From 0eb9ee1a74e58f94e576b66551b9d7f527db47b5 Mon Sep 17 00:00:00 2001
From: Yichao Yu <yyc1992@gmail.com>
Date: Fri, 7 Aug 2026 14:04:08 -0400
Subject: [PATCH] Remove unnecessary rule for hvcat_fill!

The new version in base should be type stable now
---
 src/internal_rules/core.jl | 108 +++++++++++++++++++------------------
 test/basic.jl              |  24 +++++----
 2 files changed, 68 insertions(+), 64 deletions(-)

diff --git a/src/internal_rules/core.jl b/src/internal_rules/core.jl
index 5aeca8fcb9..c1f2310abc 100644
--- a/src/internal_rules/core.jl
+++ b/src/internal_rules/core.jl
@@ -486,66 +486,68 @@ function EnzymeRules.reverse(
     return ntuple(Returns(nothing), Val(2 + length(args)))
 end
 
-# Force a rule around hvcat_fill as it is type unstable if the tuple is not of the same type (e.g., int, float, int, float)
-function EnzymeRules.augmented_primal(
-    config::EnzymeRules.RevConfig,
-    func::Const{typeof(Base.hvcat_fill!)},
-    ::Type{RT},
-    out::Annotation{AT},
-    inp::Annotation{BT},
-) where {RT,AT<:Array,BT<:Tuple}
-    primal = if EnzymeRules.needs_primal(config)
-        out.val
-    else
-        nothing
-    end
-    shadow = if EnzymeRules.needs_shadow(config)
-        out.dval
-    else
-        nothing
-    end
-    func.val(out.val, inp.val)
-    return EnzymeRules.AugmentedReturn(primal, shadow, nothing)
-end
-
-function EnzymeRules.reverse(
-    config::EnzymeRules.RevConfig,
-    func::Const{typeof(Base.hvcat_fill!)},
-    ::Type{RT},
-    _,
-    out::Annotation{AT},
-    inp::Annotation{BT},
-) where {RT,AT<:Array,BT<:Tuple}
-    nr, nc = size(out.val, 1), size(out.val, 2)
-    for b = 1:EnzymeRules.width(config)
-        da = if EnzymeRules.width(config) == 1
+if isdefined(Base, :hvcat_fill!)
+    # Force a rule around hvcat_fill as it is type unstable if the tuple is not of the same type (e.g., int, float, int, float)
+    function EnzymeRules.augmented_primal(
+        config::EnzymeRules.RevConfig,
+        func::Const{typeof(Base.hvcat_fill!)},
+        ::Type{RT},
+        out::Annotation{AT},
+        inp::Annotation{BT},
+        ) where {RT,AT<:Array,BT<:Tuple}
+        primal = if EnzymeRules.needs_primal(config)
+            out.val
+        else
+            nothing
+        end
+        shadow = if EnzymeRules.needs_shadow(config)
             out.dval
         else
-            out.dval[b]
+            nothing
         end
-        i = 1
-        j = 1
-        if (typeof(inp) <: Active)
-            dinp = ntuple(Val(length(inp.val))) do k
-                Base.@_inline_meta
-                res = da[i, j]
-                da[i, j] = 0
-                j += 1
-                if j == nc + 1
-                    i += 1
-                    j = 1
-                end
-                T = BT.parameters[k]
-                if T <: AbstractFloat
-                    T(res)
-                else
-                    T(0)
+        func.val(out.val, inp.val)
+        return EnzymeRules.AugmentedReturn(primal, shadow, nothing)
+    end
+
+    function EnzymeRules.reverse(
+        config::EnzymeRules.RevConfig,
+        func::Const{typeof(Base.hvcat_fill!)},
+        ::Type{RT},
+        _,
+        out::Annotation{AT},
+        inp::Annotation{BT},
+        ) where {RT,AT<:Array,BT<:Tuple}
+        nr, nc = size(out.val, 1), size(out.val, 2)
+        for b = 1:EnzymeRules.width(config)
+            da = if EnzymeRules.width(config) == 1
+                out.dval
+            else
+                out.dval[b]
+            end
+            i = 1
+            j = 1
+            if (typeof(inp) <: Active)
+                dinp = ntuple(Val(length(inp.val))) do k
+                    Base.@_inline_meta
+                    res = da[i, j]
+                    da[i, j] = 0
+                    j += 1
+                    if j == nc + 1
+                        i += 1
+                        j = 1
+                    end
+                    T = BT.parameters[k]
+                    if T <: AbstractFloat
+                        T(res)
+                    else
+                        T(0)
+                    end
                 end
+                return (nothing, dinp)::Tuple{Nothing,BT}
             end
-            return (nothing, dinp)::Tuple{Nothing,BT}
         end
+        return (nothing, nothing)
     end
-    return (nothing, nothing)
 end
 
 function EnzymeRules.forward(config, ::Const{typeof(Base.finalizer)}, _, f::Const, o)
diff --git a/test/basic.jl b/test/basic.jl
index 2f73e7718f..e29f75e0e4 100644
--- a/test/basic.jl
+++ b/test/basic.jl
@@ -618,17 +618,19 @@ end
 end
 
 @testset "hvcat_fill" begin
-    ar = Matrix{Float64}(undef, 2, 3)
-    dar = [1.0 2.0 3.0; 4.0 5.0 6.0]
-
-    res = first(Enzyme.autodiff(Reverse, Base.hvcat_fill!, Const, Duplicated(ar, dar), Active((1, 2.2, 3, 4.4, 5, 6.6))))
-
-    @test res[2][1] == 0
-    @test res[2][2] ≈ 2.0
-    @test res[2][3] ≈ 0
-    @test res[2][4] ≈ 4.0
-    @test res[2][5] ≈ 0
-    @test res[2][6] ≈ 6.0
+    if isdefined(Base, :hvcat_fill!)
+        ar = Matrix{Float64}(undef, 2, 3)
+        dar = [1.0 2.0 3.0; 4.0 5.0 6.0]
+
+        res = first(Enzyme.autodiff(Reverse, Base.hvcat_fill!, Const, Duplicated(ar, dar), Active((1, 2.2, 3, 4.4, 5, 6.6))))
+
+        @test res[2][1] == 0
+        @test res[2][2] ≈ 2.0
+        @test res[2][3] ≈ 0
+        @test res[2][4] ≈ 4.0
+        @test res[2][5] ≈ 0
+        @test res[2][6] ≈ 6.0
+    end
 end
 
 function named_deepcopy(x, nt)