Merge pull request #3225 from nojaf/fix-3043

Fix multiline function type in parens causing compiler error

Author: nojaf

CHANGELOG.md

@@ -8,6 +8,7 @@
 - Lambda closing parenthesis in chained method calls is no longer placed on its own line when `MultiLineLambdaClosingNewline` is enabled. [#2553](https://github.com/fsprojects/fantomas/issues/2553)
 - Long `&` (AND) patterns now break across multiple lines to respect max line length. [#1780](https://github.com/fsprojects/fantomas/issues/1780)
 - Empty array with trivia inside now has correct indentation in Stroustrup style. [#3098](https://github.com/fsprojects/fantomas/issues/3098)
+- Multiline function type inside parentheses gets extra indentation to avoid compiler error. [#3043](https://github.com/fsprojects/fantomas/issues/3043)
 
 ## [8.0.0-alpha-002] - 2025-12-15
 

src/Fantomas.Core.Tests/TypeAnnotationTests.fs

@@ -130,7 +130,11 @@ type CancellableTaskResultBuilderBase with
             task: CancellationToken -> ^TaskLike,
             continuation:
                 ('TResult1
-                    -> CancellableTaskResultCode<'TOverall, 'Error, 'TResult2>)
+                        -> CancellableTaskResultCode<
+                            'TOverall,
+                            'Error,
+                            'TResult2
+                         >)
         ) : bool =
         true
 """
@@ -580,3 +584,31 @@ XYZ.app<
         wouldSomeoneWriteThisCode
 }]
 """
+
+[<Test>]
+let ``multiline function type in parens gets extra indent, 3043`` () =
+    formatSourceString
+        """
+type MyCustomTypeWithAPrettyLongDescribingName = MyCustomConstructor1
+
+let private myFunction
+    : string
+          -> (MyCustomTypeWithAPrettyLongDescribingName -> MyCustomTypeWithAPrettyLongDescribingName -> MyCustomTypeWithAPrettyLongDescribingName)
+          -> unit =
+    fun a fn -> ()
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+type MyCustomTypeWithAPrettyLongDescribingName = MyCustomConstructor1
+
+let private myFunction
+    : string
+          -> (MyCustomTypeWithAPrettyLongDescribingName
+                  -> MyCustomTypeWithAPrettyLongDescribingName
+                  -> MyCustomTypeWithAPrettyLongDescribingName)
+          -> unit =
+    fun a fn -> ()
+"""

src/Fantomas.Core/CodePrinter.fs

@@ -3275,10 +3275,26 @@ let genType (t: Type) =
             genNode node (isSmallExpression size smallExpression longExpression) ctx
 
     | Type.Paren node ->
-        genSingleTextNode node.OpeningParen
-        +> genType node.Type
-        +> genSingleTextNode node.ClosingParen
-        |> genNode node
+        match node.Type with
+        | Type.Funs _ ->
+            let short =
+                genSingleTextNode node.OpeningParen
+                +> genType node.Type
+                +> genSingleTextNode node.ClosingParen
+
+            let long =
+                genSingleTextNode node.OpeningParen
+                +> indent
+                +> genType node.Type
+                +> unindent
+                +> genSingleTextNode node.ClosingParen
+
+            expressionFitsOnRestOfLine short long |> genNode node
+        | _ ->
+            genSingleTextNode node.OpeningParen
+            +> genType node.Type
+            +> genSingleTextNode node.ClosingParen
+            |> genNode node
     | Type.SignatureParameter node ->
         genOnelinerAttributes node.Attributes
         +> optSingle (fun id -> genSingleTextNode id +> sepColon) node.Identifier