.(D, eps(eltype(D))))
-    Δ[diagind(Δ)] .= 0
-    return (nothing, first(back(Δ)))
+
+  _conditional(d, δ) = d > δ ? sqrt(d) : zero(d)
+
+  function _pairwise_euclidean(X)
+    δ = eps(eltype(X))^2
+    D2 = pairwise(SqEuclidean(), X; dims=dims)
+    return _conditional.(D2, δ)
   end
+  D, back = pullback(_pairwise_euclidean, X)
+
+  _pairwise_pullback(Δ) = (nothing, back(Δ)...)
+  return D, _pairwise_pullback
 end

test/gradcheck.jl

@@ -1192,6 +1192,19 @@ end
         # This is impressively inaccurate, but at least it doesn't produce a NaN.
         @test first(Δ_fd) ≈ first(pb(Δ)) atol=1e-3 rtol=1e-3
       end
+
+      @testset "repeated X" begin
+        Δ = randn(P, P)
+        X = repeat(randn(rng, D), 1, P)
+
+        Δ_fd = FiniteDifferences.j′vp(
+          FiniteDifferences.central_fdm(5, 1), X -> pairwise(metric, X; dims=2), Δ, X
+        )
+        _, pb = Zygote.pullback(X -> pairwise(metric, X; dims=2), X)
+
+        # This is impressively inaccurate, but at least it doesn't produce a NaN.
+        @test first(Δ_fd) ≈ first(pb(Δ)) atol=1e-3 rtol=1e-3
+      end
     end
 
     @testset "binary pairwise - X and Y close" begin