From 15050d193d70e2e490a5a25873deaf9c3a411b0d Mon Sep 17 00:00:00 2001
From: Don Syme <dsyme@users.noreply.github.com>
Date: Mon, 1 Nov 2021 08:41:48 +0000
Subject: [PATCH] Fix #1937 (#1938)

* fix 1937

* fix format

* Use formatSourceString in unit test.

Co-authored-by: nojaf <florian.verdonck@outlook.com>
---
 src/Fantomas.Tests/OperatorTests.fs | 29 +++++++++++++++++++++++++++++
 src/Fantomas/SourceParser.fs        |  6 +++++-
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/src/Fantomas.Tests/OperatorTests.fs b/src/Fantomas.Tests/OperatorTests.fs
index e3f561e626..38cafc5a21 100644
--- a/src/Fantomas.Tests/OperatorTests.fs
+++ b/src/Fantomas.Tests/OperatorTests.fs
@@ -1219,3 +1219,32 @@ module Foo =
             | true -> id
             | false -> id)
 """
+
+[<Test>]
+let ``qualified name to active pattern, 1937`` () =
+    formatSourceString
+        false
+        """
+StringPosition.(|TrimStart|)
+(|TrimStart|)
+StringPosition.(|TrimStart|_|)
+(|TrimStart|_|)
+StringPosition.(|A|B|)
+(|A|B|)
+
+let f (|A|B|) = (|A|B|)
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+StringPosition.(|TrimStart|)
+(|TrimStart|)
+StringPosition.(|TrimStart|_|)
+(|TrimStart|_|)
+StringPosition.(|A|B|)
+(|A|B|)
+
+let f (|A|B|) = (|A|B|)
+"""
diff --git a/src/Fantomas/SourceParser.fs b/src/Fantomas/SourceParser.fs
index 22ae860276..e18a566652 100644
--- a/src/Fantomas/SourceParser.fs
+++ b/src/Fantomas/SourceParser.fs
@@ -36,7 +36,11 @@ let (|Ident|) (s: Ident) =
     | "not" -> "not"
     | "params" -> "``params``"
     | "parallel" -> "``parallel``"
-    | _ -> PrettyNaming.AddBackticksToIdentifierIfNeeded ident
+    | _ ->
+        if IsActivePatternName ident then
+            sprintf "(%s)" (DecompileOpName ident)
+        else
+            PrettyNaming.AddBackticksToIdentifierIfNeeded ident
 
 let (|LongIdent|) (li: LongIdent) =
     li