Nicer error for ambiguous methods

[penelopeysm]

Libtask.jl/src/copyable_task.jl

Lines 74 to 88 in f154425

	function build_callable(sig::Type{<:Tuple})
	if sig <: Tuple{typeof(produce),Any}
	msg = """
	Can not construct a TapedTask for a 'naked' call to `produce`.
	Please wrap the call to `produce` in a function, and construct a
	TapedTask from that function."""
	throw(ArgumentError(msg))
	end
	key = CacheKey(Base.get_world_counter(), sig)
	if haskey(mc_cache, key)
	return fresh_copy(mc_cache[key])
	else
	ir = Base.code_ircode_by_type(sig)[1][1]
	# Check whether this is a varargs call.
	isva = which(sig).isva

If Libtask attempts to construct IR for an ambiguous method, line 86 here will error with something like this. It's at least not impossible to guess where is going wrong (based on the signature in [4]), but the error message could definitely be improved here too.

ERROR: BoundsError: attempt to access 0-element Vector{Any} at index [1]
Stacktrace:
  [1] throw_boundserror(A::Vector{Any}, I::Tuple{Int64})
    @ Base ./essentials.jl:14
  [2] getindex
    @ ./essentials.jl:916 [inlined]
  [3] build_callable(sig::Type{Tuple{…}})
    @ Libtask ~/.julia/packages/Libtask/Sf4tJ/src/copyable_task.jl:86
  [4] (::Libtask.DynamicCallable{…})(::Function, ::Turing.Inference.ParticleMCMCContext{…}, ::Cauchy{…}, ::Float64, ::AbstractPPL.VarName{…}, ::DynamicPPL.VarInfo{…})
    @ Libtask ~/.julia/packages/Libtask/Sf4tJ/src/copyable_task.jl:1163
  [5] f
    @ ./REPL[6]:1 [inlined]
  [6] (::Tuple{…})(none::typeof(f), none::DynamicPPL.Model{…}, none::DynamicPPL.VarInfo{…}, none::Float64)
    @ Base.Experimental ./<missing>:0
  [7] consume
    @ ~/.julia/packages/Libtask/Sf4tJ/src/copyable_task.jl:347 [inlined]
  [8] advance!(trace::AdvancedPS.Trace{AdvancedPS.LibtaskModel{…}, AdvancedPS.TracedRNG{…}}, isref::Bool)
    @ Turing.Inference ~/ppl/lib/src/mcmc/particle_mcmc.jl:68
  [9] reweight!(pc::AdvancedPS.ParticleContainer{AdvancedPS.Trace{…}, AdvancedPS.TracedRNG{…}}, ref::Noth