Make Zero strong again (#99)

Author: YingboMa

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "0.5.4-DEV"
+version = "0.5.4"
 
 [deps]
 MuladdMacro = "46d2c3a1-f734-5fdb-9937-b9b9aeba4221"

src/differential_arithmetic.jl

@@ -47,8 +47,8 @@ for T in (:One, :AbstractThunk, :Any)
     @eval Base.:+(::Zero, b::$T) = b
     @eval Base.:+(a::$T, ::Zero) = a
 
-    @eval Base.:*(::Zero, x::$T) = zero(x)
-    @eval Base.:*(x::$T, ::Zero) = zero(x)
+    @eval Base.:*(::Zero, ::$T) = Zero()
+    @eval Base.:*(::$T, ::Zero) = Zero()
 end
 
 Base.:+(a::One, b::One) = extern(a) + extern(b)

test/differentials/zero.jl

@@ -5,8 +5,8 @@
     @test z + 1 === 1
     @test 1 + z === 1
     @test z * z === z
-    @test z * 1 === 0
-    @test 1 * z === 0
+    @test z * 1 === Zero()
+    @test 1 * z === Zero()
     for x in z
         @test x === z
     end

test/rules.jl

@@ -47,9 +47,9 @@ _second(t) = Base.tuple_type_head(Base.tuple_type_tail(t))
     @test rr1 === 1
 
     frx, nice_pushforward = frule(nice, 1, dself, 1)
-    @test nice_pushforward === 0
+    @test nice_pushforward === Zero()
     rrx, nice_pullback = rrule(nice, 1)
-    @test (NO_FIELDS, 0) === nice_pullback(1)
+    @test (NO_FIELDS, Zero()) === nice_pullback(1)
 
     sx = @SVector [1, 2]
     sy = @SVector [3, 4]