Infer type from typed array

[HarrisonGrodin]

The type of array elements should be inferred, both in the context of elements that were getindexed and inside of for loops.

For example, the code below should error, due to the fact that there is no field nonexistent in User and no method f(::User) in both cases.

julia> lintstr("""

       struct User
           email::String
       end

       f(x::Integer) = x

       users = User[]
       push!(users, User("[email protected]"))

       println(users[1].nonexistent)
       println(f(users[1]))

       for user = users
           println(user.nonexistent)
           println(f(user))
       end
       """)

Here, users is explicitly typed as User - this functionality should still work if users = [User("[email protected]")], though, as the array type is inferred from the element.

If an array is initialized with multiple types, the inferred type should be whatever Julia would promote the array to.

julia> lintstr("""

       numbers = [1, "two", 3.0]

       for number = numbers
           println(zero(number))
           println(identity(number))
       end

       """)

Here, numbers would be of eltype Any. zero(number) should error (because it is not defined for Any), but identity should pass.

[TotalVerb]

You're right that we should do more here. However we cannot in general error for

       for number = numbers
           println(zero(number))
       end

because it is a valid use case to have Any[1, 2, 3], which would also be inferred as a Vector{Any}. (Currently, we also use Any as a indicator for Lint not being able to figure out a type, and I think there would be too many spurious warnings if we started to warn about using things that are not defined on Any.)

The other suggestions are good though and should be possible in the medium term.