diff --git a/Tests/SwiftDiagnosticsTest/GroupDiagnosticsFormatterTests.swift b/T
ests/SwiftDiagnosticsTest/GroupDiagnosticsFormatterTests.swift
index eccd50dc1..461bc2333 100644
--- a/Tests/SwiftDiagnosticsTest/GroupDiagnosticsFormatterTests.swift
+++ b/Tests/SwiftDiagnosticsTest/GroupDiagnosticsFormatterTests.swift
@@ -27,6 +27,32 @@ extension SimpleDiagnosticMessage: FixItMessage {
   var fixItID: MessageID { diagnosticID }
 }
 
+struct SimpleNoteMessage: NoteMessage {
+  let message: String
+  let fixItID: MessageID
+}
+
+/// The severity of a particular marked diagnostic. This differs from
+/// DiagnosticSeverity in that notes are associated within another marked
+/// diagnostic.
+enum MarkedSeverity {
+  case error
+  case warning
+  case remark
+
+  /// Note attached to the diagnostic at a particular location
+  case note(String)
+
+  var asSeverity: DiagnosticSeverity {
+    switch self {
+      case .error: return .error
+      case .warning: return .warning
+      case .remark: return .remark
+      case .note: fatalError("Notes should never be mapped this way")
+    }
+  }
+}
+
 extension GroupedDiagnostics {
   /// Add a new test file to the group, starting with marked source and using
   /// the markers to add any suggested extra diagnostics at the marker
@@ -35,29 +61,58 @@ extension GroupedDiagnostics {
     _ markedSource: String,
     displayName: String,
     parent: (SourceFileID, AbsolutePosition)? = nil,
-    extraDiagnostics: [String: (String, DiagnosticSeverity)] = [:]
+    extraDiagnostics: [String: (String, MarkedSeverity)] = [:]
   ) -> (SourceFileID, [String: AbsolutePosition]) {
     // Parse the source file and produce parser diagnostics.
     let (markers, source) = extractMarkers(markedSource)
     let tree = Parser.parse(source: source)
     var diagnostics = ParseDiagnosticsGenerator.diagnostics(for: tree)
 
+    // Queue up notes for diagnostics.
+    var allNotes: [String: [Note]] = [:]
+    for (marker, (message, severity)) in extraDiagnostics {
+      guard case .note(let owner) = severity else {
+        continue
+      }
+
+      let pos = AbsolutePosition(utf8Offset: markers[marker]!)
+      let node = tree.token(at: pos)!.parent!
+
+      allNotes[owner, default: []].append(.init(
+        node: node,
+        message: SimpleNoteMessage(
+          message: message, fixItID: MessageID(domain: "test", id: "conjured")
+        )
+      ))
+    }
+
     // Add on any extra diagnostics provided, at their marker locations.
     for (marker, (message, severity)) in extraDiagnostics {
+      // Skip notes; they've been handled above.