Apparent Bug with BigInts in Composite Types

[ztultrebor]

import Base: convert, promote_rule

struct Thing1 <: Number
    a::BigInt
end

convert(::Type{Thing1}, x::Int) = Thing1(x)

promote_rule(::Type{Thing1}, ::Type{<:Number}) = Thing1

The following example comparison is false:

Thing1(1) == 1

Now if Thing1.a were an Int, a Float64, a Rational{Int} or a ComplexF64 the above comparison is true. Is there some explanation for this or is it a bug?

[oscardssmith]

This would work if you define ==(x::Thing1,y::Thing1). To the extent that it is a bug, it's a duplicate of #4648