generic frule for constructors?

Like for https://github.com/JuliaDiff/ChainRules.jl/issues/153
there is a generic frule for default constructors.
It relies on the arguments being the same as the fields so would have to detect that.

Its basically:

function frule(::Type{P}, args..., _, dargs...) where P
     y  = P(args...)
     dy = Composite{P}(; zip(fieldnames(P, dargs)...)
end

the more serious implementation might be:

function frule(::Type{P}, all_args...) where P
     nargs = fieldcount(P)
     length(all_args) == 2nargs + 1 || return nothing
     args = @inbounds all_args[1:nargs]
     all(typeof.(args) .== fieldtypes(P)) || return nothing
     dargs = @inbounds all_args[end-nargs : end]  # skip dself as constructors never functors
     y  = P(args...)
     dy = Composite{P}(; zip(fieldnames(P, dargs)...)
end

But this may well need to be written as a generated function if it doesn't constant fold good.

Start by reviewing ChainRules.jl issue #153 and the existing frule handling for default constructors. The issue names no implementation file or test; determine whether matching constructor arguments to field types and producing the requested primal and tangent values needs a generated function, then establish tests for the supported and rejected signatures.