`UndefVarError` when passing type with `TypeVar` to parametric function

[Socob]

When passing a type which has a TypeVar parameter to a parametric function, the result is the following unexpected UndefVarError about the function parameter:

julia> func(::Type{T}) where {T} = T
func (generic function with 1 method)

julia> func(Array{Float64, TypeVar(:N)})
ERROR: UndefVarError: `T` not defined
Stacktrace:
 [1] func(::Type{Array{Float64, N}})
   @ Main ./REPL[1]:1
 [2] top-level scope
   @ REPL[2]:1

The expected result would of course be Array{Float64, TypeVar(:N)}, not an error.

Although I’m only using exported symbols in the example above, I suspect that this might not necessarily be a supported use (?). Even if this is the case, I think at least the error should be fixed, since as it stands it’s completely misleading.

---

Info:

1. The output of versioninfo():

   Julia Version 1.10.5
   Commit 6f3fdf7b362 (2024-08-27 14:19 UTC)
   Build Info:
     Official https://julialang.org/ release
   Platform Info:
     OS: Linux (x86_64-linux-gnu)
     CPU: 16 × AMD Ryzen 7 4800H with Radeon Graphics
     WORD_SIZE: 64
     LIBM: libopenlibm
     LLVM: libLLVM-15.0.7 (ORCJIT, znver2)
   Threads: 1 default, 0 interactive, 1 GC (on 16 virtual cores)

2. How you installed Julia: juliaup

3. A minimal working example (MWE): see above

[vtjnash]

This is correct. The value of T is only defined for well-formed types, and the TypeVar constructor does not make well-formed types. You must use func(T::Type) = T if you want to return the actual input, rather that an arbitrary transform of the input (including potentially transforming it into not having a value)

[Socob]

Fair enough, but I don’t see how I’d be expected to know that T could ever end up being undefined? Like I said, a better error would be nice – I think it’d be much better if an error is thrown whenever a construction like this results in T being undefined, rather than only throwing an error when it’s used.

[vtjnash]

There is no way for it to know that you would later construct a malformed type using the private TypeVar constructor

[Socob]

I mean when calling the function. I don’t see why there couldn’t be an error if T is undefined at the beginning of the function?

[Socob]

Also, in what sense is the TypeVar constructor private? It seems to be exported by Core?

[vtjnash]

Type constructors are default private, even if the type object itself may be visible (e.g. for dispatch purposes)