add == and hash for MutableTangent

Author: oxinabox

src/tangent_types/structural_tangent.jl

@@ -5,6 +5,7 @@ Representing the type of the tangent of a `struct` `P` (or a `Tuple`/`NamedTuple
 as an object with mirroring fields.
 
 !!!!!! warning Exprimental
+    `StructuralTangent` is an experimental feature, and is part of the mutation support featureset.
     The `StructuralTangent` constructor returns a `MutableTangent` for mutable structs.
     `MutableTangent` is an experimental feature.
     Thus use of `StructuralTangent` (rather than `Tangent` directly) is also experimental.
@@ -409,7 +410,7 @@ This type represents the tangent to a mutable struct.
 It itself is also mutable.
 
 !!! warning Exprimental
-    MutableTangent is an experimental feature.
+    MutableTangent is an experimental feature, and is part of the mutation support featureset.
     While this notice remains it may have changes in behavour, and interface in any _minor_ version of ChainRulesCore.
     Exactly how it should be used (e.g. is it forward-mode only?)
 
@@ -440,4 +441,10 @@ function Base.setproperty!(tangent::MutableTangent, idx::Int, x)
     return setproperty!(tangent, name, x)
 end
 
-idx2sym(::NamedTuple{names}, idx) where names = names[idx]
+idx2sym(::NamedTuple{names}, idx) where names = names[idx]
+
+Base.hash(tangent::MutableTangent, h::UInt64) = hash(backing(tangent), h)
+function Base.:(==)(t1::MutableTangent{T1}, t2::MutableTangent{T2}) where {T1, T2}
+    typeintersect(T1, T2) == Union{} && return false
+    backing(t1)==backing(t2)
+end

test/tangent_types/structural_tangent.jl

@@ -432,14 +432,28 @@ end
         return y, ẏ
     end
 
-    obj = MDemo(99.0)
-    ∂obj = MutableTangent{MDemo}(;x=1.5)
-    frule((NoTangent(), ∂obj, NoTangent(), 10.0), setfield!, obj, :x, 95.0)
-    @test ∂obj.x == 10.0
-    @test obj.x == 95.0
-
-    frule((NoTangent(), ∂obj, NoTangent(), 20.0), setfield!, obj, 1, 96.0)
-    @test ∂obj.x == 20.0
-    @test getproperty(∂obj, 1) == 20.0
-    @test obj.x == 96.0
+    @testset "usecase" begin
+        obj = MDemo(99.0)
+        ∂obj = MutableTangent{MDemo}(;x=1.5)
+        frule((NoTangent(), ∂obj, NoTangent(), 10.0), setfield!, obj, :x, 95.0)
+        @test ∂obj.x == 10.0
+        @test obj.x == 95.0
+
+        frule((NoTangent(), ∂obj, NoTangent(), 20.0), setfield!, obj, 1, 96.0)
+        @test ∂obj.x == 20.0
+        @test getproperty(∂obj, 1) == 20.0
+        @test obj.x == 96.0
+    end
+
+    @testset "== and hash" begin
+        @test MutableTangent{Any}(x=1.0) == MutableTangent{MDemo}(x=1.0)
+        @test MutableTangent{MDemo}(x=1.0) == MutableTangent{Any}(x=1.0)
+        @test MutableTangent{Any}(x=2.0) != MutableTangent{MDemo}(x=1.0)
+        @test MutableTangent{MDemo}(x=1.0) != MutableTangent{Any}(x=2.0)
+
+        nt = (;x=1.0)
+        @test MutableTangent{typeof(nt)}(nt) != MutableTangent{MDemo}(x=1.0)
+
+        @test hash(MutableTangent{Any}(x=1.0)) == hash(MutableTangent{MDemo}(x=1.0))
+    end
 end