Julia: `=== nothing` is not `== nothing` with `--code-coverage=user`

[tkf]

I just have a very strange bug that cannot be reproduced locally _and_ replacing === nothing with == nothing (as below) fixes this bug.

diff --git a/examples/primes.jl b/examples/primes.jl
index 7dac9d2f..0f194e7f 100644
--- a/examples/primes.jl
+++ b/examples/primes.jl
@@ -11,7 +11,7 @@ using Transducers

 function sieve(xf, x)
     @info "sieve(xf, x=$x)"
-    if (@show mapfoldl(xf, right, (x,), init=nothing)) === nothing
+    if (@show mapfoldl(xf, right, (x,), init=nothing)) == nothing
         @info "$x is NOT prime"
         nothing, xf
     else

See: https://travis-ci.com/tkf/Transducers.jl/builds/119090143 (Ref https://github.com/tkf/Transducers.jl/pull/17)

Maybe it's relevant to #32135?

[tkf]

I tried other variants:

• isa Nothing works https://travis-ci.com/tkf/Transducers.jl/builds/119091015
• isnothing works https://travis-ci.com/tkf/Transducers.jl/builds/119092120
• !== x where x is not a nothing works https://travis-ci.com/tkf/Transducers.jl/builds/119092147

[StefanKarpinski]

This seems like it shouldn’t happen.

[vtjnash]

Seems like someone may have deleted the build log for the OP?

[Keno]

I can reproduce this locally if I try using Run.jl as in the travis script for that package.

[Keno]

Looks like the difference is the --code-coverage=user flag.

[Keno]

Looking at the IR, I see:

23 ┄ %44  = φ (#21 => false)::Bool
│    %45  = φ (#21 => _3, #22 => nothing)::Union{Nothing, Int64}
└───        goto #25
24 ─        goto #25
25 ┄ %48  = φ (#23 => %44)::Bool
│    %49  = φ (#23 => %45, #24 => nothing)::Union{Nothing, Int64}
└───        goto #26
26 ─ %51  = (isa)(%49, Nothing)::Bool
└───        goto #28 if not %51
27 ─ %53  = π (%49, Nothing)
└───        goto #31
28 ─ %55  = (isa)(%49, Int64)::Bool
└───        goto #30 if not %55
29 ─ %57  = π (%49, Int64)
└───        goto #31
30 ─        Core.throw(ErrorException("fatal error in type inference (type bound)"))::Union{}
└───        $(Expr(:unreachable))::Union{}
31 ┄ %61  = φ (#27 => %48, #29 => %48)::Bool

which looks like an optimizer bug, probably in type lifting.

[tkf]

using Run.jl

Ah, I thought I've tried this... But yes, I can reproduce it with Run.test() now.

Edit: maybe the easiest way to run the script is julia --startup-file=no --project=test --code-coverage=user examples/primes.jl

[tkoolen]

Maybe related to https://github.com/JuliaLang/julia/issues/30872?

[KristofferC]

Ref https://github.com/JuliaLang/julia/issues/32135#issuecomment-533975397 for another instance of this