Internal FCS error is shown on SRTP pattern for struct tuple

[Liminiens]

Repro steps

1. Write this code

let inline test (arg: ^T when ^T : struct) = 
    (^T : (member Item1: _) (arg))

[<EntryPoint>]
let main argv =
    let a = test struct (1, 2)
    0 // return an integer exit code

Expected behavior

Pattern works as ValueTuple has Item1 (?)

Actual behavior

FS0192	internal error: tcrefOfAppTy
FS0073	internal error: tcrefOfAppTy (Failure)
FS0073	internal error: tcrefOfAppTy (Failure)
FS0073	internal error: tcrefOfAppTy (Failure)
FS0073	internal error: tcrefOfAppTy (Failure)
FS0073	internal error: tcrefOfAppTy (Failure)

Related information

.NET Core SDK 2.2.101

[cartermp]

There is no current ability to use ItemX properties on struct tuples. There is an approved RFC out to do this here: https://github.com/fsharp/fslang-design/blob/master/RFCs/FS-1064-struct-tuple-equivalence.md

We'd certainly welcome the PR, but I don't think it's a priority for us given that pattern matching/deconstruction is the idiomatic way to work with tuples in F#.

That said, I'm labeling this as a regression since the error message did appear to not be an internal error at one point:

/Users/phillip/scratch/bar/Program.fs(54,13): error FS0001: A generic construct requires that the type 'struct ('a * 'b)' is a CLI or F# struct type

Though what it really should be saying is that struct (1, 2) does not support Item1 as a property

[auduchinok]

Just to add a bit of context: the initial issue mentions it's happening in Rider 2018.3.1 that uses FCS built from dev15.9 branch (with few additional patches) so the regression seems to not have appeared after recent changes in master.

[TIHan]

Edit: Yes, we should not get an internal error.

[TIHan]

Closing issue as we have resolved the internal error and now receive a proper error. Also fixes struct tuples and struct constraints in general.