`Union{Nothing,Int}` field in mutable struct, when set to `nothing`, sometimes reads as garbage `Int` in >=1.11

[danielwe]

MWE:

mutable struct Wrapper
    x::Union{Nothing,Int}
end

function findsomething(dict, inds)
    default = Wrapper(nothing)
    for i in inds
        x = get(dict, i, default).x
        if !isnothing(x)
            return x
        end
    end
    return nothing
end

for i in 1:100000
    x = findsomething(Dict(1 => Wrapper(nothing)), [2])
    if !isnothing(x)
        @error "" x
        break
    end
end

Output:

$ julia +lts     --startup-file=no bug.jl  # 1.10.9
$ julia +release --startup-file=no bug.jl  # 1.11.5
┌ Error:
│   x = 4494295104
└ @ Main ~/bug/bug.jl:19
$ julia +beta    --startup-file=no bug.jl  # 1.12-beta3
┌ Error:
│   x = 0
└ @ Main ~/bug/bug.jl:19
$ julia +nightly --startup-file=no bug.jl  # 1.13-DEV.590
┌ Error:
│   x = 2
└ @ Main ~/bug/bug.jl:19

As shown, in Julia >=1.11, default can get corrupted such that default.x reads as a garbage Int instead of nothing, even though it was initialized to nothing and never mutated.

This does not happen if Wrapper is not mutable and/or default is a global const instead of a local.

Xref https://discourse.julialang.org/t/unmutated-mutable-type-being-corrupted/129161

Platform info (except Julia version as shown above):

ulia> versioninfo()
Julia Version 1.11.5
Commit 760b2e5b739 (2025-04-14 06:53 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: macOS (arm64-apple-darwin24.0.0)
  CPU: 14 × Apple M4 Pro
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, apple-m1)
Threads: 1 default, 0 interactive, 1 GC (on 10 virtual cores)

[gbaraldi]

Bisected to #54604

[danielwe]

Just FTR: It occurred to me that the 100_000 iterations in the MWE might suggest this is a rare occurrence. It is not; empirically, it happens in 20-70% of trials. The 100_000 is just a remnant from a variant of the code where I was gathering statistics.

Also, the type instability of findsomething is irrelevant. You can change it to return true or false, and the issue remains.