Fix type inference failure in norm on structural matrix

Author: oxinabox

src/rulesets/LinearAlgebra/norm.jl

@@ -40,7 +40,10 @@ function rrule(::typeof(norm), x::AbstractArray{<:Number}, p::Real)
             end,
             # out-of-place versions
             @thunk(if isempty(x) || p == 0
-                zero.(x) .* (zero(y) * zero(real(Δy)))
+                # Note: post-julia-1.11 the zero.(Diagonal(Float64[;])) .* 0.0)
+                # only infers down to Union(Diagonal{Float64}, Matrix{Float64})
+                # rather than Diagonal{Float64}, so we cast it back.
+                maybe_withsomezeros_rewrap(x, zero.(x) .* (zero(y) * zero(real(Δy))))
             elseif p == 2
                 _norm2_back(x, y, Δy)
             elseif p == 1
@@ -72,7 +75,10 @@ function rrule(::typeof(norm), x::AbstractArray{<:Number})
             end
             ,
             @thunk(if isempty(x)
-                zero.(x) .* (zero(y) * zero(real(Δy)))
+                # Note: post-julia-1.11 the zero.(Diagonal(Float64[;])) .* 0.0)
+                # only infers down to Union(Diagonal{Float64}, Matrix{Float64})
+                # rather than Diagonal{Float64}, so we cast it back.
+                maybe_withsomezeros_rewrap(x, zero.(x) .* (zero(y) * zero(real(Δy))))
             else
                 _norm2_back(x, y, Δy)
             end)
@@ -99,7 +105,7 @@ function rrule(::typeof(norm), x::Number, p::Real)
     function norm_pullback(ȳ)
         Δy = unthunk(ȳ)
         ∂x = if iszero(Δy) || iszero(p)
-            zero(x) * zero(real(Δy))
+        zero(x) * zero(real(Δy))
         else
             signx = x isa Real ? sign(x) : x * pinv(y)
             signx * real(Δy)

src/rulesets/LinearAlgebra/utils.jl

@@ -58,7 +58,9 @@ for S in [
     :UnitLowerTriangular,
 ]
     @eval withsomezeros_rewrap(::$S, x) = $S(x)
+    @eval maybe_withsomezeros_rewrap(::$S, x) = $S(x)
 end
+maybe_withsomezeros_rewrap(::AbstractArray, x) = x
 
 # Bidiagonal, Tridiagonal have more complicated storage.
 # AdjOrTransUpperOrUnitUpperTriangular would need adjoint(parent(parent()))