Inconsistency in 1-arg and 3-arg names in :method Exprs

[timholy]

This is a re-post of timholy/Revise.jl#568:

module MyModule

function my_fun end

macro some_macro(value)
    return esc(:($MyModule.my_fun() = $value))
end

end

And then:

julia> Base.Meta.lower(MyModule, :(@some_macro 20))
:($(Expr(:thunk, CodeInfo(
    @ none within `top-level scope'
1 ─ %1 = Base.getproperty(Main.MyModule, :my_fun)
│   %2 = Core.Typeof(%1)
│   %3 = Core.svec(%2)
│   %4 = Core.svec()
│   %5 = Core.svec(%3, %4, $(QuoteNode(:(#= REPL[1]:6 =#))))
│        $(Expr(:method, nothing, :(%5), CodeInfo(
    @ REPL[1]:6 within `none'
1 ─     return 20
)))
└──      return nothing
))))

I was surprised by the nothing used for argument 1 of the 3-arg :method Expr; shouldn't it be :my_fun?

[cstjean]

Perhaps that's obvious from context, but FWIW this is on 1.6. 1.5 yields

│   %5 = Core.svec(%3, %4, $(QuoteNode(:(#= REPL[2]:8 =#))))
│        $(Expr(:method, :(Base.getproperty(Main.MyModule, :my_fun)), :(%5), CodeInfo(quote
    return 20
end)))

[Keno]

This was changed in #37287. The difference is whether or not the method can create an entirely new method or not (i.e. whether it's interpreted as an identifier or an expression).

[timholy]

To make sure I understand, you're saying this is how it should work and not a bug? If so, perhaps we should update https://docs.julialang.org/en/v1.6-dev/devdocs/ast/#Expr-types. I can do that, but I'd need to know a bit more about the conditions under which it uses nothing. I always assumed that was for anonymous functions.

[JeffBezanson]

Yes, this is how it should work. As Keno said, the difference is that f(x) = x can start with f being undefined, and assign a value to it. But M.f(x) = x requires M.f to be defined already, evaluates that expression, and uses it to determine the type to add the method to, so there is no assigned name.