Function definition causes StackOverflowError

[Keno]

I knew I was committing type crimes when I wrote this code, but feels like it should at least not stack overflow?

julia> struct Default{T}
           val::T
       end

julia> Base.convert(::Type{Default{S}}, d::Default{T}) where {S, T} = Default(convert(S, d.val))

julia> function Base.convert(TT::Type{Union{S, Default{S}}}, d::Default{T}) where {S, T}
           Default(convert(S, d.val))
       end

julia> function Base.convert(TT::Type{Union{S, Default{S}}}, b) where {S, T}
           convert(S, b)
       end

julia> Base.convert(TT::Type{Union{S, Default{S}}}, b::Union{S, Default{S}}) where {S} = b
ERROR: StackOverflowError:
Stacktrace:
 [1] top-level scope
   @ REPL[8]:1

[User-764Q]

Hi,

I tried to recreate this today on Julia 1.6.2 and got no errors so maybe it has been fixed?

[ianatol]

Successfully recreated on master and 1.6.2, will look into it

[ianatol]

You only need 2 of the definitions to cause the error. Also, trying to define it again after getting the stack overflow is successful.

julia> struct Default{T}
           val::T
       end

julia> function Base.convert(TT::Type{Union{S, Default{S}}}, d::Default{T}) where {S, T}
                  Default(convert(S, d.val))
              end

julia> Base.convert(TT::Type{Union{S, Default{S}}}, b::Union{S, Default{S}}) where {S} = b
ERROR: StackOverflowError:
Stacktrace:
 [1] top-level scope
   @ REPL[3]:1

julia> Base.convert(TT::Type{Union{S, Default{S}}}, b::Union{S, Default{S}}) where {S} = b

julia> 

[Keno]

Another case:

struct Unassigned; end
struct Matching{U>:Unassigned, V<:AbstractVector{<:Union{U, Int}}} <: AbstractVector{Union{U, Int}}
    match::V
    inv_match::Union{Nothing, V}
end
Matching{U}(v::V) where {U, V<:AbstractVector{<:Union{U, Int}}} = Matching{U, V}(v, nothing)
Matching{U}(v::V, iv::Union{V, Nothing}) where {U, V<:AbstractVector{<:Union{U, Int}}} = Matching{U, V}(v, iv)
Matching(v::V) where {U>:Unassigned, V<:AbstractVector{<:Union{U, Int}}} =
    Matching{U, V}(v, nothing)
Matching(m::Int) = Matching{Unassigned}(Union{Int, Unassigned}[unassigned for _ = 1:m], nothing)
Matching{V}(m::Matching{U}) where {U,V} = Matching{V}(convert(V, m.match),
    m.inv_match === nothing ? nothing : convert(V, m.inv_match))

gives

julia> Matching{V}(m::Matching{U}) where {U,V} = Matching{V}(convert(V, m.match),
           m.inv_match === nothing ? nothing : convert(V, m.inv_match))
ERROR: StackOverflowError:
Stacktrace:
 [1] top-level scope
   @ REPL[7]:1

Not sure if it's the same issue or not, but I guess we'll find out once one or the other is fixed: