From 75d3c056ce09314359a4ab5eea8b10c3572559fa Mon Sep 17 00:00:00 2001
From: Xianda Sun <sunxdt@gmail.com>
Date: Sat, 4 Apr 2026 15:37:41 +0100
Subject: [PATCH 1/8] fix Mooncake friendly_tangents compatibility

---
 .../utils.jl                                  | 27 ++++++++++++++--
 .../test/Back/Mooncake/test.jl                | 32 +++++++++++++++++++
 2 files changed, 57 insertions(+), 2 deletions(-)

diff --git a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
index b22d8d49b..ddf1d281d 100644
--- a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
+++ b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
@@ -11,9 +11,32 @@ end
 
 function zero_tangent_or_primal(x, backend::AnyAutoMooncake)
     if get_config(backend).friendly_tangents
-        # zero(x) but safer
-        return tangent_to_primal!!(_copy_output(x), zero_tangent(x))
+        # Mooncake 0.5.25+ replaced `tangent_to_primal!!` with the
+        # `tangent_to_friendly!!` framework. For this internal backup we still
+        # need a primal-shaped value, so use the `AsPrimal` path when
+        # available and fall back for older Mooncake releases.
+        return tangent_to_user_primal(zero_tangent(x), x)
     else
         return zero_tangent(x)
     end
 end
+
+@inline maybe_getfield(mod, name::Symbol) = isdefined(mod, name) ? getfield(mod, name) : nothing
+
+const mooncake_tangent_to_friendly = maybe_getfield(Mooncake, Symbol("tangent_to_friendly!!"))
+const mooncake_friendly_tangent_cache = maybe_getfield(Mooncake, :FriendlyTangentCache)
+const mooncake_as_primal = maybe_getfield(Mooncake, :AsPrimal)
+const mooncake_no_cache = maybe_getfield(Mooncake, :NoCache)
+
+function tangent_to_user_primal(tx, x)
+    if !isnothing(mooncake_tangent_to_friendly) &&
+       !isnothing(mooncake_friendly_tangent_cache) &&
+       !isnothing(mooncake_as_primal) &&
+       !isnothing(mooncake_no_cache)
+        dest = mooncake_friendly_tangent_cache{mooncake_as_primal}(_copy_output(x))
+        cache = isbitstype(typeof(x)) ? mooncake_no_cache() : IdDict{Any,Any}()
+        return mooncake_tangent_to_friendly(dest, x, tx, cache)
+    else
+        return tangent_to_primal!!(_copy_output(x), tx)
+    end
+end
diff --git a/DifferentiationInterface/test/Back/Mooncake/test.jl b/DifferentiationInterface/test/Back/Mooncake/test.jl
index d531e542a..b481b759c 100644
--- a/DifferentiationInterface/test/Back/Mooncake/test.jl
+++ b/DifferentiationInterface/test/Back/Mooncake/test.jl
@@ -1,6 +1,7 @@
 include("../../testutils.jl")
 
 using DifferentiationInterface, DifferentiationInterfaceTest
+using LinearAlgebra: Hermitian, SymTridiagonal, Symmetric
 using Mooncake: Mooncake
 using Test
 
@@ -80,3 +81,34 @@ test_differentiation(
     logging = LOGGING,
     excluded = SECOND_ORDER
 )
+
+@testset "Friendly tangents structured matrices" begin
+    backend = AutoMooncake(; config = Mooncake.Config(; friendly_tangents = true))
+    inputs = (
+        Symmetric([2.0 1.0; 1.0 3.0]),
+        Hermitian(ComplexF64[2 1 + im; 1 - im 3]),
+        SymTridiagonal([2.0, 3.0, 4.0], [5.0, 6.0]),
+    )
+    f(x) = real(sum(abs2, x))
+
+    @testset "$(typeof(x))" for x in inputs
+        grad = gradient(f, backend, x)
+        y, grad2 = value_and_gradient(f, backend, x)
+        pb = only(pullback(identity, backend, x, (x,)))
+
+        @test grad isa Matrix
+        @test grad2 isa Matrix
+        @test pb isa Matrix
+        @test grad == grad2
+        @test y == f(x)
+        @test pb == Matrix(x)
+
+        grad_dense = zero(Matrix(x))
+        @test gradient!(f, grad_dense, backend, x) === grad_dense
+        @test grad_dense == grad
+
+        tx_dense = (zero(Matrix(x)),)
+        @test only(pullback!(identity, tx_dense, backend, x, (x,))) === tx_dense[1]
+        @test tx_dense[1] == pb
+    end
+end

From f42023428e7e4a055c2b6f3a2d27d99fe63ed2be Mon Sep 17 00:00:00 2001
From: Xianda Sun <sunxdt@gmail.com>
Date: Sat, 4 Apr 2026 15:41:39 +0100
Subject: [PATCH 2/8] format Mooncake fix

---
 .../utils.jl                                  | 13 +++---
 .../test/Back/Mooncake/test.jl                | 41 ++++++++-----------
 2 files changed, 24 insertions(+), 30 deletions(-)

diff --git a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
index ddf1d281d..927a4cfbe 100644
--- a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
+++ b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
@@ -21,18 +21,21 @@ function zero_tangent_or_primal(x, backend::AnyAutoMooncake)
     end
 end
 
-@inline maybe_getfield(mod, name::Symbol) = isdefined(mod, name) ? getfield(mod, name) : nothing
+@inline maybe_getfield(mod, name::Symbol) =
+    isdefined(mod, name) ? getfield(mod, name) : nothing
 
-const mooncake_tangent_to_friendly = maybe_getfield(Mooncake, Symbol("tangent_to_friendly!!"))
+const mooncake_tangent_to_friendly = maybe_getfield(
+    Mooncake, Symbol("tangent_to_friendly!!")
+)
 const mooncake_friendly_tangent_cache = maybe_getfield(Mooncake, :FriendlyTangentCache)
 const mooncake_as_primal = maybe_getfield(Mooncake, :AsPrimal)
 const mooncake_no_cache = maybe_getfield(Mooncake, :NoCache)
 
 function tangent_to_user_primal(tx, x)
     if !isnothing(mooncake_tangent_to_friendly) &&
-       !isnothing(mooncake_friendly_tangent_cache) &&
-       !isnothing(mooncake_as_primal) &&
-       !isnothing(mooncake_no_cache)
+        !isnothing(mooncake_friendly_tangent_cache) &&
+        !isnothing(mooncake_as_primal) &&
+        !isnothing(mooncake_no_cache)
         dest = mooncake_friendly_tangent_cache{mooncake_as_primal}(_copy_output(x))
         cache = isbitstype(typeof(x)) ? mooncake_no_cache() : IdDict{Any,Any}()
         return mooncake_tangent_to_friendly(dest, x, tx, cache)
diff --git a/DifferentiationInterface/test/Back/Mooncake/test.jl b/DifferentiationInterface/test/Back/Mooncake/test.jl
index b481b759c..eb296a567 100644
--- a/DifferentiationInterface/test/Back/Mooncake/test.jl
+++ b/DifferentiationInterface/test/Back/Mooncake/test.jl
@@ -13,8 +13,8 @@ nomatrix(scens) = filter(s -> !(s.x isa AbstractMatrix) && !(s.y isa AbstractMat
 backends = [
     AutoMooncake(),
     AutoMooncakeForward(),
-    AutoMooncake(; config = Mooncake.Config(; friendly_tangents = true)),
-    AutoMooncakeForward(; config = Mooncake.Config(; friendly_tangents = true)),
+    AutoMooncake(; config=Mooncake.Config(; friendly_tangents=true)),
+    AutoMooncakeForward(; config=Mooncake.Config(; friendly_tangents=true)),
 ]
 
 for backend in backends
@@ -23,31 +23,25 @@ for backend in backends
 end
 
 test_differentiation(
-    backends[3:4],
-    default_scenarios();
-    excluded = SECOND_ORDER,
-    logging = LOGGING,
+    backends[3:4], default_scenarios(); excluded=SECOND_ORDER, logging=LOGGING
 );
 
 test_differentiation(
     backends[3:4],
     nomatrix(
         default_scenarios(;
-            include_normal = false,
-            include_constantified = true,
-            include_cachified = true,
-            use_tuples = true
-        )
+            include_normal=false,
+            include_constantified=true,
+            include_cachified=true,
+            use_tuples=true,
+        ),
     );
-    excluded = SECOND_ORDER,
-    logging = LOGGING,
+    excluded=SECOND_ORDER,
+    logging=LOGGING,
 );
 
 test_differentiation(
-    backends[1:2],
-    nomatrix(default_scenarios());
-    excluded = SECOND_ORDER,
-    logging = LOGGING,
+    backends[1:2], nomatrix(default_scenarios()); excluded=SECOND_ORDER, logging=LOGGING
 );
 
 EXCLUDED = @static if VERSION ≥ v"1.11-" && VERSION ≤ v"1.12-"
@@ -63,12 +57,12 @@ end
 test_differentiation(
     [SecondOrder(AutoMooncakeForward(), AutoMooncake())],
     nomatrix(default_scenarios());
-    excluded = EXCLUDED,
-    logging = LOGGING,
+    excluded=EXCLUDED,
+    logging=LOGGING,
 )
 
 @testset "NamedTuples" begin
-    ps = (; A = rand(5), B = rand(5))
+    ps = (; A=rand(5), B=rand(5))
     myfun(ps) = sum(ps.A .* ps.B)
     grad = gradient(myfun, backends[1], ps)
     @test grad.A == ps.B
@@ -76,14 +70,11 @@ test_differentiation(
 end
 
 test_differentiation(
-    backends[3:4],
-    nomatrix(static_scenarios());
-    logging = LOGGING,
-    excluded = SECOND_ORDER
+    backends[3:4], nomatrix(static_scenarios()); logging=LOGGING, excluded=SECOND_ORDER
 )
 
 @testset "Friendly tangents structured matrices" begin
-    backend = AutoMooncake(; config = Mooncake.Config(; friendly_tangents = true))
+    backend = AutoMooncake(; config=Mooncake.Config(; friendly_tangents=true))
     inputs = (
         Symmetric([2.0 1.0; 1.0 3.0]),
         Hermitian(ComplexF64[2 1 + im; 1 - im 3]),

From 8235e6c60398e7e9afe676ab5afb1792ce614c34 Mon Sep 17 00:00:00 2001
From: Xianda Sun <sunxdt@gmail.com>
Date: Sat, 4 Apr 2026 15:46:04 +0100
Subject: [PATCH 3/8] remove Mooncake test formatting churn

---
 .../test/Back/Mooncake/test.jl                | 41 +++++++++++--------
 1 file changed, 25 insertions(+), 16 deletions(-)

diff --git a/DifferentiationInterface/test/Back/Mooncake/test.jl b/DifferentiationInterface/test/Back/Mooncake/test.jl
index eb296a567..b481b759c 100644
--- a/DifferentiationInterface/test/Back/Mooncake/test.jl
+++ b/DifferentiationInterface/test/Back/Mooncake/test.jl
@@ -13,8 +13,8 @@ nomatrix(scens) = filter(s -> !(s.x isa AbstractMatrix) && !(s.y isa AbstractMat
 backends = [
     AutoMooncake(),
     AutoMooncakeForward(),
-    AutoMooncake(; config=Mooncake.Config(; friendly_tangents=true)),
-    AutoMooncakeForward(; config=Mooncake.Config(; friendly_tangents=true)),
+    AutoMooncake(; config = Mooncake.Config(; friendly_tangents = true)),
+    AutoMooncakeForward(; config = Mooncake.Config(; friendly_tangents = true)),
 ]
 
 for backend in backends
@@ -23,25 +23,31 @@ for backend in backends
 end
 
 test_differentiation(
-    backends[3:4], default_scenarios(); excluded=SECOND_ORDER, logging=LOGGING
+    backends[3:4],
+    default_scenarios();
+    excluded = SECOND_ORDER,
+    logging = LOGGING,
 );
 
 test_differentiation(
     backends[3:4],
     nomatrix(
         default_scenarios(;
-            include_normal=false,
-            include_constantified=true,
-            include_cachified=true,
-            use_tuples=true,
-        ),
+            include_normal = false,
+            include_constantified = true,
+            include_cachified = true,
+            use_tuples = true
+        )
     );
-    excluded=SECOND_ORDER,
-    logging=LOGGING,
+    excluded = SECOND_ORDER,
+    logging = LOGGING,
 );
 
 test_differentiation(
-    backends[1:2], nomatrix(default_scenarios()); excluded=SECOND_ORDER, logging=LOGGING
+    backends[1:2],
+    nomatrix(default_scenarios());
+    excluded = SECOND_ORDER,
+    logging = LOGGING,
 );
 
 EXCLUDED = @static if VERSION ≥ v"1.11-" && VERSION ≤ v"1.12-"
@@ -57,12 +63,12 @@ end
 test_differentiation(
     [SecondOrder(AutoMooncakeForward(), AutoMooncake())],
     nomatrix(default_scenarios());
-    excluded=EXCLUDED,
-    logging=LOGGING,
+    excluded = EXCLUDED,
+    logging = LOGGING,
 )
 
 @testset "NamedTuples" begin
-    ps = (; A=rand(5), B=rand(5))
+    ps = (; A = rand(5), B = rand(5))
     myfun(ps) = sum(ps.A .* ps.B)
     grad = gradient(myfun, backends[1], ps)
     @test grad.A == ps.B
@@ -70,11 +76,14 @@ test_differentiation(
 end
 
 test_differentiation(
-    backends[3:4], nomatrix(static_scenarios()); logging=LOGGING, excluded=SECOND_ORDER
+    backends[3:4],
+    nomatrix(static_scenarios());
+    logging = LOGGING,
+    excluded = SECOND_ORDER
 )
 
 @testset "Friendly tangents structured matrices" begin
-    backend = AutoMooncake(; config=Mooncake.Config(; friendly_tangents=true))
+    backend = AutoMooncake(; config = Mooncake.Config(; friendly_tangents = true))
     inputs = (
         Symmetric([2.0 1.0; 1.0 3.0]),
         Hermitian(ComplexF64[2 1 + im; 1 - im 3]),

From a478578e57a8a0c958c103376b4dea0156cd837f Mon Sep 17 00:00:00 2001
From: Xianda Sun <sunxdt@gmail.com>
Date: Fri, 10 Apr 2026 08:12:26 +0100
Subject: [PATCH 4/8] ci: retrigger after Mooncake v0.5.26 release


From 65997c462b8b493756bb37c1d7019fa7f6f92c81 Mon Sep 17 00:00:00 2001
From: Xianda Sun <sunxdt@gmail.com>
Date: Fri, 10 Apr 2026 08:25:15 +0100
Subject: [PATCH 5/8] style: apply Runic formatting to utils.jl

---
 .../ext/DifferentiationInterfaceMooncakeExt/utils.jl      | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
index 927a4cfbe..2184e9ecb 100644
--- a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
+++ b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
@@ -33,11 +33,11 @@ const mooncake_no_cache = maybe_getfield(Mooncake, :NoCache)
 
 function tangent_to_user_primal(tx, x)
     if !isnothing(mooncake_tangent_to_friendly) &&
-        !isnothing(mooncake_friendly_tangent_cache) &&
-        !isnothing(mooncake_as_primal) &&
-        !isnothing(mooncake_no_cache)
+            !isnothing(mooncake_friendly_tangent_cache) &&
+            !isnothing(mooncake_as_primal) &&
+            !isnothing(mooncake_no_cache)
         dest = mooncake_friendly_tangent_cache{mooncake_as_primal}(_copy_output(x))
-        cache = isbitstype(typeof(x)) ? mooncake_no_cache() : IdDict{Any,Any}()
+        cache = isbitstype(typeof(x)) ? mooncake_no_cache() : IdDict{Any, Any}()
         return mooncake_tangent_to_friendly(dest, x, tx, cache)
     else
         return tangent_to_primal!!(_copy_output(x), tx)

From 106f50fc7b1b77e9f350a45ad4fc2b867864dc8c Mon Sep 17 00:00:00 2001
From: Xianda Sun <sunxdt@gmail.com>
Date: Fri, 10 Apr 2026 10:41:25 +0100
Subject: [PATCH 6/8] test: skip friendly_tangents static_scenarios on Julia
 1.11
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Mooncake returns raw Tangent objects instead of friendly arrays for
StaticArrays on Julia 1.11. This is an upstream bug — skip the test
until it is fixed.
---
 .../test/Back/Mooncake/test.jl                    | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/DifferentiationInterface/test/Back/Mooncake/test.jl b/DifferentiationInterface/test/Back/Mooncake/test.jl
index b481b759c..7e5a291a7 100644
--- a/DifferentiationInterface/test/Back/Mooncake/test.jl
+++ b/DifferentiationInterface/test/Back/Mooncake/test.jl
@@ -75,12 +75,15 @@ test_differentiation(
     @test grad.B == ps.A
 end
 
-test_differentiation(
-    backends[3:4],
-    nomatrix(static_scenarios());
-    logging = LOGGING,
-    excluded = SECOND_ORDER
-)
+# friendly_tangents + StaticArrays broken on Julia 1.11 (upstream Mooncake bug)
+@static if !(VERSION ≥ v"1.11-" && VERSION < v"1.12-")
+    test_differentiation(
+        backends[3:4],
+        nomatrix(static_scenarios());
+        logging = LOGGING,
+        excluded = SECOND_ORDER,
+    )
+end
 
 @testset "Friendly tangents structured matrices" begin
     backend = AutoMooncake(; config = Mooncake.Config(; friendly_tangents = true))

From 7043da2edaf2fb58870902246784eb175d66aa5b Mon Sep 17 00:00:00 2001
From: Xianda Sun <sunxdt@gmail.com>
Date: Fri, 10 Apr 2026 10:44:47 +0100
Subject: [PATCH 7/8] fix: convert raw Mooncake.Tangent in pullback/gradient
 results

On Julia 1.11, Mooncake may return raw Tangent objects instead of
friendly arrays for StaticArrays even with friendly_tangents=true.
Add _maybe_to_primal dispatch as a safety net that converts leaked
Tangent objects to primal-shaped values, no-op otherwise.
---
 .../DifferentiationInterfaceMooncakeExt/onearg.jl |  8 ++++----
 .../DifferentiationInterfaceMooncakeExt/twoarg.jl |  4 ++--
 .../DifferentiationInterfaceMooncakeExt/utils.jl  |  5 +++++
 .../test/Back/Mooncake/test.jl                    | 15 ++++++---------
 4 files changed, 17 insertions(+), 15 deletions(-)

diff --git a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/onearg.jl b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/onearg.jl
index 2514cdc40..2bbf49f1c 100644
--- a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/onearg.jl
+++ b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/onearg.jl
@@ -35,7 +35,7 @@ function DI.value_and_pullback(
     new_y, (_, new_dx) = value_and_pullback!!(
         prep.cache, dy, f, x, map(DI.unwrap, contexts)...; prep.args_to_zero
     )
-    return new_y, (_copy_output(new_dx),)
+    return new_y, (_maybe_to_primal(new_dx, x),)
 end
 
 function DI.value_and_pullback(
@@ -51,7 +51,7 @@ function DI.value_and_pullback(
         y, (_, new_dx) = value_and_pullback!!(
             prep.cache, dy, f, x, map(DI.unwrap, contexts)...; prep.args_to_zero
         )
-        y, _copy_output(new_dx)
+        y, _maybe_to_primal(new_dx, x)
     end
     y = first(ys_and_tx[1])
     tx = map(last, ys_and_tx)
@@ -134,7 +134,7 @@ function DI.value_and_gradient(
         prep.cache, f, x, map(DI.unwrap, contexts)...;
         prep.args_to_zero
     )
-    return y, _copy_output(new_grad)
+    return y, _maybe_to_primal(new_grad, x)
 end
 
 function DI.value_and_gradient!(
@@ -150,7 +150,7 @@ function DI.value_and_gradient!(
         prep.cache, f, x, map(DI.unwrap, contexts)...;
         prep.args_to_zero
     )
-    copyto!(grad, new_grad)
+    copyto!(grad, _maybe_to_primal(new_grad, x))
     return y, grad
 end
 
diff --git a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/twoarg.jl b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/twoarg.jl
index 2b55131b9..ed7f4ca9c 100644
--- a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/twoarg.jl
+++ b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/twoarg.jl
@@ -64,7 +64,7 @@ function DI.value_and_pullback(
         prep.args_to_zero
     )
     copyto!(y, y_after)
-    return y, (_copy_output(dx),)
+    return y, (_maybe_to_primal(dx, x),)
 end
 
 function DI.value_and_pullback(
@@ -90,7 +90,7 @@ function DI.value_and_pullback(
             prep.args_to_zero
         )
         copyto!(y, y_after)
-        _copy_output(dx)
+        _maybe_to_primal(dx, x)
     end
     return y, tx
 end
diff --git a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
index 2184e9ecb..5be9097b3 100644
--- a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
+++ b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
@@ -21,6 +21,11 @@ function zero_tangent_or_primal(x, backend::AnyAutoMooncake)
     end
 end
 
+# Safety net: if Mooncake returns a raw Tangent (e.g. Julia 1.11 + StaticArrays),
+# convert it to a primal-shaped value. No-op for already-converted results.
+_maybe_to_primal(tx, x) = _copy_output(tx)
+_maybe_to_primal(tx::Mooncake.Tangent, x) = tangent_to_user_primal(tx, x)
+
 @inline maybe_getfield(mod, name::Symbol) =
     isdefined(mod, name) ? getfield(mod, name) : nothing
 
diff --git a/DifferentiationInterface/test/Back/Mooncake/test.jl b/DifferentiationInterface/test/Back/Mooncake/test.jl
index 7e5a291a7..313c0d2bf 100644
--- a/DifferentiationInterface/test/Back/Mooncake/test.jl
+++ b/DifferentiationInterface/test/Back/Mooncake/test.jl
@@ -75,15 +75,12 @@ test_differentiation(
     @test grad.B == ps.A
 end
 
-# friendly_tangents + StaticArrays broken on Julia 1.11 (upstream Mooncake bug)
-@static if !(VERSION ≥ v"1.11-" && VERSION < v"1.12-")
-    test_differentiation(
-        backends[3:4],
-        nomatrix(static_scenarios());
-        logging = LOGGING,
-        excluded = SECOND_ORDER,
-    )
-end
+test_differentiation(
+    backends[3:4],
+    nomatrix(static_scenarios());
+    logging = LOGGING,
+    excluded = SECOND_ORDER,
+)
 
 @testset "Friendly tangents structured matrices" begin
     backend = AutoMooncake(; config = Mooncake.Config(; friendly_tangents = true))

From ce72bafde06c0427f022cdd93703a7d95c4ed6b7 Mon Sep 17 00:00:00 2001
From: Xianda Sun <sunxdt@gmail.com>
Date: Fri, 10 Apr 2026 11:57:34 +0100
Subject: [PATCH 8/8] fix: handle MutableTangent and forward mode tangent leaks

Also convert leaked Mooncake.MutableTangent (e.g. MVector tangents)
and apply _maybe_to_primal in forward mode (pushforward) paths.
---
 .../ext/DifferentiationInterfaceMooncakeExt/forward_onearg.jl | 2 +-
 .../ext/DifferentiationInterfaceMooncakeExt/forward_twoarg.jl | 4 ++--
 .../ext/DifferentiationInterfaceMooncakeExt/utils.jl          | 1 +
 3 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/forward_onearg.jl b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/forward_onearg.jl
index c470b6473..cab7f84d6 100644
--- a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/forward_onearg.jl
+++ b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/forward_onearg.jl
@@ -41,7 +41,7 @@ function DI.value_and_pushforward(
             map(first_unwrap, contexts, prep.context_tangents)...,
         )
         y = first(y_and_dy)
-        dy = _copy_output(last(y_and_dy))
+        dy = _maybe_to_primal(last(y_and_dy), y)
         return y, dy
     end
     y = _copy_output(first(ys_and_ty[1]))
diff --git a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/forward_twoarg.jl b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/forward_twoarg.jl
index 3c75f530b..8ebb6ef99 100644
--- a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/forward_twoarg.jl
+++ b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/forward_twoarg.jl
@@ -55,7 +55,7 @@ function DI.value_and_pushforward(
             (x, dx),
             map(first_unwrap, contexts, prep.context_tangents)...,
         )
-        return _copy_output(new_dy)
+        return _maybe_to_primal(new_dy, y)
     end
     return y, ty
 end
@@ -93,7 +93,7 @@ function DI.value_and_pushforward!(
             (x, dx),
             map(first_unwrap, contexts, prep.context_tangents)...,
         )
-        copyto!(dy, new_dy)
+        copyto!(dy, _maybe_to_primal(new_dy, y))
     end
     return y, ty
 end
diff --git a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
index 5be9097b3..beeb6f611 100644
--- a/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
+++ b/DifferentiationInterface/ext/DifferentiationInterfaceMooncakeExt/utils.jl
@@ -25,6 +25,7 @@ end
 # convert it to a primal-shaped value. No-op for already-converted results.
 _maybe_to_primal(tx, x) = _copy_output(tx)
 _maybe_to_primal(tx::Mooncake.Tangent, x) = tangent_to_user_primal(tx, x)
+_maybe_to_primal(tx::Mooncake.MutableTangent, x) = tangent_to_user_primal(tx, x)
 
 @inline maybe_getfield(mod, name::Symbol) =
     isdefined(mod, name) ? getfield(mod, name) : nothing