Also accept alignment from if keyword in genKeepIdentIfThenElse. (#2975)

* Also accept alignment from if keyword in genKeepIdentIfThenElse.

* Add additional test

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+### Changed
+* ExperimentalKeepIndentInBranch should fire when first branch was all on one line. [#2973](https://github.com/fsprojects/fantomas/issues/2973)
+
 ## 6.2.3 - 2023-11-2
 
 ### Fixed

src/Fantomas.Core.Tests/KeepIndentInBranchTests.fs

@@ -2183,3 +2183,83 @@ module Foo =
 
         failwith ""
 """
+
+[<Test>]
+let ``if keyword and elseBranch align, 2973`` () =
+    formatSourceString
+        false
+        """
+module Program =
+    let main _ =
+        if false then 1 else
+        printfn "hi!"
+        0
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+module Program =
+    let main _ =
+        if false then
+            1
+        else
+
+        printfn "hi!"
+        0
+"""
+
+[<Test>]
+let ``elif keyword and elseBranch align`` () =
+    formatSourceString
+        false
+        """
+if false then
+    1
+elif false then 2 else
+printfn "hi!"
+0
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+if false then
+    1
+elif false then
+    2
+else
+
+printfn "hi!"
+0
+"""
+
+[<Test>]
+let ``elseBranch aligns with if keyword in computation expression`` () =
+    formatSourceString
+        false
+        """
+async {
+    if not (proc.Start ()) then return Error "failed to start" else
+    use stdout = proc.StandardOutput
+    let! ct = Async.CancellationToken
+    return! Async.AwaitTask (stdout.ReadToEndAsync ct)
+}
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+async {
+    if not (proc.Start()) then
+        return Error "failed to start"
+    else
+
+    use stdout = proc.StandardOutput
+    let! ct = Async.CancellationToken
+    return! Async.AwaitTask(stdout.ReadToEndAsync ct)
+}
+"""

src/Fantomas.Core/CodePrinter.fs

@@ -1298,7 +1298,7 @@ let genExpr (e: Expr) =
                     indentSepNlnUnindent (genExpr node.ThenExpr)
                     +> sepNln
                     +> genSingleTextNode node.Else
-                    +> genKeepIdent node.Else node.ElseExpr
+                    +> genKeepIdentIfThenElse node.If.Node node.Else node.ElseExpr
 
                 // Check if the `if expr then` is already multiline or cross the max_line_length.
                 let isMultiline =
@@ -1378,7 +1378,12 @@ let genExpr (e: Expr) =
                 +> indentSepNlnUnindent (genExpr node.ThenExpr)
                 |> genNode node)
             +> optSingle
-                (fun (elseNode, elseExpr) -> sepNln +> genSingleTextNode elseNode +> genKeepIdent elseNode elseExpr)
+                (fun (elseNode, elseExpr) ->
+                    let genKeepIdent =
+                        let branch = List.last node.Branches
+                        genKeepIdentIfThenElse branch.If.Node elseNode elseExpr
+
+                    sepNln +> genSingleTextNode elseNode +> genKeepIdent)
                 node.Else
 
         ifElseCtx areAllShort shortExpr longExpr
@@ -2073,7 +2078,7 @@ let genClause (isLastItem: bool) (node: MatchClauseNode) =
                                 | None -> Pattern.Node node.Pattern
                                 | Some bar -> bar
 
-                            genKeepIdent startNode node.BodyExpr
+                            genKeepIdentMatchClause startNode node.BodyExpr
 
                         expressionFitsOnRestOfLine (sepSpace +> genExpr node.BodyExpr) long
 
@@ -2253,7 +2258,19 @@ let genExprInMultilineInfixExpr (e: Expr) =
     | Expr.Record _ -> atCurrentColumnIndent (genExpr e)
     | _ -> genExpr e
 
-let genKeepIdent (startNode: Node) (e: Expr) ctx =
+let genKeepIdentIfThenElse (ifKeyword: Node) (elseKeyword: Node) (e: Expr) ctx =
+    let exprNode = Expr.Node e
+
+    if
+        ctx.Config.ExperimentalKeepIndentInBranch
+        && (elseKeyword.Range.StartColumn = exprNode.Range.StartColumn
+            || ifKeyword.Range.StartColumn = exprNode.Range.StartColumn)
+    then
+        (sepNln +> sepNlnUnlessContentBefore exprNode +> genExpr e) ctx
+    else
+        indentSepNlnUnindent (genExpr e) ctx
+
+let genKeepIdentMatchClause (startNode: Node) (e: Expr) ctx =
     let exprNode = Expr.Node e
 
     if