Cthulhu should display ambiguous method matches

[Keno]

julia> struct _CtorSlotNarrowVal end

julia> Base.convert(::Type{_CtorSlotNarrowVal}, ::Any) = _CtorSlotNarrowVal()

julia> mutable struct _CtorSlotNarrowHolder
           x::_CtorSlotNarrowVal
       end

julia> Base.infer_effects(convert, Tuple{Type{_CtorSlotNarrowHolder}, Any})
(+c,+e,!n,+t,+s,+m,+u,+o,+r)

I was investigating where that nothrow taint was coming from. Turns out:

julia> Base.methods_including_ambiguous(convert, Tuple{Type{_CtorSlotNarrowVal}, Any})
# 2 methods for generic function "convert" from Base:
 [1] convert(::Type{T}, x::T) where T
     @ Base_compiler.jl:133
 [2] convert(::Type{_CtorSlotNarrowVal}, ::Any)
     @ REPL[9]:1

julia> convert(_CtorSlotNarrowVal, _CtorSlotNarrowVal())
ERROR: MethodError: convert(::Type{_CtorSlotNarrowVal}, ::_CtorSlotNarrowVal) is ambiguous.

Candidates:
  convert(::Type{_CtorSlotNarrowVal}, ::Any)
    @ Main REPL[16]:1
  convert(::Type{T}, x::T) where T
    @ Base Base_compiler.jl:133

Possible fix, define
  convert(::Type{_CtorSlotNarrowVal}, ::_CtorSlotNarrowVal)

Stacktrace:
 [1] top-level scope
   @ REPL[19]:1

So it's smarter than me, but I feel like this should have been annotated in the callsite display.