Workaround for strange compiler error

src/spectator/matchers/dummy_match_data.cr

@@ -0,0 +1,30 @@
+require "./match_data"
+
+module Spectator::Matchers
+  # Match data that does nothing useful.
+  # This is a workaround for a compilation issue - hopefully temporary.
+  # This might be the same issue that `DummyExample` solves.
+  # Without this, the Crystal compiler complains:
+  # ```text
+  # undefined method 'Spectator::Matchers::MatchData#message()'
+  # ```
+  # The method is clearly defined by `MatchData`, just abstract.
+  # Additionally, there are implementations of `MatchData` -
+  # see `NilMatcher::MatchData`.
+  private struct DummyMatchData < MatchData
+    # Dummy values.
+    def values
+      {expected: nil, actual: nil}
+    end
+
+    # Dummy message.
+    def message
+      "DUMMY"
+    end
+
+    # Dummy message.
+    def negated_message
+      "DUMMY"
+    end
+  end
+end