reserve syntax that could be used for computed field types

[JeffBezanson]

This will allows us to implement #18466 in a non-breaking way. Example:

julia> struct Foo{T}
           x::T
           y::eltype(T)
       end
ERROR: syntax: unsupported field type expression "eltype(T)"

Specifically, this disallows call expressions containing type parameters.

[JeffBezanson]

Putting this in NEWS might just add noise --- I don't believe there's anything useful you can do with this syntax currently.

[o314]

What about

 struct Foo{T}
   x::T
   y::synthetic(T)
end

or

 struct Foo{T}
   x::T
   y::generated(T)
end

My 2cts

[o314]

@JeffBezanson

I don't believe there's anything useful you can do with this syntax currently.

Will be very interesting to embed into a graph (LightGraphs.jl ?) or db (JuliaDB ?) library to handle the (re)computing of derived properties, metrics etc.

When the time has come, of course...

[yuyichao]

Putting this in NEWS might just add noise --- I don't believe there's anything useful you can do with this syntax currently.

Actually https://github.com/simonster/StructsOfArrays.jl/pull/7/files#diff-04c6e90faac2675aa89e2176d2eec7d8R71

[yuyichao]

What it does is still possible assuming this is still allowed. It's nonetheless something useful that can be done currently with the syntax though.

[JeffBezanson]

Ok, you're right. Maybe this should be a deprecation warning instead? Or continue to allow the current syntax, and require something like field::@defer f(Vector{T}) in the future for deferred field types?

[mauro3]

Was the triage decision to use field::@defer f(Vector{T})? Or why was this reverted?

[yuyichao]

require something like field::@defer f(Vector{T}) in the future for deferred field types

That keeps the semantics consistent with super types, which I assume will probably never support deferred computed types.

[iamed2]

@generated struct Foo{T}
    x::T
    y::eltype(T)
end

Maybe?