Explicitly don't support mul! for adjoint plans

Author: gaurav-arya

src/definitions.jl

@@ -643,3 +643,6 @@ function _mul(p::AdjointPlan{T}, x::AbstractArray, ::RealInverseProjectionStyle)
 end
 
 plan_inv(p::AdjointPlan) = adjoint(plan_inv(p.p))
+function LinearAlgebra.mul!(y::AbstractArray, p::AdjointPlan, x::AbstractArray)
+    throw(MethodError(LinearAlgebra.mul!, "mul! is not supported for adjoint plans"))
+end

test/runtests.jl

@@ -246,6 +246,7 @@ end
                 @test size(Pinv') == AbstractFFTs.output_size(Pinv) 
                 @test dot(x, Pinv * y) ≈ dot(Pinv' * x, y)
                 @test dot(x, Pinv \ y) ≈ dot(Pinv' \ x, y)
+                @test_throws MethodError mul!(x, P', y)
             end
         end
     end