Soundness regression in 4.5.0-beta-001. Nested lambdas are stripped

[seanamos]

Issue created from fantomas-online

Code

let x =
    fun _ ->
        fun _ -> "hello"

Result

let x = fun _ -> "hello"

Problem description

I noticed this with the latest shipped version of Ionide which includes 4.5.0-beta-001.
When a lambda is nested as in the above example, one of the lambdas will be removed by fantomas.
This obviously breaks the source code.

Extra information

• The formatted result breaks my code.
• The formatted result gives compiler warnings.
• I or my company would be willing to help fix this.

Options

Fantomas Master at 06/12/2021 10:31:17 - 8f71594

Default Fantomas configuration

[nojaf]

Hello Sean, thank you for reporting this issue.
I believe the problem lies in the change in the |Lambda|_| active pattern:
See

fantomas/src/Fantomas/SourceParser.fs

Line 1252 in 8f71594

| SynExpr.Match (matchSeqPoint = NoDebugPointAtInvisibleBinding; clauses = [ Clause (_, expr, _) ])

It appears the processing of the parsedData is wrong somehow.