UndevVarError from ScopedValues.get on abstract-typed ScopedValue due to isassigned change

[iamed2]

Version 1.6.1 introduced a change that caused behaviour that works on 1.6.0 to error. This behaviour also works with the implementation in Julia 1.12.6 and 1.14.0-DEV.2090.

It only affects ScopedValues with an abstract type parameter.

Expected behaviour:

julia> const MY_SCOPED_ABSTRACT_INT = ScopedValue{Integer}()
ScopedValue{Integer}()

julia> ScopedValues.get(MY_SCOPED_ABSTRACT_INT)

julia> @with MY_SCOPED_ABSTRACT_INT => 3 begin
           ScopedValues.get(MY_SCOPED_ABSTRACT_INT)
       end
Some(3)

Behaviour with ScopedValues.jl 1.6.1 on Julia 1.10.11:

julia> const MY_SCOPED_ABSTRACT_INT = ScopedValue{Integer}()
ScopedValue{Integer}(undefined)

julia> ScopedValues.get(MY_SCOPED_ABSTRACT_INT)

julia> @with MY_SCOPED_ABSTRACT_INT => 3 begin
           ScopedValues.get(MY_SCOPED_ABSTRACT_INT)
       end
ERROR: UndefRefError: access to undefined reference
Stacktrace:
 [1] getproperty
   @ ./Base.jl:37 [inlined]
 [2] get(val::ScopedValue{Integer})
   @ ScopedValues ~/.julia/packages/ScopedValues/Ac0AN/src/ScopedValues.jl:139
 [3] macro expansion
   @ REPL[12]:2 [inlined]
 [4] macro expansion
   @ ~/.julia/packages/ScopedValues/Ac0AN/src/ScopedValues.jl:213 [inlined]
 [5] top-level scope
   @ REPL[12]:1

This behaviour is used by AWS.jl for with_aws_config and current_aws_config, and so users of AWS.jl 1.96+ on ScopedValues.jl 1.6.1 encounter this error when using with_aws_config.

[iamed2]

I believe the fix would be to copy from https://github.com/JuliaLang/julia/blob/c26f5d43da7961b9ead912a886af20fc78fb8ef9/base/scopedvalues.jl#L203-L218: doing more checks on whether has_default is true, and only grabbing the default field in those cases.

I think it's possible this may only work with concrete types due to compiler optimizations causing get to avoid accessing val.default? I can't see why it would succeed otherwise.

[vchuravy]

Sorry about that, can you open a PR?

[vchuravy]

cc: @JamesWrigley

[iamed2]

Sorry about that, can you open a PR?

Sorry, took a break and just got back to my desk. Thank you for implementing a fix!