Unhelpful error message for `lowerkey`

[jakobjpeters]

This doesn't work.

julia> struct A end

julia> JSON.json(A() => 1)
ERROR: ArgumentError: No key representation for A. Define StructUtils.lowerkey(::A)
...

julia> StructUtils.lowerkey(::A) = "a";

julia> JSON.json(A() => 1)
ERROR: ArgumentError: No key representation for A. Define StructUtils.lowerkey(::A)

This works, but it's unclear if this is the intended fix.

julia> StructUtils.lowerkey(::JSON.JSONStyle, ::A) = "a";

julia> JSON.json(A() => 1)
"{\"a\":1}"

error source:

JSON.jl/src/write.jl

Line 238 in 0c9b900

StructUtils.lowerkey(::JSONStyle, x) = throw(ArgumentError("No key representation for $(typeof(x)). Define StructUtils.lowerkey(::$(typeof(x)))"))