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Better error message for convert(Type{Union{}}, ...) #40951

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eschnett opened this issue May 26, 2021 · 7 comments
Open

Better error message for convert(Type{Union{}}, ...) #40951

eschnett opened this issue May 26, 2021 · 7 comments
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error messages good first issue

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@eschnett
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@eschnett eschnett commented May 26, 2021

I received this error message:

  MethodError: convert(::Type{Union{}}, ::Vector{Float32}) is ambiguous. Candidates:
    convert(::Type{T}, a::AbstractArray) where T<:Array in Base at array.jl:532
    convert(T::Type{var"#s79"} where var"#s79"<:BitArray, a::AbstractArray) in Base at bitarray.jl:577
    convert(T::Type{var"#s832"} where var"#s832"<:SparseArrays.SparseVector, m::AbstractVector{T} where T) in SparseArrays at /Users/eschnett/julia-1.6/share/julia/stdlib/v1.6/SparseArrays/src/sparsevector.jl:453
    convert(T::Type{var"#s832"} where var"#s832"<:SharedArrays.SharedArray, a::Array) in SharedArrays at /Users/eschnett/julia-1.6/share/julia/stdlib/v1.6/SharedArrays/src/SharedArrays.jl:377
    convert(::Type{Union{}}, x) in Base at essentials.jl:203
    convert(::Type{T}, arg) where T<:VecElement in Base at baseext.jl:8
  Possible fix, define
    convert(::Type{Union{}}, ::Vector{T} where T)

This is a confusing error message. Would it be possible to catch this as a special case in the definition of convert, and emit an error such as "Converting to Union{}" does not make sense; check your call site"?
@JeffBezanson
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@JeffBezanson JeffBezanson commented May 26, 2021

Yes, we already have a function show_convert_error to give better printing of method errors involving convert. This case can be added there.
@jakobnissen
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@jakobnissen jakobnissen commented May 27, 2021

Ref #36706
@sanjibansg
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@sanjibansg sanjibansg commented May 27, 2021

I would like to contribute to this issue, will be grateful for guidance to approach this. @JeffBezanson @eschnett @jakobnissen
@ArunSanganal
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@ArunSanganal ArunSanganal commented Jun 2, 2021

@kahanikaar are you working on this issue?
@Pramodh-G
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@Pramodh-G Pramodh-G commented Jun 16, 2021

Hi @eschnett, thanks for filing the issue. Could you please help me by providing a MWE to reproduce it? It would help me understand what's going on behind the scenes here.
@eschnett
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@eschnett eschnett commented Jun 16, 2021

Try this:

convert(Union{}, Float32[])
@ArunSanganal
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@ArunSanganal ArunSanganal commented Jun 20, 2021

Not quite sure but does fixing this issue start with modifying this?

julia/base/essentials.jl

Line 216 in 87e08d9

convert(::Type{Union{}}, x) = throw(MethodError(convert, (Union{}, x)))

I am kinda new to error handling so any help would be appreciated. Logic says whenever this is encountered, we need to redirect it to show_convert_error function as @JeffBezanson mentioned.
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@eschnett @JeffBezanson @jakobnissen @sanjibansg @Pramodh-G @ArunSanganal