Assignment `T{TT} = Int` should error

[mauro3]

I think this should throw an error (or does it make sense?):

julia> T{TT} = Int                                                                                                                                                       
Int64                                                                                                                                                                    

instead it semi-works:

julia> T                                                                                                                                                                 
Int64                                                                                                                                                                    

thus the parameter TT is, I think, just silently ignored.

Here what it gets lowered to:

julia> Meta.lower(Main, :(T{TT} = Int))
:($(Expr(:thunk, CodeInfo(:(begin
      const T
      TT = (Core.TypeVar)(:TT)
      SSAValue(0) = (Core.UnionAll)(TT, Int)
      T = SSAValue(0)
      return SSAValue(0)
  end)))))

[brenhinkeller]

Does still reproduce, but no idea if this is a problem or not

[StefanKarpinski]

@JeffBezanson, any opinion on this? Bug or feature?

[vtjnash]

Probably breaking now, though might be interesting in the future to require that (some or all) where clauses will error if they do not produce a UnionAll result

[StefanKarpinski]

Are there any legitimately useful applications of this? If not, then we could consider it a minor change and PkgEval disallowing it.

[adienes]

if it's not possible to remove this (although I'm not sure what it's supposed to mean and imo looks like it should be an error), maybe a candidate for #54903