Fix fsprojects#1937 (fsprojects#1938)

* fix 1937

* fix format

* Use formatSourceString in unit test.

Co-authored-by: nojaf <florian.verdonck@outlook.com>

src/Fantomas.Tests/OperatorTests.fs

@@ -1256,3 +1256,32 @@ let subtractTwo = + {literalValue}
         $"""
 let subtractTwo = + {literalValue}
 """
+
+[<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|)
+"""

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