Fix rrules for Symmetric and Diagonal constructors (#23)

Currently these definitions are extending `rrule(::typeof(T), x)` where
`T` is a type. However, `typeof(Diagonal) == UnionAll`, which means this
is not defining the method it looks like it might be defining. The only
reason this worked when originally implemented was that one of the
`rrule` definitions was for `rrule(UnionAll, Matrix)` and the other for
`rrule(UnionAll, Vector)`, so dispatch still worked.

This replaces these problematic `::typeof(T)`s with `::Type{<:T}`.

Author: ararslan

src/rules/linalg/diagonal.jl

@@ -1,2 +1,2 @@
-rrule(::typeof(Diagonal), d::AbstractVector) = Diagonal(d), Rule(diag)
+rrule(::Type{<:Diagonal}, d::AbstractVector) = Diagonal(d), Rule(diag)
 rrule(::typeof(diag), A::AbstractMatrix) = diag(A), Rule(Diagonal)

src/rules/linalg/symmetric.jl

@@ -1,4 +1,4 @@
-rrule(::typeof(Symmetric), A::AbstractMatrix) = Symmetric(A), Rule(_symmetric_back)
+rrule(::Type{<:Symmetric}, A::AbstractMatrix) = Symmetric(A), Rule(_symmetric_back)
 
 _symmetric_back(ΔΩ) = UpperTriangular(ΔΩ) + LowerTriangular(ΔΩ)' - Diagonal(ΔΩ)
 _symmetric_back(ΔΩ::Union{Diagonal, UpperTriangular}) = ΔΩ

test/rules/linalg/diagonal.jl

@@ -2,7 +2,10 @@
     @testset "Diagonal" begin
         rng, N = MersenneTwister(123456), 3
         rrule_test(Diagonal, randn(rng, N, N), (randn(rng, N), randn(rng, N)))
-        rrule_test(Diagonal, Diagonal(randn(rng, N)), (randn(rng, N), randn(rng, N)))
+        D = Diagonal(randn(rng, N))
+        rrule_test(Diagonal, D, (randn(rng, N), randn(rng, N)))
+        # Concrete type instead of UnionAll
+        rrule_test(typeof(D), D, (randn(rng, N), randn(rng, N)))
     end
     @testset "diag" begin
         rng, N = MersenneTwister(123456), 7