split_marker crash

Urrgh. I have this panic in a clean validate (sh validate -fast) on my branch wip/T15809. ghc-stage1: panic! (the 'impossible' happened) (GHC version 8.7.20181119 for x86_64-unknown-linux): split_marker_entry Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug libraries/ghc-prim/ghc.mk:4: recipe for target 'libraries/ghc-prim/dist-install/build/GHC/Types.o' failed Yes, it's a branch not identical to HEAD, but I have done nothing concerning split_markers. Sigh. What should I do? Simon