Julia: UnionAll clashing with type parameter for function

[ranjanan]

On Julia master:

julia> struct Pullback{S, T}
               t::T
             end

julia> function Base.show(io::IO, ::Type{<:Pullback{S}}) where S 
                S
              end

julia> P = Pullback{Tuple{Type{T}}, T} where T
Error showing value of type UnionAll:
ERROR: UndefVarError: S not defined
Stacktrace:
 [1] show(::IOContext{REPL.Terminals.TTYTerminal}, ::Type{
SYSTEM (REPL): showing an error caused an error
ERROR: UndefVarError: S not defined
Stacktrace:
 [1] show(::IOContext{REPL.Terminals.TTYTerminal}, ::Type{
SYSTEM (REPL): caught exception of type UndefVarError while trying to handle a nested exception; giving up

cc: @vchuravy, major props for helping me track this down

This code is part of Zygote.jl so cc: @MikeInnes

[Keno]

The initial error isn't a bug. The type parameter is undefined. Obviously the subsequent errors are incorrect.

[KristofferC]

https://github.com/FluxML/Zygote.jl/issues/134 should maybe be reopened.

Overloading show on type and messing up will make showing errors fail, since they use that.

[Keno]

I'm wondering if we should try using the fallback printing for the exception if it throws an error again.