Heuristic for checking if a Method always errors

[mrufsvold]

@oxinabox mused here that it should be possible to use effect analysis to figure out if a method always, sometimes, or never errors.

[RFC/WIP] DuckDispatch.jl

I feel we should really be able to use effects (and a little compile-time code analysisis) to break methods down into Always throws, vs May throw, vs Never throws Probably even to getting a list of all exception types it might throw – should be a reasonable use of abstract interpetetation. Just having “Always throws” or not is enough to eliminate a ton of the false positives – namely the ones that that come from stub definitions that just throw “NotImplementedException”. And that should just …

I don’t know enough about the internals to dig into that at the moment, but it did lead me to wonder if there was a more basic heuristic I could use to at least tell if a method always errors.

I’m wondering if code_typed infers that the return type is Union{} does that guarantee that the function will error? Like this:

julia> err_f() = error("")
err_f (generic function with 1 method)

julia> code_typed(err_f, ()) |> only
CodeInfo(
1 ─     invoke Main.error(""::String)::Union{}
└──     unreachable
) => Union{}

[jar1]

Distinguish "will throw" from "may throw"

[nsajko]

@mrufsvold

infers that the return type is Union{} does that guarantee that the function will error?

An empty union return type means that the function may not ever return a value, so it either runs infinitely or throws.