Add regression test for #15655: error codes 999 and 3217 are distinct (#19480)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: abonie <20281641+abonie@users.noreply.github.com>

Author: 3 people

tests/FSharp.Compiler.ComponentTests/ErrorMessages/DiagnosticRegressionTests.fs

@@ -3,6 +3,21 @@ module ErrorMessages.DiagnosticRegressionTests
 open Xunit
 open FSharp.Test.Compiler
 
+// https://github.com/dotnet/fsharp/issues/15655
+[<Fact>]
+let ``Issue 15655 - error codes 999 and 3217 are distinct`` () =
+    // Verify that notAFunctionButMaybeIndexerErrorCode (3217) is distinct from packageManagerError (999)
+    // by triggering a "not a function but maybe indexer" error
+    FSharp
+        """
+let d = System.Collections.Generic.Dictionary<string,int>()
+let v = d ["key"]
+        """
+    |> typecheck
+    |> shouldFail
+    |> withErrorCode 3217
+
+    
 // https://github.com/dotnet/fsharp/issues/13697
 [<Fact>]
 let ``Issue 13697 - typeof with out of scope type in attribute should report FS0039`` () =
@@ -17,7 +32,8 @@ type Vehicle() = class end
     |> withDiagnostics
         [ (Error 39, Line 3, Col 14, Line 3, Col 28, "The type 'OutOfScopeType' is not defined.")
           (Error 267, Line 3, Col 7, Line 3, Col 29, "This is not a valid constant expression or custom attribute value") ]
-          
+
+
 // https://github.com/dotnet/fsharp/issues/16410
 [<Fact>]
 let ``Issue 16410 - no spurious FS3570 warning with KeyValue active pattern`` () =