error FS0073: internal error: Undefined or unsolved type variable when using generic type annotations on DU of DUs with static members requiring 2+ inputs #6648
Description
When attempting to create a discriminated union with a generic type annotation whose cases are types of other generic discriminated unions, the following error FS0073: internal error: Undefined or unsolved type variable: ^_?155636 is thrown after adding static members that require more than 1 input (e.g. op_Subtraction, in the provided example).
Repro steps
Provide the steps required to reproduce the problem
- Run the following code in VS2019 FSI
type SomeUnion1<'T> =
| SomeCase1a of 'T[]
| SomeCase1b of 'T[]
static member inline (~-) x =
match x with
| SomeCase1a a ->
a |> Array.map (fun b -> -b) |> SomeCase1a
| SomeCase1b b ->
b |> Array.map (fun c -> -c) |> SomeCase1b
static member inline (-) (a, b) =
match a, b with
| SomeCase1a x, SomeCase1a y
| SomeCase1a x, SomeCase1b y ->
(x, y) ||> Array.map2 (fun u v -> u - v) |> SomeCase1a
| SomeCase1b x, SomeCase1a y
| SomeCase1b x, SomeCase1b y ->
(x, y) ||> Array.map2 (fun u v -> u - v) |> SomeCase1b
type SomeUnion2<'T> =
| SomeCase2a of 'T[][]
| SomeCase2b of 'T[][]
static member inline (~-) x =
match x with
| SomeCase2a a ->
a |> Array.map (Array.map (fun b -> -b)) |> SomeCase2a
| SomeCase2b b ->
b |> Array.map (Array.map (fun c -> -c)) |> SomeCase2b
static member inline (-) (a, b) =
match a, b with
| SomeCase2a x, SomeCase2a y
| SomeCase2a x, SomeCase2b y ->
(x, y) ||> Array.map2 (Array.map2 (fun u v -> u - v)) |> SomeCase2a
| SomeCase2b x, SomeCase2a y
| SomeCase2b x, SomeCase2b y ->
(x, y) ||> Array.map2 (Array.map2 (fun u v -> u - v)) |> SomeCase2b
type UnionOfUnions<'T> =
| ParentCase1 of SomeUnion1<'T>
| ParentCase2 of SomeUnion2<'T>
static member inline (~-) x =
match x with
| ParentCase1 a -> -a |> ParentCase1
| ParentCase2 a -> -a |> ParentCase2
static member inline (-) (a, b) =
match a, b with
| ParentCase1 x, ParentCase1 y -> x - y |> ParentCase1
| ParentCase2 x, ParentCase2 y -> x - y |> ParentCase2
| _, _ -> failwith "Cannot subtract these"Expected behavior
The expected behavior presumably should not throw the internal error. The following example, without the op_Subtraction static member, successfully runs in FSI:
type SomeUnion1<'T> =
| SomeCase1a of 'T[]
| SomeCase1b of 'T[]
static member inline (~-) x =
match x with
| SomeCase1a a ->
a |> Array.map (fun b -> -b) |> SomeCase1a
| SomeCase1b b ->
b |> Array.map (fun c -> -c) |> SomeCase1b
type SomeUnion2<'T> =
| SomeCase2a of 'T[][]
| SomeCase2b of 'T[][]
static member inline (~-) x =
match x with
| SomeCase2a a ->
a |> Array.map (Array.map (fun b -> -b)) |> SomeCase2a
| SomeCase2b b ->
b |> Array.map (Array.map (fun c -> -c)) |> SomeCase2b
type UnionOfUnions<'T> =
| ParentCase1 of SomeUnion1<'T>
| ParentCase2 of SomeUnion2<'T>
static member inline (~-) x =
match x with
| ParentCase1 a -> -a |> ParentCase1
| ParentCase2 a -> -a |> ParentCase2The following input examples can confirm this:
> let case1bTest = Array.init 5 (fun x -> double (x-1)) |> SomeCase1b |> ParentCase1
val case1bTest : UnionOfUnions<double> =
ParentCase1 (SomeCase1b [|-1.0; 0.0; 1.0; 2.0; 3.0|])
> -case1bTest;;
val it : UnionOfUnions<double> =
ParentCase1 (SomeCase1b [|1.0; 0.0; -1.0; -2.0; -3.0|])
> let case2aTest = Array.init 6 (fun x -> double (x+2)) |> Array.chunkBySize 2 |> SomeCase2a |> ParentCase2
val case2aTest : UnionOfUnions<double> =
ParentCase2 (SomeCase2a [|[|2.0; 3.0|]; [|4.0; 5.0|]; [|6.0; 7.0|]|])
> -case2aTest;;
val it : UnionOfUnions<double> =
ParentCase2 (SomeCase2a [|[|-2.0; -3.0|]; [|-4.0; -5.0|]; [|-6.0; -7.0|]|])Actual behavior
Running the code snippet given in the Repro section of this issue yields an error:
> error FS0073: internal error: Undefined or unsolved type variable: ^_?155636Known workarounds
This problem may be circumvented by not using generic type parameters.
Related information
Provide any related information
- Operating system - Windows 10
- Branch - F# 4.6
- .NET Runtime, CoreCLR or Mono Version
- Editing Tools (e.g. Visual Studio Version) - VS2019 16.0.1
- Links to F# RFCs or entries on https://github.com/fsharp/fslang-suggestions
- Links to performance testing scripts
- Indications of severity