Iterating over a non-iterable object causes a stack overflow

[rdeits]

I think this is the reason Julia is essentially hanging in JuliaMath/SpecialFunctions.jl#107 and JuliaPackaging/CMakeWrapper.jl#14 because some other issue is causing us to try to iterate over a Symbol.

Here's a simple reproduction:

julia> struct Foo; end

julia> iterate(Foo())
┌ Warning: The start/next/done iteration protocol is deprecated. Use `iterate` instead.
│   caller = iterate(::Foo) at essentials.jl:853
└ @ Base ./essentials.jl:853

That warning prints once and then Julia hangs for a very long time (I haven't the patience to find out how long). Interrupting it with ctrl+C shows the stack overflow:

^CERROR: InterruptException:
Stacktrace:
 ... (the last 2 lines are repeated 10917 more times)
 [21842] backtrace() at ./stacktraces.jl:167
 [21842] macro expansion at ./deprecated.jl:72 [inlined]
 [21842] macro expansion at ./logging.jl:311 [inlined]
 [21842] depwarn(::String, ::Symbol) at ./deprecated.jl:68
 [21842] start(::Foo) at ./essentials.jl:879
 [21842] iterate(::Foo) at ./essentials.jl:853
 [21842] start(::Foo) at ./essentials.jl:880
 [21842] iterate(::Foo) at ./essentials.jl:853

it looks like the fallback start() is calling iterate() and the fallback iterate() is calling start().

[rdeits]

Looks like this is probably fixed by #28391