Friends

After 10 days of working on !15719 I have suddenly started getting

/home/simonpj/code/HEAD-19/libraries/ghc-boot-th-next/../ghc-internal/src/GHC/Internal/Classes.hs:10:17: error: [GHC-04924]
    Unknown flag in  {-# OPTIONS_GHC #-} pragma: -fdefines-known-key-names
   |
10 | {-# OPTIONS_GHC -fdefines-known-key-names #-}
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^

I have no idea why this was fine until 10am this morning, and has suddenly started happening now.  

How does GHC decide what runtime flags it allows?  `-fdefines-known-key-names` is in GHC.Driver.Session.fFlagsDeps.

I'm boggled that it worked until 10am and then abruptly stopped.

Any advice?

Simon