inference: if it doesn't know how to quack, it can't be a duck

[KristofferC]

It would be cool if inference could figure out that if you call a method on an object inferred as a union, inference could ignore all types that are known to throw with that method for future expressions in that block. For example:

function find_first_above_5(v::Vector{Union{Nothing, Float64}})
    for x in v
        if x > 5.0
            return x # x > 5.0 is MethodError for Nothing so can assume ::Float64
        end
    end
    return 0.0
end

x = Union{Nothing, Float64}[1.0, 2.0, 3.0, 6.0, 4.0]
find_first_above_5(x)
# 6.0

Core.Compiler.return_types(find_first_above_5, Tuple{Vector{Union{Nothing, Float64}}})
# 1-element Array{Any,1}:
#  Union{Nothing, Float64}

code_warntype(find_first_above_5, Tuple{Vector{Union{Nothing, Float64}}})

Here, a return value of Float64 could be inferred.