fibheaps is broken in mode=slow

$ make -C gc mode=slow
...
expected stderr not matched by reality
--- /run/user/1000/no_stderr20579	2019-12-13 00:56:57.928824266 +0000
+++ /run/user/1000/runtest20579.2	2019-12-13 00:56:57.937824444 +0000
@@ -0,0 +1,2 @@
+fibheaps: Main.lhs:(296,21)-(297,37): Non-exhaustive patterns in lambda
+