Fails to descend into Type constructors

[mbauman]

For example:

julia> @descend Array{Int}(undef, 2)
ERROR: MethodError: no method matching _descend(::Nothing; params=Core.Compiler.Params(Core.Compiler.InferenceResult[], 0x0000000000006741, true, true, true, false, 100, 1000, 400, 4, 4, 8, 3, 32), iswarn=false)
Closest candidates are:
  _descend(::Any, ::Any; params, kwargs...) at /home/mbauman/.julia/packages/Cthulhu/c2nCD/src/Cthulhu.jl:202
  _descend(::Core.MethodInstance; iswarn, params, optimize, kwargs...) at /home/mbauman/.julia/packages/Cthulhu/c2nCD/src/Cthulhu.jl:112
Stacktrace:
 [1] #_descend#37(::Core.Compiler.Params, ::Base.Iterators.Pairs{Symbol,Bool,Tuple{Symbol},NamedTuple{(:iswarn,),Tuple{Bool}}}, ::typeof(Cthulhu._descend), ::Any, ::Any) at /home/mbauman/.julia/packages/Cthulhu/c2nCD/src/Cthulhu.jl:203
 [2] #_descend at ./none:0 [inlined]
 [3] #_descend_with_error_handling#31(::Base.Iterators.Pairs{Symbol,Bool,Tuple{Symbol},NamedTuple{(:iswarn,),Tuple{Bool}}}, ::typeof(Cthulhu._descend_with_error_handling), ::Type, ::Any) at /home/mbauman/.julia/packages/Cthulhu/c2nCD/src/Cthulhu.jl:88
 [4] #_descend_with_error_handling at ./none:0 [inlined]
 [5] #descend_code_typed#30 at /home/mbauman/.julia/packages/Cthulhu/c2nCD/src/Cthulhu.jl:62 [inlined]
 [6] descend_code_typed(::Type, ::Any) at /home/mbauman/.julia/packages/Cthulhu/c2nCD/src/Cthulhu.jl:62
 [7] top-level scope at REPL[32]:1

Of course this example wouldn't be terribly interesting, but this came from a different example where the constructor was actually doing something I wanted to look at.

[vchuravy]

Interesting.

julia> Base._methods_by_ftype(Tuple{Array{Int}, typeof(undef), Int}, -1, typemax(UInt))
0-element Array{Any,1}

julia> code_typed(Array{Int}, (typeof(undef), Int))
1-element Array{Any,1}:
 CodeInfo(
1 ─ %1 = $(Expr(:foreigncall, :(:jl_alloc_array_1d), Array{Int64,1}, svec(Any, Int64), :(:ccall), 2, Array{Int64,1}, :(m), :(m)))::Array{Int64,1}
└──      return %1
) => Array{Int64,1}

The former method is what Cthulhu uses.

[vchuravy]

Ah:

Base._methods(Array{Int}, Tuple{typeof(undef), Int}, -1, typemax(UInt))
1-element Array{Any,1}:
 svec(Tuple{Type{Array{Int64,N} where N},UndefInitializer,Int64}, svec(Int64), (::Type{Array{T,N} where N})(::UndefInitializer, m::Int64) where T in Core at boot.jl:416)

[mbauman]

Thanks!