Allow typed bindings in CE `let!`s without parentheses

[baronfel]

I propose we

allow CE let-bindings to accept typed patterns without required parenthesis:

async {
  let! value: int = 
      async {
        return 10
      }
   return value + 1
}

The existing way of approaching this problem in F# is

to wrap the type pattern in parenthesis:

async {
  let! (value: int) = 
      async {
        return 10
      }
   return value + 1
}

Pros and Cons

The advantages of making this adjustment to F# are

it removes a syntactical foot-gun that is frustrating and confusing to users.

The disadvantages of making this adjustment to F# are

it is a slight special casing of the allowable patterns in CE bindings and so may make the parser/lexer/checker more complicated

Extra information

Estimated cost (XS, S, M, L, XL, XXL): S to M

Related suggestions:

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• [x] This is a language change and not purely a tooling change (e.g. compiler bug, editor support, warning/error messages, new warning, non-breaking optimisation) belonging to the compiler and tooling repository
• [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it
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Please tick all that apply:

• [x] This is not a breaking change to the F# language design - existing code does not compile so the change would be safe
• [ ] I or my company would be willing to help implement and/or test this

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[brianrourkeboll]

It's interesting how these two examples are currently represented differently in the AST—compare the SynBinding's headPat, returnInfo, and expr:

let (x : int) = 3

https://fsprojects.github.io/fantomas-tools/#/ast?data=N4KABGBEAmCmBmBLAdrAzpAXFSAacUiaAYmolmPAIYA2as%2BEkAxgPZwWQ2wAuYAFAA8w2FDwCUYALxgAzHgKRYggA5Vk0CjwBOAV1ggAvkA

let x : int = 3

https://fsprojects.github.io/fantomas-tools/#/ast?data=N4KABGBEAmCmBmBLAdrAzpAXFSAacUiaAYmolmPAIYA2as%2BEkAxgPZwWQ2wAuYAHmGwo%2BAXjABmPAUix%2BAByrJoFHgCcArrBABfIA

Separately, there are also several expression contexts where an unparenthesized type annotation is currently disallowed that I came across in https://github.com/dotnet/fsharp/pull/16079:

https://github.com/dotnet/fsharp/blob/b077e0912d36d3d8114387369f01c9f78fc1e605/src/Compiler/Service/ServiceAnalysis.fs#L1236-L1259

I have neither looked into the compiler nor reasoned from first principles as to why that might be for each case, but if somebody looks at addressing this issue, it may be worth looking into those as well.

At least for some other pattern contexts there are good reasons why parentheses are and must be required, since, e.g., a trailing ->, |, or & token could otherwise be interpreted as part of the type:

fun (x : …) -> …
function (x : …) -> …
match … with (x : …) -> …
(x : int) | x
(x : int) & y