Expressions other than properties/method calls/indexing are allowed after _. in dot-lambdas

[brianrourkeboll]

Expressions other than properties/method calls/indexing are allowed after _. in dot-lambdas (see fsharp/fslang-suggestions#506, fsharp/fslang-design#710). I think this is a bug in #13907, unless there's something I'm missing from fsharp/fslang-design#710.

Repro steps

1. Write a dot-lambda and put a literal or most any other atomic expression after the dot. This effectively means any arbitrarily-complex expression, as long as it's parenthesized.
2. _.expr compiles as fun _ -> expr, i.e., basically fun x -> ignore x; expr.

let _ = _.3
let _ = _.1e-04
let _ = _."🙃"
let _ = _.[||]
let _ = _.{||}
let _ = _.typeof<int>
let _ = _.null
let _ = _.__SOURCE_DIRECTORY__
let _ = _.(<@ 1 @>)
let _ = _.(nameof nameof)
let _ = _.struct (1, 2, 3)
let _ = _.{ new System.IDisposable with member _.Dispose () = () }
let _ = _.(while true do ())
let _ = _.(let x = 3 in x + x)

Expected behavior

This was surprising to me. I would have expected only properties, method calls, or indexing to be allowed.

Actual behavior

Any arbitrary expression can be used on the right-hand side of a dot-lambda (_.).

Known workarounds

N/A.

Related information

This is in main and in the latest .NET 8 SDK.