Inference regression in julia 1.7

[jw3126]

The following snipped infers in 1.6 and even 1.7-rc1 but does not anymore. Came up here:

using Accessors
using Test
struct TT{A,B}
    a::A
    b::B
end
lens = @optic(_.b.a.b[2])
obj = TT(2, TT(TT(3,(4,4)), 2))
@inferred modify(identity, obj, lens)

[JeffBezanson]

Could you try bisecting this?

[jw3126]

@JeffBezanson bisected to e8caa27

[maleadt]

MWE:

r(b) = r(typeof(b))
r(::Type) = nothing
r(::Nothing) = nonexistent

struct C{t,I} end
r(::Type{C{c,d}}) where {c,d} = f(r(c), e)
f(::Nothing, :) = nothing
f(g, :) = h

k(b, j, :) = l
k(b, j, ::Nothing) = b

i(b, j) = k(b, j, r(j))
struct A{d} end
Base.return_types(i, (Int64, C{C{C{A, Tuple}, Tuple}}))

[vtjnash]

i, k, and f there are unrelated, and are just playing the role of a type-unstable conditional

This is now explicitly disallowed in inference:

r(::Type{C{c,d}}) where {c,d} = f(r(c), e)

Since it is inferring / calling:

r(::Type{C{C{C{A}}}}) -> r(::Type{C{C{A}}}) -> r(::Type{C{A}}) -> r(::Type{C{A}}) -> r(::Type{A}) -> nothing

And we decided (e8caa27) that this pattern is not something we care about, relative to other patterns.

[jw3126]

And we decided (e8caa27) that this pattern is not something we care about, relative to other patterns.

Does this mean this bug won't be fixed?

[jw3126]

And we decided (e8caa27) that this pattern is not something we care about, relative to other patterns.

This pattern can arise in the wild, when you combine trait based dispatch with recursion. This is the cause in Accessors.jl:

OpticStyle(optic) = OpticStyle(typeof(optic))
OpticStyle(::Type{MyOptic}) = ...
doit(optic) = _doit(optic, OpticStyle(optic))
function _doit(optic::ComposedOptic, ::SomeStyle)
    x1 = doit(optic.inner)
    ...
    x2 = doit(optic.outer)
    ...
end

A workaround is to replace

OpticStyle(::Type{MyOptic}) = ...

by

OpticStyle(::MyOptic) = ...

[vtjnash]

Ah, interesting. I wonder if we can do something about that too.

[cscherrer]

Maybe try

@generated function OpticStyle(optic::T) where {T}
    OpticStyle(T)
end

This worked for me in a similar situation.

[jw3126]

Maybe try

@generated function OpticStyle(optic::T) where {T}
    OpticStyle(T)
end

This worked for me in a similar situation.

If I do this, it seems dispatch selects the wrong method in some situations. I thought

@generated function OpticStyle(optic::T) where {T}
    OpticStyle(T)
end

and

function OpticStyle(optic::T) where {T}
    OpticStyle(T)
end

have exactly the same semantics, but seems not to be the case.

Base.@pure OpticStyle(obj) = OpticStyle(typeof(obj)) 

solves the problem, but will probably lead to bugs.